CBSE Class 6 Science Advanced Chemistry Matter and Its Segregation Notes Set 01

Matter and Its Segregation

Introduction

Everything around us, whether we can touch it, see it or not, has some mass and takes up some room. A book on the table occupies space, which is its volume, and it presses down on our hand with its weight, which tells us it has mass. Air cannot be seen, but a balloon blown full of air gets bigger and also weighs more than an empty one. So air too takes up space and has mass.

1.1 Classification of Matter

Matter is anything that has mass and occupies space. All materials and substances are made of matter, and matter itself is built of extremely small particles such as atoms and molecules.

Classification means putting similar things into the same group on the basis of some common property. For example, pencil, pen, chalk and ink can be put in a "writing" group, while shirt, skirt, blouse and trousers go in a "clothes" group. In the same way, all matter is sorted into pure substances and mixtures.

Pure substance: all the particles in it are of the same kind. A pure substance is either an element (for example gold) or a compound (for example water). Each pure substance has its own fixed properties, such as a definite melting point and boiling point.

Mixture: a material in which the particles are not all of the same kind. Most of the things we find in nature, such as soil, sea water and air, are mixtures. The parts of a mixture can be present in any proportion.

Types of Mixtures

Homogeneous mixture: the composition is the same all through the mixture. If a spoon of salt is stirred into a glass of water, the salt disappears from view and the first sip tastes as salty as the last. Salt in water, juice, tea and air are homogeneous mixtures. Solutions are of this kind.

Heterogeneous mixture: the composition is not the same everywhere, so the different parts can be seen and they often settle if left alone. Sand stirred in water, mud in water, oil in water and soil are heterogeneous mixtures.

1.2 Separation of the Components of a Mixture

Often we have to take the parts of a mixture apart. We remove impurities from water to make it drinkable, we use a strainer to take tea leaves out of tea, gardeners pick stones out of soil and we remove husk and stones from rice or dal before cooking.

Why do we separate the components of a mixture?

1. To get two useful things from one mixture, for example butter from milk by churning.
2. To remove harmful parts or impurities, for example stones and husk from rice or dal.
3. To throw away a part that has no use, for example tea leaves after the tea is made.

How is separation done? Many substances in nature occur mixed with others. To use them, we separate them with the help of a difference in some physical property of the parts, such as size, weight, magnetic property, solubility, melting point or boiling point.

(a) Separating Two or More Solids

Solid parts can be separated without difficulty when they differ clearly in size or colour. A difference in magnetic nature or in solubility in a given liquid can also be used. The following methods are used.

(i) Handpicking. When the parts of the mixture differ in size, shape or colour and the unwanted part is small in quantity, it is simply picked out by hand. Stones are removed from rice or pulses in this way.

(ii) Threshing. This is the method of freeing the grain from the dried stalks of the crop. The stalks are beaten so that the grains fall out. It may be done by bullocks walking over the stalks, or by machines when the quantity of grain is large.

Activity 1. Aim: to separate the parts of a mixture of different solids. Take a spoonful each of dal of different colours (yellow, green, black, pink and so on), mix them in a bowl and try to pick them out colour by colour. Conclusion: a mixture of dals of different colours can be separated by hand.

(iii) Winnowing. It separates a heavier solid from a lighter one by using wind or moving air. Farmers pour the mixture of grain and husk from a height. The breeze blows the light husk away to one side, while the heavier grain falls straight down and forms a heap. The husk is later used, for example, as cattle fodder.

(iv) Sieving. A sieve is a vessel with a net or holes through which the smaller particles pass and the bigger ones stay behind. In a flour mill, wheat is poured on a slanting sieve to remove leftover stalks, husk and stones before grinding. At building sites sieves separate pebbles and stones from sand.

(v) Magnetic separation. It can be used if one part of the mixture is a magnetic substance, that is, one that a magnet attracts. In an iron ore, iron particles are pulled out by a magnet, and the non-magnetic impurities are left behind. A mixture of iron filings and sulphur, or of iron filings and sand, can be separated in this way.

Activity 2. Aim: to separate a mixture of solids using a magnet. Put some sand and iron filings in a bowl and mix them. Roll a bar magnet slowly over the mixture; the iron filings stick to the magnet. Brush them off into a separate place and repeat until no iron filings remain in the sand. Conclusion: a magnetic substance can be separated from a non-magnetic one with a magnet.

(vi) Sublimation. Some solids, on heating, turn straight into vapour without becoming liquid first. This is called sublimation. If one part of a solid mixture sublimes and the other does not, the mixture can be separated by heating it. Iodine, camphor, naphthalene, ammonium chloride and dry ice (solid carbon dioxide) are substances that sublime. For example, ammonium chloride can be separated from sodium chloride (common salt) by sublimation, because only ammonium chloride changes to vapour. The vapour turns back into solid on a cool surface.

The figure shows the changes of state of water. Ice can change directly into steam (sublimation) and steam can change directly back into ice (deposition), besides the changes through the liquid state.

(b) Separating an Insoluble Solid from a Liquid

Chalk powder, sand and dust do not dissolve in water. Such solids can be separated from the liquid by sedimentation and decantation, by loading, or by filtration.

(i) Sedimentation and decantation. Sometimes light impurities like dust or soil in rice or pulses cannot be removed by winnowing or handpicking. The grain is then washed with water: the impurities go into the water and make it muddy. When a heavier part of a mixture settles at the bottom after water is added, the process is sedimentation. When the water above, with the dirt, is then poured off without disturbing the settled part, the process is decantation.

(ii) Loading. Very tiny particles of an insoluble solid stay floating in the liquid for a long time because they are so light, and sedimentation would take too long. In loading, a substance such as alum is added; it makes the floating particles stick together, become heavier and settle quickly. Loading is mainly used to get clear water from muddy water.

