Matter: Grade 7 Science – Science Playground

Hold a brick in one hand and a pillow in the other. The pillow is bigger, but the brick pulls your hand down much harder. Drop the brick into a bucket of water and it sinks, but a block of wood the same size floats. Why? And what are the brick, the pillow, the water and the air around them all made of?

In this unit you will measure how tightly matter is packed (its density), find out why some things float and others sink, look inside the tiny atoms that everything is built from, and learn to tell a change you can undo, like melting ice, from one you can’t, like burning wood.

What matter is

Look around you: air, water, soil, stones, wood, your own body. Do all of them take up space? Do they all have mass? And could you catch light or sound in a box, the way you catch air in a balloon or a football?

Figure 9.1 Matter is found in three states. In a solid the particles are packed tightly in a fixed pattern, so a solid keeps its shape. In a liquid they are still close together but can slide past one another, so a liquid flows and takes the shape of its container. In a gas they are far apart and move everywhere, so a gas spreads out to fill any space.

Anything that has mass and occupies space (has volume) is called matter. Air, water, soil, stone and wood are all matter, in one of three states: solid, liquid or gas (Figure 9.1).

Light, heat, sound and shadows are not matter. They have no mass and take up no space. Light, heat and sound are forms of energy; a shadow is only a place where light doesn’t reach. You can catch air in a balloon, but you can’t fill a box with light and weigh it.

Is each one matter? Decide, then check. A few are tricky.

a cricket ball

0 right of 0 · 1 of 12

Ask yourself: does it have mass, and does it take up space?

Right. A cricket ball has mass (it is heavy in your hand) and takes up space: matter.

Not quite. A cricket ball has mass (it is heavy in your hand) and takes up space: matter.

Right. Air is a gas, and gases are matter: the air pushes the balloon out (volume), and a blown-up balloon is a little heavier than an empty one (mass).

Not quite. Air is a gas, and gases are matter: the air pushes the balloon out (volume), and a blown-up balloon is a little heavier than an empty one (mass).

Right. Steam is water as a gas. Water has mass and volume in every state, so steam is matter.

Not quite. Steam is water as a gas. Water has mass and volume in every state, so steam is matter.

Right. Sunlight is a form of energy, not a substance. It has no mass, and you can’t fill a bottle with it and weigh it.

Not quite. Sunlight is a form of energy, not a substance. It has no mass, and you can’t fill a bottle with it and weigh it.

Right. A shadow is only a place where light is blocked. There is nothing there to weigh: not matter.

Not quite. A shadow is only a place where light is blocked. There is nothing there to weigh: not matter.

Right. Heat is energy moving from a hot thing to a cooler one. The stove is matter; its heat is not.

Not quite. Heat is energy moving from a hot thing to a cooler one. The stove is matter; its heat is not.

Right. Sound is a vibration passed along through air. The air and the drum are matter, but the sound itself has no mass or volume.

Not quite. Sound is a vibration passed along through air. The air and the drum are matter, but the sound itself has no mass or volume.

Right. Tricky! You smell momos because tiny particles from the food float through the air into your nose. Those particles are matter.

Not quite. Tricky! You smell momos because tiny particles from the food float through the air into your nose. Those particles are matter.

Right. Smoke is tiny bits of solid and gas rising from the burning incense. It has mass and takes up space: matter.

Not quite. Smoke is tiny bits of solid and gas rising from the burning incense. It has mass and takes up space: matter.

Right. A rainbow is sunlight split into colors by raindrops. The drops are matter, but the rainbow is light: not matter.

Not quite. A rainbow is sunlight split into colors by raindrops. The drops are matter, but the rainbow is light: not matter.

Right. Petrol is a liquid. It has mass and fills the tank: matter.

Not quite. Petrol is a liquid. It has mass and fills the tank: matter.

Right. An idea has no mass and takes up no space. Your brain is matter; the idea is not.

Not quite. An idea has no mass and takes up no space. Your brain is matter; the idea is not.

Some substances have a lot of mass in a small space: a small lump of iron, gold or mercury is surprisingly heavy. Others have little mass even though they take up a lot of space: paper, plastic foam, cotton wool. The difference is in how closely their particles are packed and how heavy each particle is.

Which of these is not matter?

Pick an answer to see why.

9.1 Density of matter

Figure 9.2 An iron block and a wooden block of exactly the same size on a beam balance. The iron side goes down: the same volume holds more mass of iron than of wood.

At normal temperature, substances are found as solids, liquids or gases. The particles are packed less closely in a liquid than in a solid, and much less closely in a gas than in a liquid. So equal volumes of different substances don’t have equal masses: the particles are packed more or less closely, and some particles are heavier than others. How much mass is packed into each bit of space is called density.

