Solutions-NCERT-Class-8-Science-Curiosity-Chapter-9-The Amazing World of Solutes, Solvents, and Solutions-CBSE

It appears that a group of people are gathered at the seashore to collect salt from the sand.

This scene represents a freedom movement protest, where people are breaking the salt law by making salt themselves.

When you add too much sugar to your tea, it stops dissolving because the tea becomes a saturated solution. This means the tea cannot dissolve any more sugar at that temperature, so the extra sugar settles at the bottom.

To solve this problem:

• Heat the tea: Warm tea can dissolve more sugar.
• Add more tea or water: This increases the amount of solvent, so more sugar can dissolve.

So, increasing temperature or the amount of liquid helps the extra sugar dissolve.

This happened because of different properties of water and oil.

• Water is a polar solvent. Its molecules have partial positive (H⁺) and partial negative (O⁻) charges.
• Oil is non-polar. It has no charges to attract or stabilize ions/polar molecules.
• Polar substances dissolve in polar solvents, non-polar substances dissolve in non-polar solvents.

• Since sugar and salt are polar/ionic, they don’t dissolve in non-polar oil, but dissolved in polar solvent water

Water is considered a good solvent :

• Water can dissolve many substances such as salts, sugars, acids, and bases.
• Its polar nature helps it dissolve both solid and some liquid substances.
• Because it dissolves many materials, water is called a universal (good) solvent.

Water bottles are usually tall and cylindrical instead of spherical for several practical reasons:

• Easy to hold and grip
• Stability : Cylindrical bottles can stand upright easily. A spherical bottle would roll and fall.
• Easy to store and pack : Cylindrical bottles fit well in bags, cupboards, refrigerators, and bottle holders. Spherical bottles waste space.
• Easy to pour and drink from : The tall shape allows better control while pouring or drinking.

So, water bottles are cylindrical because they are convenient, stable, space-saving, and practical for daily use.

• Yes, just as water can act as a solvent in liquid solutions, gases can also form solution.
• Air is a gaseous solution. Since nitrogen is present in the largest amount in the air, it is considered as the solvent, while oxygen, argon, carbon dioxide, and other gases are considered as solutes.

• If we keep adding more salt to a fixed amount of water, the salt will dissolve only up to a certain limit. After this point, the solution becomes saturated and no more salt can dissolve. Any extra salt added will remain undissolved and settle at the bottom of the container.
• No more solute can be dissolved into the saturated solution at that specific temperature.

Yes, gases also dissolve in water. For example, oxygen and carbon dioxide dissolve in water, which helps aquatic plants and animals survive. However, gases dissolve in much smaller amounts than solids, and their solubility depends on temperature and pressure.

• This happens mainly because of the difference in density of the substances.
• Sawdust is less dense than water, so it floats on the surface.
• Sand is denser than water, so it sinks to the bottom.
• The size and shape of particles also affect how quickly they sink or float.

• If 1 litre of oil weighs only about 910 grams, it means the density of oil is 0.91 g/mL.
• Since 1 litre of water weighs 1000 grams (density = 1 g/mL), the density of oil is less than that of water. This is why oil floats on water.

Measuring cylinders are narrow and tall so that small changes in volume cause a noticeable change in liquid level. This makes the reading more accurate and easier to measure. In wider containers like beakers, the same volume change causes only a small rise in level, leading to more reading errors.

In case of coloured liquids the mark on the measuring cylinder should coincide with the top of the meniscus!

False: Oxygen is more soluble in cold water.

False: Mixture of sand and water is not a solution. Sand does not dissolve in water but settles down.

False: The amount of space occupied by any object is called its volume.

False: A saturated solution has more solute dissolved than an unsaturated solution.

True.

placed; rise

solute, solvent

Decrease

Saturated

(ii) Water is denser than oil

Density = \(\frac{mass}{volume} = \frac{225}{90}\) = 2.5 g/cm³ 

The stone sculpture has a density of 2.5 g/cm³. Since this exceeds water's density of 1 g/cm³, it will sink.

(a) The most appropriate statement is (iii): “No more solute can be dissolved into the saturated solution at that temperature.”

• A saturated solution means it has dissolved the maximum amount of solute possible at a given temperature.
• Any extra solute added will remain undissolved because the solvent cannot accommodate more.

(b) Why the others are not appropriate

(i) Incorrect: A saturated solution cannot dissolve more solute at the same temperature; that’s the definition of saturation.

(ii) Incorrect: This describes a saturated solution, not an unsaturated one. An unsaturated solution can still dissolve more solute.

(iv) Incorrect: Saturated solutions can form at any temperature, not only at high temperatures. The saturation point depends on solubility at that specific temperature.

The bottle’s total capacity is 2 litres = 2000 mL.

I have already poured in 500 mL.

So, remaining capacity = 2000 mL − 500 mL = 1500 mL

The bottle can hold 1500 mL (or 1.5 litres) more water.

Density = \(\frac{mass}{volume} = \frac{400}{40}\) = 10 g/cm³ 

An unpeeled orange floats, while a peeled orange sinks, because of density and trapped air.

Unpeeled orange:

• The peel is thick and porous, with tiny air pockets inside it.
• These air pockets increase the orange’s volume without adding much mass.
• This makes the average density of the orange less than water, so it floats.

Peeled orange:

• When the peel is removed, the air pockets are gone.
• The remaining fruit is denser than water.
• Its average density becomes greater than water, so it sinks.

Density of object A = \(\frac{mass}{volume} = \frac{200}{40}\) = 5 g/cm³ 

Density of object B = \(\frac{mass}{volume} = \frac{240}{60}\) = 4 g/cm³ 

Object A is denser, having more density than object B.

When Reema flattens the clay into a thin sheet, its mass remains 120 g and its volume remains 60 cm³ (only the shape changes).

Therefore, the density stays the same (2 g/cm³). Changing the shape does not affect density, since density depends only on mass and volume, not on form.

We know, Density = \(\frac{mass}{volume}\)

Therefore, Volume = \(\frac{mass}{density} = \frac{600}{7.9}\) = 75.94 cm³

The density of the water decreases.

• When the water is heated by the beaker containing hot water (70 °C), the particles of the water tend to move away and spread.
• This spreading results in an increase in the water's volume (as indicated by the rising water level in the glass tube).
• Since density is defined as Mass/Volume, and the mass does not change while the volume increases, the density of the substance decreases upon heating.