NCERT Solutions for Class 8 Science Chapter 9 The Amazing World of Solutes Solvents and Solutions (PDF) – 2027

People are collecting salt from the seashore. Sea water is a solution — salt is the solute and water is the solvent. The sun and wind evaporate the water, but the salt does not evaporate. So the solution left behind becomes more and more concentrated until it is saturated; after that the extra salt is left on the sand as white crystals, which the people are picking up.

The tea has become a saturated solution of sugar at that temperature, so the extra sugar cannot dissolve and settles at the bottom of the cup.

To solve it: (i) heat the tea — the solubility of sugar increases with temperature, so the extra sugar dissolves; or (ii) add more tea (more solvent), so there is more water to dissolve the same sugar. Stirring alone will not help.

Water particles attract the particles of sugar and salt strongly, pull them away from the crystal and spread them evenly — so sugar and salt dissolve in water. Oil particles hardly attract sugar or salt particles, so the crystals are not pulled apart and they do not dissolve in oil.

Water is called a good solvent because it dissolves a very large number of substances — salt, sugar, ORS, many medicines and even gases such as oxygen and carbon dioxide.

Because a tall cylinder is easy to stand, hold, pack and pour from. Its flat base lets it stand upright without rolling; its small width fits in one hand; cylinders pack side by side with little wasted space; and the water level shows at a glance how much is left. A spherical bottle would roll, be hard to grip, leave gaps when packed and be awkward to pour from.

It is a uniform mixture. The sugar and salt have dissolved completely in the water, so the liquid is clear and you cannot see the sugar or the salt separately anywhere in the glass.

No. Chalk powder does not dissolve. It stays as tiny solid grains that make the water look milky, and if the glass is left standing, the grains slowly settle at the bottom. This is a non-uniform mixture, like sand in water or sawdust in water (Fig. 9.2).

Yes. Air is a uniform mixture of nitrogen, oxygen, carbon dioxide, water vapour and other gases, and a uniform mixture is a solution. The gases are spread evenly, no component can be seen or picked out separately, and a sample of air taken from anywhere in this room has the same composition.

The first few spoons disappear completely. Somewhere around the sixth to the eighth spoon in half a tumbler of water (roughly 100 mL), the grains stop vanishing and a layer of salt stays at the bottom however long you stir. A typical filled Table 9.1 looks like this.

Your own number may differ from seven, and it should. It depends on how much water you took, how big your spoon is, and how warm the room is — so record what you observe.

In half a glass tumbler of water — about 100 mL — you will usually manage six to eight level teaspoons before salt starts staying at the bottom. Write down your own count, because it is the count for your volume of water, your spoon and your room temperature.

It shows that a fixed amount of water has a definite limit — it cannot dissolve salt endlessly. Up to that limit the solution is unsaturated; at the limit it is saturated at that temperature, and any extra salt stays undissolved at the bottom.

At first every spoonful dissolves and the solution gets steadily more concentrated. Then a stage arrives when the added salt stops dissolving and settles at the bottom. From that moment the solution is saturated at that temperature, and no matter how much more salt you pour in, none of it will dissolve — the extra simply piles up as solid.

The 4 spoons in 50 mL — and by a factor of four.

Concentration means the amount of solute in a fixed quantity of solution, so bring both to the same volume before comparing.

Yes — strongly. For most solids, solubility increases as temperature increases. Activity 9.2 shows it directly: baking soda that would not dissolve at 20 °C dissolves at 50 °C, and more still dissolves at 70 °C.

A solution that is saturated at one temperature behaves as an unsaturated solution when it is heated.

It dissolves. The layer of solid baking soda lying at the bottom of the beaker disappears as the water warms from 20 °C to 50 °C, and the liquid becomes clear again.

The same thing happens again — the undissolved baking soda dissolves. After the solution was made saturated once more at 50 °C by adding fresh baking soda, heating it to 70 °C clears that solid too.

Put the three stages side by side and the pattern is unmistakable:

For most solids, solubility in a liquid increases as the temperature rises. Water at 70 °C dissolves more baking soda than water at 50 °C, and water at 20 °C dissolves the least of the three.

Two statements follow, and both are worth writing down:

• A saturated solution at one temperature behaves as an unsaturated solution if the temperature is raised — the leftover solid then dissolves.
• Any figure quoted for solubility must carry a temperature with it, otherwise it does not say anything definite.

Asima Chatterjee’s father was a doctor with a keen interest in plants and botany, and she shared this interest. She was also drawn to India’s long tradition of herbal medicine — Ayurveda, Siddha and other systems. She wanted to find out which chemical compounds in medicinal plants actually cure disease. Using solvents and solutions, she extracted and isolated such compounds and helped develop anti-epileptic and anti-malarial drugs.

Yes. Many gases, including oxygen, dissolve in water — though oxygen dissolves only to a small extent.

Small as that amount is, everything living in a pond, a river or the sea depends on it. Fishes, aquatic plants and other water organisms take their oxygen from the water itself, not from the air above it, and it is this dissolved oxygen that keeps them alive.

It is a uniform mixture — that is, a solution. The gases dissolve evenly through the water; you cannot see them, they do not float up as visible bubbles, and a sample taken from any depth has the same dissolved gas in it.

Yes, and in the opposite way to solids: the solubility of gases generally decreases as the temperature increases. Cold water holds more dissolved oxygen; as water warms up, the dissolved oxygen escapes.