(iii) Filtration. The separation of insoluble solid particles from a liquid by passing the mixture through a filter is called filtration. The filter lets the liquid through and stops the solid. A filter may be filter paper, fine muslin cloth or a fine mesh. The solid left on the filter is the residue and the clear liquid that passes through is the filtrate. A tea strainer is a common filter.

Activity 3. Aim: to separate insoluble solids from water by sedimentation and decantation. Take three test tubes. Put sand and water in tube A, rice and water in tube B and dal and water in tube C. Let them stand until the solids settle. Tilt each tube slowly and pour the water into another tube without disturbing the solid. Observation: sand, rice and dal all settle; the water poured from the sand tube is not perfectly clear, as some fine sand is still floating in it. Conclusion: an insoluble solid can be separated from a liquid by sedimentation and decantation.

(c) Separating a Soluble Solid from a Liquid

(i) Evaporation. Evaporation is the change of a liquid into vapour at a temperature below its boiling point. It goes on wherever there is water. Sea water has many salts dissolved in it, including common salt. It is collected in shallow pits and left in the sun. The sun warms it, the water slowly evaporates and in a few days only the solid salts remain. Common salt is then obtained from these by further purification.

(ii) Condensation. The drops of water that gather under the lid of a vessel in which milk or vegetables have been boiled are formed because the hot food gives out water vapour; when the vapour touches the cooler lid it turns back into liquid droplets. The change of a vapour into a liquid on cooling is called condensation. In the same way, droplets form on the outside of a bottle of cold water.

1.3 Other Special Techniques

Centrifugation. This technique speeds up the settling of very small particles hanging in a liquid. Its principle is that something spun fast is pushed outwards, away from the centre of rotation. The mixture is put in a tube which is spun at high speed in a centrifuge machine. The solid particles are driven to the bottom of the tube and the clear liquid above (supernatant liquid) can be poured off. Cream is obtained from milk by this technique, and the spinner of a washing machine dries clothes on the same principle.

Separating funnel. Two liquids that do not mix with each other (immiscible liquids), such as oil and water, are separated with a separating funnel. The method depends on the difference in density of the two liquids. The funnel is a glass vessel with a tap (stopcock) at the bottom. The mixture is poured in and kept still, so that the denser liquid forms the lower layer and the lighter one floats on top. The tap is opened to let the lower layer run out and is closed when it has all come out, leaving the upper liquid in the funnel. A mixture of kerosene oil and water is an example, with water below and kerosene above.

1.4 Can Water Dissolve Any Amount of a Substance?

Many substances dissolve in water and form a solution. A solution made with water is called an aqueous solution. (A solution made with some other liquid, such as iodine in alcohol, is a non-aqueous solution.)

Activity. Half fill a beaker with water and add one teaspoon of salt, stirring until it dissolves. Keep adding a spoon at a time and stirring. After a few spoons, some salt is left undissolved at the bottom. This shows that the water cannot dissolve any more salt, and the solution is now saturated.

If we now add a little more salt to this saturated solution and heat it, the salt dissolves. This solution holds more solute than a saturated solution normally would and is called a supersaturated solution. On cooling, the extra salt appears again at the bottom.

A solution that holds less solute than is needed to saturate it is an unsaturated solution.

The substance that dissolves is the solute and the liquid in which it dissolves is the solvent. How much solute dissolves in a given amount of solvent depends on the nature of the solute, the solvent and the temperature.

Add to Your Knowledge

Crystallisation. This is the process of getting an impure solid compound in the form of pure crystals. It works because a growing crystal leaves the impurities behind in the solution. Crystallisation is a better way of purifying a solid than simple evaporation, for two reasons.

1. Evaporation heats the solution right up to dryness. Some solids break down on such heating, and others, like sugar, get charred.
2. In both methods, the impure solid is first dissolved and the solution filtered to remove insoluble impurities. Some soluble impurities still remain. On evaporating to dryness they get deposited along with the solid and spoil it. If the solution is instead left to crystallise, pure crystals form and the impurities stay in the liquid.

Fractional distillation. It is the method of separating two miscible liquids (liquids that mix) by heating the mixture at a controlled temperature. The liquid with the lower boiling point turns to vapour first, rises up the fractionating column, is cooled in the condenser and is collected as a liquid. Petrol, diesel and kerosene are obtained from petroleum, a mixture of many liquids, by this process.

Points to Remember

1. Classification means grouping similar things together.
2. A homogeneous mixture has uniform composition throughout; a heterogeneous mixture does not.
3. Differences in size, colour, magnetic nature and solubility are used to separate substances.
4. Handpicking separates solids by hand, for example stones from rice.
5. Threshing separates grains from stalks by beating them on a hard surface; machines are used for large quantities.
6. Winnowing separates grain from husk using wind.
7. A sieve is a plate or net with holes; it is used to remove particles of different sizes and, at building sites, to separate pebbles and stones from sand.
8. Iodine, camphor, naphthalene, ammonium chloride and dry ice (solid \(\ce{CO2}\)) sublime.
9. Decantation is pouring off the clear liquid after sedimentation, without disturbing the sediment.
10. Loading is adding alum to make suspended particles settle faster, mainly to clear muddy water.
11. Filtration separates insoluble solid particles from a liquid using a filter, such as a tea strainer.
12. Evaporation changes a liquid into vapour below its boiling point; condensation changes vapour into liquid on cooling.
13. Centrifugation is used to get cream from milk.
14. A separating funnel separates two immiscible liquids.
15. A solution in which no more solute can dissolve is saturated; one with less solute than that is unsaturated; one with more is supersaturated.

Concept Map