Activity

9.1: comparing the density of different substances

You need: a hollow box shaped like a cuboid (a small cardboard box), plastic bags, a beam balance, and sand, soil, husk and sawdust.

Measure the length, breadth and height of the box and work out its volume (length × breadth × height).
Fill the box completely with sand, level the top, and empty it into a plastic bag.
Do the same with soil, husk and sawdust, each in its own bag.
Measure the mass of each bag on the beam balance (use the same kind of bag each time, or subtract the bag’s own mass).
Divide each mass by the volume of the box and complete the table.

S.N.	Name of matter	Mass of matter (g)	Mass ÷ volume (g/cm³)
1.	sand
2.	soil
3.	husk
4.	sawdust

a.Why do equal volumes of these substances have different masses?
b.What is the density of sand, soil, husk and sawdust?

What you should see

The boxful of sand is the heaviest and the boxful of husk the lightest, though every one fills the same box. Typical results: dry sand about 1.5–1.6 g/cm³, garden soil about 1.1–1.3 g/cm³, sawdust about 0.2 g/cm³, rice husk about 0.1 g/cm³ (yours will differ a little: it depends how damp and how tightly packed they are).

a.The grains of sand are hard and heavy and pack closely together; husk and sawdust are light, and are full of air between the bits. So the same space holds very different masses.
b.Each density is the mass of that boxful divided by the volume of the box.

Mass per unit volume of a substance is called its density: the mass of 1 cm³ (or of 1 m³) of it.

$\text{density } (D) = \frac{\text{mass of the object } (m)}{\text{volume of the object } (V)} \qquad D = \frac{m}{V}$

If mass is measured in kilograms (kg) and volume in cubic meters (m³), density is in kilograms per cubic meter (kg/m³). This is the SI unit of density.
If mass is measured in grams (g) and volume in cubic centimeters (cm³), density is in grams per cubic centimeter (g/cm³). This is the CGS unit of density.

To change g/cm³ into kg/m³, multiply by 1000. Water has a density of 1 g/cm³, which is 1000 kg/m³.

Numerical problems

The same three steps work for every problem: write down what is given (with units), write the formula, then put in the numbers.

Relative density

To compare how dense a substance is, scientists compare it with water at 4 °C, whose density is 1 g/cm³ (1000 kg/m³). (Water is densest at 4 °C, so that temperature is chosen.)

The ratio of the density of a substance to the density of water at 4 °C is called the relative density of the substance. It tells you how many times heavier the substance is than the same volume of water.

$\text{relative density} = \frac{\text{density of the substance}}{\text{density of water at 4 °C}}$

Relative density has no unit: it is one density divided by another, so the units cancel. In g/cm³, the relative density is simply the same number as the density, because water’s density is 1 g/cm³.

S.N.	Substance	Density (kg/m³)	Density (g/cm³)	Relative density
1.	ice	920	0.92	0.92
2.	aluminium	2700	2.7	2.7
3.	iron	7800	7.8	7.8
4.	kerosene	800	0.8	0.8
5.	mercury	13 600	13.6	13.6
6.	gold	19 300	19.3	19.3

Floating and sinking

The rule is simple:

If its relative density is more than 1 (denser than water), a substance sinks in water.
If its relative density is less than 1 (less dense than water), it floats.

The same rule works in any liquid: a substance sinks in a liquid if it is denser than the liquid, and floats if it is less dense than the liquid.

Why? An object in a liquid pushes some liquid out of the way, and the liquid pushes back up on it. A floating object sinks only until the liquid it has pushed aside weighs as much as the object itself. The less dense the object, the less of it needs to be under the surface. An object denser than the liquid can’t push aside enough, even when it is completely under, so it sinks.

Pick an object and a liquid. Try ice in water and then in kerosene, and an egg in water and then in very salty water.

Object:

Liquid:

Density of the object: 0.92 g/cm³
Density of the liquid: 1.00 g/cm³
Relative density of the object: 0.92

Floats. The cork is less dense than the water (0.24 < 1.00 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 24% of it is under the surface.

Floats. The cork is less dense than the very salty water (0.24 < 1.15 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 21% of it is under the surface.

Floats. The cork is less dense than the kerosene (0.24 < 0.80 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 30% of it is under the surface.

Floats. The dry wood is less dense than the water (0.60 < 1.00 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 60% of it is under the surface.

Floats. The dry wood is less dense than the very salty water (0.60 < 1.15 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 52% of it is under the surface.

Floats. The dry wood is less dense than the kerosene (0.60 < 0.80 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 75% of it is under the surface.

Floats. The ice is less dense than the water (0.92 < 1.00 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 92% of it is under the surface.

