Matter

Look around you. Soil, salt, water, wood, iron and air all have weight, and all take up space. Anything that has weight (mass) and takes up space is called matter.

Some kinds of matter are pure. A pure substance is made of only one kind of matter, the same all the way through. Water, salt, sugar, iron and copper are pure substances.

Other kinds of matter are impure: two or more substances mixed together. Clay, milk and air are impure. An impure substance is called a mixture.

Think of the things you see every day: soil, rice, milk, curd, tea and air. Is each made of one substance, or of several mixed together?

To make tea, we mix water, milk, tea leaves and sugar. Muddy water has soil, sand and bits of rubbish mixed in it. Air is nitrogen, oxygen, carbon dioxide and other gases mixed together.

A mixture is two or more substances mixed together physically: they are just mixed, and they don’t join to make a new substance. Each one keeps its own properties. The sugar in tea is still sweet, and the water is still wet.

The substances that make up a mixture are called its components (or ingredients). Water, milk, tea leaves and sugar are the components of tea.

Types of mixture

Dirty rice may have small stones, bits of brick and glass, unhusked grains (dhan) and husk mixed in it. You can see and pick out each of these. But you can’t see the nitrogen, oxygen and carbon dioxide mixed in air. So mixtures are of two types, by how well their components are mixed:

• a.homogeneous mixtures
• b.heterogeneous mixtures

Salt and water mix evenly, and you can’t see or pick out the salt with your eyes. Sand and water don’t mix evenly, and you can see the sand.

A mixture in which the components are mixed evenly all through, so that they can’t be seen or told apart with the naked eye, is a homogeneous mixture. Every part of it is the same. Examples: salt water, sugar water, soda water, clear fruit drinks (sarbat) and air.

A mixture in which the components are not mixed evenly, so that they can be seen and told apart with the naked eye, is a heterogeneous mixture. Its parts differ from place to place. Examples: sandy water, muddy river water, and rice mixed with stones or husk.

Forms of mixture

Each component of a mixture can be a solid, a liquid or a gas. So mixtures come in six forms:

Notice that the two liquid mixtures behave differently. Milk and water mix evenly, but kerosene doesn’t mix with water at all: it floats on top in a layer of its own.

Not every mixture can be separated in the same way. We pick a method that suits the properties of the substances in it: their size, how heavy they are, whether they dissolve, and the temperature at which they boil. Mixtures can be separated by filtering, decantation, evaporation, distillation and other methods, as you will see next.

Methods of separating mixtures

Every mixture holds two or more components, and we may not want all of them. So we separate out the part we need. Stones, husk, unhusked grains and bits of brick or glass in rice are unwanted, so we pick them out to get clean rice. Separating a mixture to get the pure, useful substances from it is called separation of mixtures.

You already use several methods at home:

None of these can take salt out of salt water: it is dissolved, and its particles are far too small to catch. For mixtures like that we need the methods below.

Evaporation

Evaporation is the method of separating a dissolved solid from a liquid by heating the mixture, so that the liquid turns into vapor and goes off, and the solid is left behind. The mixture is heated slowly. The water goes off as vapor, and the salt stays in the pan. This is how salt is made from sea water, in shallow ponds where the Sun and wind do the heating.

Distillation

Heat water, and at a certain temperature it boils and turns into steam. Cool the steam, and it turns back into water. So water can be taken out of a mixture as steam, and the steam cooled back into water in another vessel.

Salt water can be separated this way. Salt needs a very high temperature to boil, so when the mixture is heated, the water turns into steam long before the salt. The steam is led away and cooled, and it turns back into pure water.

Distillation is the method of separating a liquid from a mixture by heating it until it turns into vapor, then cooling the vapor so that it turns back into liquid, and collecting it in another vessel. In the laboratory, the steam is cooled in a special glass tube with cold water flowing round it, called a condenser.

Use of distillation as a local technology

In some communities, people make raksi, a strong alcoholic drink, at home by distillation. Cooked millet (or rice) is mixed with marcha, a starter cake full of yeasts and molds, and left for some days to ferment: the molds and yeasts slowly turn the grain’s starch into alcohol. Then the fermented mash is heated in a home still (Figure 9.4). The alcohol turns into vapor sooner than the water does. The vapor meets the cold bottom of a pot of cold water, turns back into liquid, and is collected separately as raksi. As the mash gets hotter, more and more water vapor comes over too, so raksi is never pure alcohol.

Distillation is also used to take sweet-smelling oils out of plants: steam is passed through the leaves or roots, and the oil comes over with it. Nepal makes such oils from lemongrass, wintergreen (dhasingre) and jatamansi. And in some dry places by the sea, distillation turns sea water into drinking water.