Because of density — the mass packed into unit volume. Sawdust is less dense than water, so it floats; sand is much denser than water, so it sinks.

The husk is less dense than water and the rice grain is denser, so the husk stays on the surface while the rice settles.

The husk is a thin, dry, papery covering with air trapped in it, so it has very little mass for its volume. The rice grain is solid and compact, so the same volume of rice has much more mass.

Density is the mass present in a unit volume of a substance.

Two properties of density are worth fixing in your mind straight away:

• It does not depend on the shape or the size of the piece you have. A gram of iron and a kilogram of iron have exactly the same density.
• It does depend on temperature and pressure. Pressure matters mainly for gases; for solids and liquids its effect is negligible.

It tells us the oil’s density directly — and it is less than that of water.

0.91 g/mL is less than the density of water (about 1 g/mL). So the density of oil is less than that of water — that is why oil floats on water.

It means the buttermilk inside occupies a space of 200 millilitres. The number on the pack is a volume, not a mass — it tells you how much room the liquid fills, not how heavy it is.

Only as accurately as their smallest division allows, and that depends on the size of the cylinder — a bigger cylinder has a coarser scale.

So, to measure 70 mL of water, a 100 mL measuring cylinder is the best choice — it measures in one step and reads up to 1 mL.

100 mL. The cylinder in Fig. 9.16 is marked 100 mL at the top of its scale, and that top mark is the largest volume it can measure in one filling.

10 mL. On the cylinder in Fig. 9.16 the numbered marks run 10, 20, 30 … up to 100, so the step from one numbered mark to the next — 10 mL to 20 mL, or 40 mL to 50 mL — is 10 mL every time.

10. Count the fine lines between 10 mL and 20 mL on Fig. 9.16 — there are ten equal spaces from one numbered mark to the next.

1 mL. Ten millilitres are shared equally among ten divisions.

1 mL — for the 100 mL measuring cylinder shown in Fig. 9.16.

Because in a narrow tube the same small volume of liquid produces a much larger rise in level, so the marks can be spaced far enough apart to be read accurately.

For a coloured liquid you read the mark that coincides with the top of the meniscus — the opposite of what you do for water.

In both cases the rule that never changes is: keep your eye exactly level with the surface you are reading.

The water level rises — from 50 mL to 55 mL in Fig. 9.19. The stone has pushed aside, or displaced, exactly its own volume of water, so the rise in level is the volume of the stone.

Rows 2 and 3 are sample readings — fill in whatever your own objects give. With the mass from Activity 9.3 you can now finish the calculation the chapter sets out:

Yes — and the layers are arranged in a clear order: the density increases as you go from the surface towards the centre. The outermost layer, the crust, is the lightest of them all.

Because ice is less dense than liquid water — the same mass of water takes up more space after it freezes.

The steps are these:

• Water is densest at 4 °C. Cool it below that and it starts behaving unusually.
• At 0 °C it turns into ice, and its structure changes — the particles arrange themselves in a pattern that takes up more space. This is called expansion.
• The mass has not changed, but the volume has increased. Since Density = Mass/Volume, the density falls.
• Being lighter for the same volume, ice floats on the water below it.

Dissolve plenty of common salt in the water and stir until no more will dissolve. The egg that sank in tap water will lift off the bottom and float in the salt solution.

Dissolving salt adds a lot of mass to the water but increases its volume only slightly, so the density of the liquid increases. Once the salt solution is denser than the egg, the egg floats.

(ii) Water is denser than oil.

The most appropriate statement is (iii) No more solute can be dissolved into the saturated solution at that temperature. That is exactly the definition of saturation.

The peel is full of tiny air pockets. It adds a great deal of volume to the orange while adding very little mass, so the whole unpeeled fruit has an average density below 1 g/cm³ and floats (Fig. 9.25a). Remove the peel and you throw away that light, air-filled layer; what is left is dense juicy pulp with a density a little above 1 g/cm³, so it sinks (Fig. 9.25b).

Nothing at all — the density stays exactly the same, 2 g/cm³.

Rearrange the density formula to make volume the subject.

Check it the other way round: 75.95 cm³ × 7.9 g/cm³ = 600.0 g. ✔

The density of the water decreases. The rise in the glass tube is the visible sign that the water has expanded.

The Dead Sea is a salt lake between Israel and Jordan. River water flows in, but there is no outlet and the hot sun evaporates the water quickly, so the salt keeps collecting. Its water is now an extremely concentrated salt solution — about ten times saltier than sea water. Fish and plants cannot survive in such a concentrated solution, and very little oxygen dissolves in it, so there is almost no aquatic life except some microbes.

Similar water bodies: Great Salt Lake (USA), Lake Assal (Djibouti), and in India, Sambhar Lake (Rajasthan) and Pangong Tso (Ladakh).

Method: Take 50 mL each of water, vinegar and cooking oil in three glasses at room temperature. Add common salt to each, one level spoon at a time, stirring in the same way, and count how many spoons dissolve before salt starts settling. Sample observations:

Conclusion: Salt is highly soluble in water, nearly as soluble in vinegar (which is mostly water), and insoluble in oil.

Points for both sides:

Sample conclusion: Water dissolves more everyday substances than any other single liquid, so it is rightly called a very versatile solvent. But it cannot dissolve oil, grease, wax or most paints, so other solvents are also needed for some jobs.