Floats. The ice is less dense than the very salty water (0.92 < 1.15 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 80% of it is under the surface.

Sinks. The ice is denser than the kerosene (0.92 > 0.80 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The egg is denser than the water (1.08 > 1.00 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Floats. The egg is less dense than the very salty water (1.08 < 1.15 g/cm³), so it sinks only until it has pushed aside its own weight of liquid: about 94% of it is under the surface.

Sinks. The egg is denser than the kerosene (1.08 > 0.80 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The aluminium is denser than the water (2.70 > 1.00 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The aluminium is denser than the very salty water (2.70 > 1.15 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The aluminium is denser than the kerosene (2.70 > 0.80 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The iron is denser than the water (7.80 > 1.00 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The iron is denser than the very salty water (7.80 > 1.15 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The iron is denser than the kerosene (7.80 > 0.80 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The gold is denser than the water (19.30 > 1.00 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The gold is denser than the very salty water (19.30 > 1.15 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

Sinks. The gold is denser than the kerosene (19.30 > 0.80 g/cm³), so even with all of it under the surface it can’t push aside its own weight of liquid.

A piece of candle wax (relative density 0.9) is dropped into kerosene (relative density 0.8). What happens?

Pick an answer to see why.

A story from science

Archimedes and the king’s crown

More than 2200 years ago, the story goes, King Hiero of Syracuse gave a goldsmith some gold to make a crown. When the crown came back, the king suspected that the goldsmith had kept some of the gold and mixed in cheaper silver. He asked the scientist Archimedes to find out, without damaging the crown.

Archimedes knew that silver is less dense than gold, so a crown with silver in it would be bigger than a pure gold crown of the same mass. The trouble was measuring the volume of something so oddly shaped. Stepping into a full bath, he noticed the water spill over the edge, and realized that an object pushes aside exactly its own volume of water. He is said to have run home shouting “Eureka!” (“I have found it!”). The crown pushed aside more water than an equal mass of pure gold: the goldsmith had cheated.

9.2 Element and compound

Element

Break a piece of copper into smaller and smaller bits by any means you like, and every bit is still copper, with all the properties of copper. Copper can’t be split by any chemical process into anything simpler. Substances like this are called elements.

An element is a pure substance that can’t be broken down into simpler substances by any chemical process. Iron, gold, silver, copper, oxygen, hydrogen and sulphur are elements.

Scientists know 118 elements. About 92 of them are found in nature (the elements up to uranium); the rest have been made in laboratories, a few atoms at a time.

Elements are found in all three states. At normal temperature:

solid: iron, gold, silver, copper, carbon, sulphur, and most others;
liquid: only mercury and bromine;
gas: hydrogen, oxygen, nitrogen, chlorine, and the noble gases such as helium, neon and argon.

Figure 9.3 Kinds of matter. A pure substance is either an element (one kind of atom, such as copper) or a compound (one kind of molecule, made of different atoms joined chemically, such as water). Substances mixed physically, without joining, make a mixture: homogeneous if it is the same all through (salt water), heterogeneous if you can see different parts (sand in water).

Figure 9.3 sorts matter into these groups. A mixture is made of two or more substances mixed together without joining chemically. Each keeps its own properties, and they can be mixed in any amounts: salt water can be a little salty or very salty. A mixture that is the same all through, like salt water, air or sharbat, is homogeneous; one in which the parts can be seen, like sand in water or muddy water, is heterogeneous.

Atom

If an element is broken down further and further, in the end we reach the smallest particles of all. These smallest particles are called atoms.

An atom is the smallest particle of an element that can take part in a chemical reaction. When iron reacts with another substance, it is iron atoms that take part.

All the atoms of one element are alike, and different from the atoms of every other element: an oxygen atom is different from a hydrogen atom. Since there are 118 elements, there are 118 kinds of atoms. Every substance in the universe, from your fingernail to the Sun, is made of these 118 kinds, in different combinations.

Structure of an atom

Small as it is, an atom is made of even smaller parts, called sub-atomic particles: electrons, protons and neutrons.

An atom has two parts:

the nucleus, the tiny center of the atom, where the protons and neutrons are;
the shells (also called orbits) round the nucleus, where the electrons move.

Figure 9.4 An atom of helium. In the nucleus are 2 protons (+) and 2 neutrons (no charge); 2 electrons (−) move round the nucleus in a shell. (Not to scale: the nucleus is about 100 000 times smaller across than the whole atom.)

Proton

A proton is a positively charged (+) sub-atomic particle, found in the nucleus. Its mass is almost exactly the mass of one hydrogen atom, and is taken as 1 atomic mass unit (amu).