Chromatography

Have you ever spilled a drop of water on a page written with a fountain pen or felt pen? The ink spreads and blurs, and the letters can’t be read. Look closely and you may see rings of different colors at its edges.

Many inks and colored substances are mixtures of several colors (dyes). When water moves through paper, cloth or chalk, it carries the dyes along with it. But some dyes travel quickly and others slowly, so they come apart. Separating the colors in a mixture by letting a liquid carry them through paper, each at its own speed, is called paper chromatography. It is used to separate and identify colored substances.

Application of chromatography. Chromatography is used in hospitals, laboratories and research centers:

1. In laboratories and research centers, to separate and identify many kinds of chemicals.
2. In hospitals, to separate and identify the chemicals in urine and blood (for example, to find a medicine or a poison).
3. To separate and identify the colors (dyes) in a mixture, such as the colors added to sweets and drinks.

Centrifuging

A mixture may contain both heavy and light substances that don’t dissolve. Spun round very fast, the heavier parts move to the bottom (the outside of the circle), and the lighter parts stay above them. Separating a mixture by spinning it very fast is called centrifuging. It is used to separate cream and butter from milk and curd, and to test blood in laboratories.

Leave muddy water to stand and the mud settles in time. Spinning does the same job, but much faster and much better.

Today, laboratories use machines called centrifuges, spun by an electric motor. The mixture is put in test tubes, and the tubes are placed in special holders inside. The motor spins them very fast, thousands of turns a minute. Afterwards, the heavy substances are at the bottom of each test tube, and the light ones at the top.

Have you seen curd churned in a village, with a theki and a madani? Churning is the simplest local way to get butter from curd. Curd is put in a wooden pot, the theki. A wooden churning stick, the madani, stands in it. A rope (neti) is wound round the stick, and pulling its two ends in turn spins the stick fast, one way and then the other. After a while, lumps of butter appear. Water is added and the butter floats up; it is then lifted out by hand. The liquid left behind is buttermilk (mohi). Heating the butter turns it into ghee.

Today, in towns, butter is also made by shaking milk or curd in machines with electric motors, such as electric churners and blenders.

The three mixtures differ because their particles are of different sizes. By the size of their particles, mixtures are of three types: solution, colloid and suspension.

Solution

Have you ever drunk Jeevan Jal when you had diarrhea? What is it made of? Can you see what is in it? Discuss with your friends.

A solution is a mixture of two or more substances mixed evenly (homogeneously). The substances in it can’t be seen or told apart. In sugar water, for example, you can’t see the sugar.

A solution is made of a solute and a solvent:

• The solute is the substance that dissolves. In sugar water, sugar is the solute.
• The solvent is the substance in which the solute dissolves, making an even mixture. In sugar water, water is the solvent.

Solution = Solute + Solvent

A solution can have more than one solute in the same solvent. In lemon water (nimbu pani), made of water, lemon juice and sugar, water is the solvent, and the lemon juice and sugar are both solutes. Jeevan Jal has several solutes, salts and glucose, all dissolved in clean water.

Colloid

Milk, gum and blood are mixtures of another kind, called colloids.

How big are the particles? In a solution, the dissolved particles are tiny: less than 10⁻⁷ cm in diameter (one ten-millionth of a centimeter across). In a colloid, they are a little bigger: from 10⁻⁷ cm to 10⁻⁵ cm across. That is still far too small to see, even with a hand lens, and too small to be caught by filter paper. From outside, a colloid looks like a solution, evenly mixed. But its particles are big enough to scatter light, so it is counted as a kind of heterogeneous mixture.

Suspension

Fill a beaker half full of water. Stir in some chalk powder, flour (maida) or lime (chuna), and keep stirring for a while. What kind of mixture do you get, homogeneous or heterogeneous? Discuss.

It is heterogeneous: it looks cloudy, you can see bits in it, and if you leave it, the bits slowly settle on the bottom. Such a mixture is called a suspension. Its particles are bigger than a colloid’s: more than 10⁻⁵ cm across. They can be caught by filter paper, and many of them can be seen.

Use of solution in our daily life

We see farmers putting manure on their fields to help plants grow. But do plants eat manure, as we eat rice? Of course not. Plants take in their nutrients through their roots, dissolved in water as solutions. People and animals also take in many of the substances they need as solutions. In short, solutions are useful in our daily life in these ways:

1. Plants take in, through their roots, the water and the mineral salts they need to make their food, as a solution.
2. The food we eat is digested, and medicines dissolve; both are taken into the blood as solutions.
3. Animals and plants that live in water breathe the oxygen dissolved in it.
4. Many medicines, drinks and juices are sold as solutions.

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

homogeneous · distillation · bottom · evaporation · mixture · components

2. Choose the correct answer

3. Write true or false

4. Differentiate between

5. Give reasons

6. Answer the following questions