Exam practice

volume (density = mass ÷ volume)

less

sinking

1/1837

K (from its Latin name, kalium)

chemical

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

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 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.

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.

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.

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.

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.

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.

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.

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

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.)

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

• 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.

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

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.

• 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.

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.

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).

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

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.