Neutron

A neutron has no charge: it is neutral. It is also found in the nucleus. Its mass is almost the same as a proton’s, so it too has a mass of about 1 amu.

Electron

An electron is a negatively charged (−) particle. It moves round the nucleus in a fixed path called a shell or orbit. Its mass is tiny compared with a proton’s or a neutron’s: about 1/1837 of the mass of a hydrogen atom (1/1836 of a proton’s), so small that it is usually ignored.

Sub-atomic particle	Symbol	Mass	Charge	Where it is
proton	p⁺	1 amu	positive (+)	nucleus
electron	e⁻	1/1837 amu	negative (−)	shells
neutron	n⁰	1 amu	none (0)	nucleus

An atom has as many electrons as protons. The positive charges of the protons and the negative charges of the electrons cancel, so an atom as a whole has no charge: it is electrically neutral.

Which two particles are found in the nucleus of an atom?

Pick an answer to see why.

Symbols of elements

Each element has a symbol: one capital letter, or a capital letter followed by a small one. Usually it comes from the element’s English name (H for hydrogen, Ca for calcium), but a few come from the old Latin names: Na from natrium (sodium) and K from kalium (potassium).

S.N.	Element	Symbol	Protons	Electrons	Neutrons
1.	hydrogen	H	1	1	0
2.	helium	He	2	2	2
3.	lithium	Li	3	3	4
4.	beryllium	Be	4	4	5
5.	boron	B	5	5	6
6.	carbon	C	6	6	6
7.	nitrogen	N	7	7	7
8.	oxygen	O	8	8	8
9.	fluorine	F	9	9	10
10.	neon	Ne	10	10	10
11.	sodium (natrium)	Na	11	11	12
12.	magnesium	Mg	12	12	12
13.	aluminium	Al	13	13	14
14.	silicon	Si	14	14	14
15.	phosphorus	P	15	15	16
16.	sulphur	S	16	16	16
17.	chlorine	Cl	17	17	18
18.	argon	Ar	18	18	22
19.	potassium (kalium)	K	19	19	20
20.	calcium	Ca	20	20	20

The neutron numbers are for each element’s most common kind of atom.

Two numbers describe an atom:

$\text{atomic number} = \text{number of protons} = \text{number of electrons}$
$\text{atomic mass} = \text{number of protons} + \text{number of neutrons}$

The atomic number is the number of protons in the nucleus; in a neutral atom it is also the number of electrons. Every element has its own atomic number: every atom with 8 protons is oxygen. The atomic mass (more exactly called the mass number) adds up the particles in the nucleus, since the electrons weigh almost nothing.

Figure 9.5 Models of an oxygen atom (8 protons, 8 neutrons; electrons 2 and 6 in its two shells) and a chlorine atom (17 protons, 18 neutrons; electrons 2, 8 and 7 in three shells). Each shell holds only so many electrons; you will learn the rule in Grade 8.

Project

Make a model of an atom

Look at the models of an oxygen atom and a chlorine atom in Figure 9.5.

Take a chart paper. Draw a small circle in the middle with a pencil compass and stick a colored thread along it with glue: this is the nucleus.
Draw a bigger circle round it with the same center and stick a thread along it: the first shell. Add more shells as the atom needs.
Take three kinds of pulses found at home or in the market, such as black gram (maas), soyabean, split red gram (rahar) or black-eyed beans (bodi). Let one kind stand for protons, one for neutrons and one for electrons: for example, black gram for protons, soyabean for neutrons and split red gram for electrons.
Stick the right number of each in the right place to make a model of a helium atom (2 protons and 2 neutrons in the nucleus, 2 electrons in the first shell).
In the same way, make a model of an oxygen atom (8 protons and 8 neutrons in the nucleus; 2 electrons in the first shell and 6 in the second).

Molecule

Atoms usually don’t stay alone: they join together in small groups. A group of two or more atoms joined together is called a molecule. Water, air, sugar and much of the matter around you are made of molecules.

The molecules of an element are made of atoms of one kind: a hydrogen molecule (H₂) is two hydrogen atoms; an oxygen molecule (O₂) is two oxygen atoms; nitrogen (N₂) and chlorine (Cl₂) are the same.
The molecules of a compound are made of atoms of different kinds: a water molecule (H₂O) is one oxygen atom joined to two hydrogen atoms; a hydrogen chloride molecule (HCl) is one hydrogen atom and one chlorine atom.

Figure 9.6 Molecules drawn as balls (a space-filling model). The atoms’ sizes and the distances between their centers are to one scale, so joined atoms overlap. On the left, molecules of elements: two atoms of the same kind. On the right, molecules of compounds: atoms of different kinds. In water, the two hydrogen atoms sit on one side of the oxygen atom, at an angle of 104.5°.

