The Amazing World of Solutes, Solvents, and Solutions

What do you think is happening in the picture below?

Answer

In the picture, a group of people have gathered near the seashore where salt is being collected. The salty water from the sea is left to dry, and after the water evaporates, the salt remains behind. The people are collecting this salt that has formed near the shore.

What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?

Answer

When too much sugar is added to tea, a stage comes when the sugar stops dissolving and begins to settle at the bottom. This happens because the tea has reached its limit and cannot dissolve any more sugar at that temperature, that is, it has become a saturated solution.

This problem can be solved by heating the tea. On increasing the temperature, the solubility increases, so the tea can dissolve more sugar and the undissolved sugar dissolves completely.

Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?

Answer

Sugar and salt dissolve in water because they mix evenly with water and form a uniform mixture, that is, a solution. They do not dissolve in oil because oil does not form such a uniform mixture with them.

Water is considered a good solvent because it can dissolve a large number of substances, including many solids, liquids, and gases, to form solutions. Due to this ability to dissolve many substances, water is one of the most useful solvents.

Why are water bottles usually tall and cylindrical in shape instead of spherical?

Answer

Water bottles are usually made tall and cylindrical because this shape makes them easy to hold, store, and pour from. A tall, cylindrical bottle can be placed easily on a flat surface and stored conveniently side by side.

A spherical bottle, on the other hand, would roll around, would be difficult to keep stable on a surface, and would be harder to hold and pour water from. Hence, a tall cylindrical shape is more practical and convenient.

State whether the statements given below are True [T] or False [F]. Correct the false statement(s).

1. Oxygen gas is more soluble in hot water rather than in cold water.
2. A mixture of sand and water is a solution.
3. The amount of space occupied by any object is called its mass.
4. An unsaturated solution has more solute dissolved than a saturated solution.
5. The presence of different gases in the atmosphere is also a uniform mixture.

Answer

1. False
   Corrected Statement — Oxygen gas is more soluble in cold water than in hot water, because the solubility of gases generally decreases with an increase in temperature.
2. False
   Corrected Statement — A mixture of sand and water is a non-uniform mixture, not a solution, because the components are not evenly distributed.
3. False
   Corrected Statement — The amount of space occupied by any object is called its volume.
4. False
   Corrected Statement — An unsaturated solution can dissolve more solute at a given temperature, whereas a saturated solution cannot dissolve any more solute at that temperature.
5. True

Fill in the blanks.

1. The volume of a solid can be measured by the method of displacement, where the solid is ............... in water and the ............... in water level is measured.
2. The maximum amount of ............... dissolved in ............... at a particular temperature is called solubility at that temperature.
3. Generally, the density ............... with increase in temperature.
4. The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a ............... solution of glucose.

Answer

1. The volume of a solid can be measured by the method of displacement, where the solid is immersed (lowered) in water and the rise (change) in water level is measured.
2. The maximum amount of solute dissolved in a fixed quantity of solvent at a particular temperature is called solubility at that temperature.
3. Generally, the density decreases with increase in temperature.
4. The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.

You pour oil into a glass containing some water. The oil floats on top. What does this tell you?

1. Oil is denser than water
2. Water is denser than oil
3. Oil and water have the same density
4. Oil dissolves in water

Answer

Water is denser than oil.

Reason — Since the oil floats on top of the water, it means that oil is lighter, that is, less dense than water. Therefore, water is denser than oil.

A stone sculpture weighs 225 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.

Answer

Given, mass of the stone sculpture = 225 g and volume = 90 cm³.

Density $= \dfrac{\text{Mass}}{\text{Volume}} = \dfrac{225 \text{ g}}{90 \text{ cm}^3} = 2.5$ g/cm³

The density of the stone sculpture is 2.5 g/cm³, which is greater than the density of water (1 g/cm³). Therefore, the stone sculpture will sink in water.

Which one of the following is the most appropriate statement, and why are the other statements not appropriate?

1. A saturated solution can still dissolve more solute at a given temperature.
2. An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature.
3. No more solute can be dissolved into the saturated solution at that temperature.
4. A saturated solution forms only at high temperatures.

Answer

The most appropriate statement is:

No more solute can be dissolved into the saturated solution at that temperature.

Reason — A saturated solution is one in which the maximum amount of solute has been dissolved and no more solute can be dissolved at that temperature. Hence, this statement is correct.

The other statements are not appropriate because:

Option 1 — A saturated solution cannot dissolve more solute at a given temperature, as it has already reached its limit.

Option 2 — An unsaturated solution can still dissolve more solute, so it has not dissolved the maximum amount possible.

Option 4 — A saturated solution can form at any temperature, not only at high temperatures.

You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?

Answer

Total volume of the bottle = 2 litres = 2000 mL.

Volume of water already poured = 500 mL.

Remaining volume the bottle can hold = 2000 mL − 500 mL = 1500 mL.

So, the bottle can hold 1500 mL (1.5 litres) more water.

An object has a mass of 400 g and a volume of 40 cm³. What is its density?

Answer

Given, mass of the object = 400 g and volume = 40 cm³.

Density $= \dfrac{\text{Mass}}{\text{Volume}} = \dfrac{400 \text{ g}}{40 \text{ cm}^3} = 10$ g/cm³

The density of the object is 10 g/cm³.

Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.

Answer

An unpeeled orange floats on water because its peel contains tiny air pockets. These air pockets increase the volume of the orange without much increase in mass. As a result, the overall density of the unpeeled orange becomes less than that of water, so it floats.

When the orange is peeled, these air pockets are removed along with the peel. The peeled orange becomes more compact, and its overall density becomes greater than that of water. As a result, the peeled orange sinks.

Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?

Answer

Density of Object A $= \dfrac{\text{Mass}}{\text{Volume}}$

$= \dfrac{200 \text{ g}}{40 \text{ cm}^3} = 5$ g/cm³

Density of Object B $= \dfrac{\text{Mass}}{\text{Volume}}$

$= \dfrac{240 \text{ g}}{60 \text{ cm}^3} = 4$ g/cm³

Since the density of Object A (5 g/cm³) is greater than the density of Object B (4 g/cm³), Object A is denser.

Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.

Answer

The density of the clay will remain the same.

When the clay is flattened into a thin sheet, only its shape changes. Assuming no clay is added or removed and no air gaps are formed, its mass (120 g) and volume (60 cm³) remain unchanged. Since the density of a substance is independent of its shape or size, and depends on its mass and volume, the density of the clay does not change.

Density $= \dfrac{\text{Mass}}{\text{Volume}} = \dfrac{120 \text{ g}}{60 \text{ cm}^3} = 2$ g/cm³

So, the density remains 2 g/cm³ in both cases.

A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?

Answer

Given, mass of the iron block = 600 g and density = 7.9 g/cm³.

Since Density $= \dfrac{\text{Mass}}{\text{Volume}}$,

we get Volume $= \dfrac{\text{Mass}}{\text{Density}}$.

Volume $= \dfrac{600 \text{ g}}{7.9 \text{ g/cm}^3} = 75.95$ cm³ $\approx 76$ cm³

So, the volume of the iron block is about 76 cm³.

You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70 °C), the water level in the glass tube rises. How does it affect the density?

Answer

When the test tube is kept in a beaker containing hot water, the water inside the test tube gets heated. On heating, the particles of water move away and spread out, so the volume of the water increases. This is why the water level in the glass tube rises.

Since the mass of the water remains the same but its volume increases, the density of the water decreases.
This is because Density $= \dfrac{\text{Mass}}{\text{Volume}}$, so when the volume increases while mass remains the same, the density becomes less.

Hence, heating the water causes its density to decrease.