A molecule has all the properties of its substance. A spoonful of sugar tastes sweet, and so does each molecule of sugar. Split a sugar molecule into its atoms (carbon, hydrogen and oxygen), and they are not sweet at all. So:

A molecule is the smallest particle of an element or a compound that has all the properties of that substance.

The molecule of an element is made of the same kind of atoms (two oxygen atoms make an oxygen molecule). The molecule of a compound is made of two or more different kinds of atoms (one oxygen atom and two hydrogen atoms make a water molecule). In nature, elements join to form compounds, and compounds break down into new substances. That is how matter changes.

Compound

Figure 9.7 From atoms to compounds. Single atoms (H, O, Cl) join into molecules of elements (H₂, O₂, Cl₂), or with atoms of other kinds into molecules of compounds (H₂O, HCl).

When atoms of two or more elements react together, a new substance is formed: this is called a chemical reaction. A compound is the substance formed when two or more elements combine chemically, in a fixed ratio by number of atoms. Water is formed from hydrogen and oxygen, always two hydrogen atoms to one oxygen atom. Common salt is formed from sodium and chlorine, always one to one. Water (H₂O) and common salt (sodium chloride, NaCl) are compounds.

A compound is a new substance with new properties, often nothing like the elements it is made from. Sodium is a soft metal that reacts violently with water, and chlorine is a poisonous yellow-green gas, yet together they make the salt you sprinkle on your food.

How is a compound different from a mixture?

Pick an answer to see why.

9.3 Change in matter

Activity

9.4: two kinds of change

Take a piece of ice and a small piece of wood. Warm the ice. Then, with your teacher, burn the piece of wood. Watch both.

a.What differences do you find between the two? Think about the state, the structure and the properties of each.
b.List some similar changes that happen in daily life, and discuss them in class.

What you should see

a.The ice melts into water: only its state changes. The water is still the same substance, and it freezes back into ice in a freezer. The wood burns: it turns into black charcoal, gray ash, smoke and gases, all new substances with new properties, and no way exists to turn ash back into wood.
b.Like melting ice: water boiling, ghee melting, salt dissolving. Like burning wood: food cooking, a candle wick burning, iron rusting, fruit rotting.

Matter changes all the time, because of heat, light, pressure, electricity and so on. In some changes only the state, shape, size or color changes; in others the substance itself changes into a new one with new properties. So there are two kinds of change: physical change and chemical change.

Physical change

Figure 9.8 Water to vapor and back. Water boils in the kettle; its vapor rises from the spout, meets the cold plate and turns back into drops of water, which drip into the beaker. (Steam itself is invisible: the white cloud you see near a kettle is tiny drops already formed again.)

A change in which the color, volume, shape, size, state or density of matter changes, but not its chemical nature, is called a physical change. No new substance is formed.

Physical changes are all around us: a big lump of salt is crushed into fine powder; ice or snow melts into water; water freezes into ice; a log is chopped into firewood; spices are ground into powder; salt dissolves in water to make salt water.

Make a list of other physical changes in your daily life and present it in class.

Characteristics of physical changes

a.The state, shape or size of the matter may change, but its chemical nature stays the same.
b.No new substance is formed.
c.The change can usually be reversed.
d.It is a temporary change.
e.The mass of the matter doesn’t change.
f.Physical changes are brought about by heat, light, pressure, electricity and so on.
g.The molecules themselves don’t change.

Chemical change

Activity

9.6: what happens when magnesium burns?

Do this only with your teacher.

You need: a strip of magnesium ribbon, tongs, matches or a lighter, a spirit lamp or Bunsen burner, and a watch glass.

Hold the magnesium ribbon with the tongs and light its end in the flame.
Hold it over the watch glass as it burns, so that the ash falls into the glass.
Compare the magnesium ribbon with what is left in the watch glass.
- a.Are the properties of the magnesium ribbon and its ash the same?
- b.What kind of change is this?

Careful: magnesium burns with a dazzling white light. Don’t stare at it.

What you should see

The ribbon burns with a brilliant white flame and leaves a soft white powder.

a.No. The ribbon was a shiny, silvery-gray, bendable metal; the powder is white, dull and crumbly. It is a new substance, magnesium oxide.
b.A chemical change: magnesium has joined with oxygen from the air.

magnesium + oxygen → magnesium oxide

Figure 9.9 Burning magnesium ribbon. Held in tongs over a watch glass, the ribbon burns with a dazzling white light, and white magnesium oxide falls into the glass.

A coil of magnesium ribbon burning on a metal plate with a blinding white flame and white smoke, white ash lying around it — **Figure 9.10** A coil of magnesium ribbon burning. Its white smoke and the white ash round it are magnesium oxide. The light is so bright that magnesium was once used in camera flashes, and it is still used in fireworks. Photo: Capt. John Yossarian, CC BY-SA 3.0

A change in which a substance turns into a new substance with new properties is called a chemical change. In it, the molecules themselves change: their atoms separate and join up in new ways.

Burning magnesium into magnesium oxide is a chemical change: the magnesium reacts with oxygen in the air. Other chemical changes: iron rusting, fruit rotting, food being digested and used (“burnt”) in our bodies, photosynthesis in leaves, wood burning, waste turning into compost, and hydrogen and oxygen joining to form water.

Make a list of other chemical changes in your daily life and present it in class.

Characteristics of chemical changes

a.New substances with different properties are formed.
b.It is a permanent change: the new substance can’t easily be turned back into the original one.
c.The molecules themselves change: their atoms are rearranged.
d.Some chemical changes give out heat, and some take in heat.

Signs that a chemical change has happened: a new color, a gas given off (bubbles or a smell), heat or light given out, or a solid appearing.

Comparing physical and chemical changes

Physical change	Chemical change
1.The state, shape or size may change; the substance stays the same.	1.The substance changes: its properties change, and its state may too.
2.The change can usually be reversed.	2.The change can’t usually be reversed.
3.No new substance is formed.	3.A new substance is formed.
4.It is a temporary change.	4.It is a permanent change.
5.The molecules don’t change.	5.The molecules change: atoms join up in new ways.
6.Example: ice melting.	6.Example: wood burning.

Sort each change. Ask yourself: is a new substance made?

Ice melting in a glass

Rice burning at the bottom of the pot

Chopping wood into firewood

An iron gate rusting

Milk turning into curd

Sugar dissolving in tea

Grinding spices on a silauto

Cooking an egg

Steam from a kettle turning into drops on a lid

A leaf making food in sunlight

All sorted!

Physical change

ice melting
chopping wood
sugar dissolving
grinding spices
steam to drops

Chemical change

rice burning
rusting
milk to curd
cooking an egg
photosynthesis

Is it a physical change or a chemical change? Tap a group.

Right. Ice and water are the same substance, H₂O. Only the state changes, and freezing turns it back.

Not quite. Is the water from melted ice a new substance, or still H₂O?

Right. Black, bitter carbon and smoke are new substances. You can’t turn them back into rice.

Not quite. Can burnt rice ever become white rice again?

Right. Each small piece is still wood. Only the size and shape change; no new substance is made.

Not quite. Is a small piece of wood a different substance from a big log?

Right. Rust is a new, brown, crumbly substance (iron oxide) made from iron, oxygen and water.

Not quite. Is rust the same stuff as shiny iron?

Right. Bacteria turn the milk’s sugar into lactic acid, a new substance. Curd can’t be turned back into milk.

Not quite. Can you turn curd back into fresh milk?

Right. The sugar is still sugar: the tea tastes sweet, and boiling the water away would leave the sugar behind.

Not quite. Does the tea still taste of sugar?

Right. Powdered jeera is still jeera. Only the size of the pieces changes.

Not quite. Is jeera powder a different substance from whole jeera?

Right. Heat changes the egg’s proteins for good: the clear liquid turns white and solid, and no cooling makes it raw again.

Not quite. Can a boiled egg ever become raw again?

Right. Steam and the drops are both water. Only the state changes, from gas to liquid.

Not quite. Are the drops on the lid a new substance, or water?

Right. Photosynthesis makes new substances, sugar and oxygen, from carbon dioxide and water.

Not quite. Does the leaf make anything new from carbon dioxide and water?

Right. Photosynthesis makes new substances, sugar and oxygen, from carbon dioxide and water. Every one is sorted.

Which of these is a chemical change?

Pick an answer to see why.

Thick links of an old iron chain covered in orange-brown rust, which is flaking off in layers — **Figure 9.11** Rust on an old iron chain. Iron, oxygen and water have made a new, crumbly substance, which flakes off in layers and lets the rust eat deeper. Photo: W.carter, CC0

Project

How raksi is made

In many parts of Nepal, raksi is made from cooked grains such as millet or rice. The cooked grain is mixed with marcha, a starter cake full of yeasts and molds, and left for some days to ferment. The fermented mash is then heated: the alcohol turns into vapor, which is led away, cooled, and turns back into liquid alcohol. Find out (by watching, from books or the internet, or by asking your elders) which steps are physical changes and which are chemical. Make a presentation and discuss it in class.

What you should see

Cooking the grain: chemical (heat changes the starch and proteins).
Fermentation with marcha: chemical. The molds turn starch into sugar, and the yeasts turn sugar into alcohol and carbon dioxide: new substances.
Heating the mash until the alcohol turns to vapor: physical (a change of state).
Cooling the vapor back into liquid: physical (condensation, as on the cold plate in Activity 9.5).

This is a science study of a process, not a recipe: alcohol harms a growing body and brain, and isn’t for children.

Exercise

1. Fill in the blanks with suitable words from the box

1/1837 · Na · volume · chemical · less · K · more · sinking

The density of a substance depends on its mass and __________.

Answer

volume (density = mass ÷ volume)

A substance which has __________ density than water floats in water.

Answer

less

A __________ body has more density than water.

Answer

sinking

The mass of an electron is __________ amu.

Answer

1/1837

Potassium is represented by the symbol __________.

Answer

K (from its Latin name, kalium)

Rusting of iron is a __________ change.

Answer

chemical

2. Match the following

Match each atom with its atomic number.

Name of atom	Atomic number
potassium	8
helium	16
boron	19
sulphur	20
silicon	5
oxygen	14
	2

Answer

Name of atom	Atomic number
potassium	19
helium	2
boron	5
sulphur	16
silicon	14
oxygen	8

(20, calcium’s atomic number, is left over.)

3. Choose the best answer

a. What is the density of water?

Pick an answer to see why.

b. Which is the correct formula for density?

Pick an answer to see why.

c. Which sub-atomic particle is positively charged?

Pick an answer to see why.

d. Which of the following best defines a molecule?

Pick an answer to see why.

e. Which group shows only physical changes?

Pick an answer to see why.

f. Which of the following is true for an element?

Pick an answer to see why.

g. Which statement is true for a compound?

Pick an answer to see why.

h. Which of the following is true for potassium?

Pick an answer to see why.

4. Differentiate between

Chemical change and physical change

Answer

Chemical change	Physical change
A new substance with new properties is formed.	No new substance is formed.
It is permanent: it can’t usually be reversed.	It is temporary: it can usually be reversed.
The molecules change.	The molecules stay the same.
Example: burning wood, rusting.	Example: melting ice, dissolving salt.

Element and compound

Answer

Element	Compound
A pure substance that can’t be broken down into simpler substances by a chemical process.	A pure substance formed by two or more elements combined chemically.
Made of only one kind of atom.	Made of two or more kinds of atoms, in a fixed ratio.
Example: oxygen, iron, copper.	Example: water, common salt.

Density and relative density

Answer

Density	Relative density
Mass per unit volume of a substance.	The density of a substance divided by the density of water at 4 °C.
D = m ÷ V	RD = density of substance ÷ density of water at 4 °C
Has a unit: kg/m³ (SI) or g/cm³ (CGS).	Has no unit: it is a ratio.
Example: iron, 7800 kg/m³.	Example: iron, 7.8.

5. Give reasons

An egg sinks in pure water but floats in salty water.

Answer

An egg is a little denser than pure water, so it sinks. Dissolving a lot of salt makes the water denser, until it is denser than the egg; then the egg floats. An object floats in a liquid that is denser than itself.

An atom is electrically neutral.

Answer

An atom has the same number of protons (positive) and electrons (negative). Their charges are equal and opposite, so they cancel, and the atom as a whole has no charge.

Breaking a lump of salt into fine powder is a physical change.

Answer

The powder is still salt, with the same taste and properties: only the size of the pieces has changed, and no new substance is formed. A change in which no new substance is formed is a physical change.

Forming water from hydrogen and oxygen is a chemical change.

Answer

Hydrogen and oxygen are gases; the water formed from them is a new substance, a liquid with completely different properties. The atoms join in a new way (two hydrogen atoms to one oxygen atom). A change that forms a new substance is a chemical change.

An iron frame must be painted regularly, but an aluminium frame needs no paint.

Answer

Iron rusts when air and moisture reach it, and rust slowly eats the frame away, so it is kept painted. Aluminium also reacts with air, but it at once forms a very thin, tough, invisible layer of aluminium oxide that sticks to the metal and stops any further reaction. So aluminium protects itself.

6. Answer the following questions

Draw the structure of an atom, showing protons, electrons and neutrons.

Answer

Draw a small circle in the middle for the nucleus, with protons (marked +) and neutrons inside it. Draw one or more circles round it for the shells, with electrons (marked −) on them. Label each part, as in Figure 9.4: helium has 2 protons and 2 neutrons in the nucleus and 2 electrons in the shell.

What is density? What is the mass of aluminium of volume 2 m³? (The density of aluminium is 2700 kg/m³.)

Answer

Density is the mass per unit volume of a substance.

Given: V = 2 m³, D = 2700 kg/m³, m = ?

$m = D \times V = 2700 \text{ kg/m}^3 \times 2 \text{ m}^3 = 5400 \text{ kg}$

The mass of the aluminium is 5400 kg.

What is relative density? If the densities of pure milk and of water at 4 °C are 1030 kg/m³ and 1000 kg/m³, what is the relative density of pure milk?

Answer

Relative density is the ratio of the density of a substance to the density of water at 4 °C.

$\text{RD of milk} = \frac{1030 \text{ kg/m}^3}{1000 \text{ kg/m}^3} = 1.03$

The relative density of pure milk is 1.03.

Balls of cork, wood and aluminium of the same size are put into water, as in Figure 9.12.

i.Why does cork float in water, but aluminium sink?
ii.If the density of aluminium is 2.7 g/cm³, find its relative density.

Figure 9.12 Balls of cork, wood and aluminium, all the same size, in water.

Answer

i. Cork (density about 0.24 g/cm³) is less dense than water (1 g/cm³), so it floats, mostly above the surface. Aluminium (2.7 g/cm³) is denser than water, so it sinks. (Wood, about 0.7 g/cm³, floats too, but lower in the water than cork, because it is denser than cork.)

ii.
$\text{RD of aluminium} = \frac{2.7 \text{ g/cm}^3}{1 \text{ g/cm}^3} = 2.7$

The relative density of an object is 0.92. Does it sink or float in water? Explain why.

Answer

It floats. A relative density of 0.92 means the object is less dense than water (0.92 is less than 1): the same volume of it weighs only 0.92 times as much as water. Anything less dense than water floats. (Ice has exactly this relative density, and floats with about 92 % of it under the surface.)

What is an element? Write the names and symbols of the elements with atomic numbers 7, 13 and 17.

Answer

An element is a pure substance that can’t be broken down into simpler substances by any chemical process.

7: nitrogen, N
13: aluminium, Al
17: chlorine, Cl

Using Figure 9.13, describe how elements, molecules, compounds and mixtures are related.

Figure 9.13 Atoms, molecules of elements, molecules of compounds, and a mixture.

Answer

An element is made of one kind of atom (H, O, Cl).
Atoms join to form molecules. Two atoms of the same element make a molecule of that element (H₂, O₂, Cl₂).
When atoms of different elements join chemically, in a fixed ratio, they make molecules of a compound (H₂O, HCl), a new substance.
When molecules of different substances are only mixed together, not joined, they make a mixture: here hydrogen, oxygen and water molecules, each keeping its own properties.

Which atoms are there in a molecule of water? How many of each?

Answer

A water molecule (H₂O) has two hydrogen atoms and one oxygen atom.

What are sub-atomic particles? Draw an atom and label its sub-atomic particles.

Answer

Sub-atomic particles are the particles an atom is made of: protons (positive, in the nucleus), neutrons (no charge, in the nucleus) and electrons (negative, moving round the nucleus in shells). Draw them as in Figure 9.4, labeling the nucleus, a proton, a neutron, an electron and a shell.

Name the state of each kind of matter in Figure 9.14 from how closely its particles are packed.

Figure 9.14 Matter A, B and C: how closely the particles are packed.

Answer

Matter A is a gas: its particles are few and far apart.
Matter B is a liquid: its particles are close together but jumbled, free to slide past each other.
Matter C is a solid: its particles are packed tightly in a regular pattern.

What kind of change is called a physical change? Give examples.

Answer

A change in which the state, shape, size or color of a substance changes, but no new substance is formed, is a physical change. Examples: ice melting, water boiling, salt dissolving in water, wood being chopped, spices being ground.

What is a chemical change? Give any three examples from daily life.

Answer

A change in which a substance turns into a new substance with new properties is a chemical change. Examples: cooking rice, iron rusting, milk turning into curd (also: wood burning, fruit rotting, food being digested).

Sort these into physical and chemical changes:

i.evaporation of water
ii.boiling an egg
iii.condensation of vapor
iv.burning fossil fuels
v.dissolving sugar in water

Answer

Physical change	Chemical change
i.evaporation of water	ii.boiling an egg
iii.condensation of vapor	iv.burning fossil fuels
v.dissolving sugar in water

(Boiling an egg changes its proteins for good: the clear white turns solid and white, and nothing turns it back.)

Which element has atomic number 18? Make a model of its atom.

Answer

Argon (Ar). An argon atom has 18 protons and (in its most common kind) 22 neutrons in the nucleus, and 18 electrons: 2 in the first shell, 8 in the second and 8 in the third. Make the model as in the project above.

Figure 9.15 An argon atom: 18 protons and 22 neutrons in the nucleus; 18 electrons in three shells, 2, 8 and 8.

Worksheet to print (PDF, 4 pages) Answer key for teachers

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