Q1. What will happen to the size of rain water puddles in a hot sunny day?

Solution

On a hot sunny day, due to high temperature, the rate of evaporation of water will be faster. Therefore, water from the rain water puddles will dry up and the puddles will decrease in size.

Q2. Explain which one will cause more severe burns – boiling water at 100oC or steam?

Solution

Particles in steam, that is, water vapour at 373 K (100oC) have more energy than boiling water at the same temperature because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.

Q3. How does wind speed affect the rate of evaporation? Elaborate.

Solution

The rate of evaporation of a liquid increases with increasing wind speed. When the speed of wind increases, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings. This increases the rate of evaporation of water.

Q4. What is meant by particulate nature of matter? List any four characteristics of particles of matter.

Solution

Particulate nature of matter means that all matter is made up of discrete tiny particles. Characteristics of particles of matter: 1. Particles of matter are very small. 2. Particles of matter have space between them. 3. Particles of matter are continuously moving. 4. Particles of mater attract each other.

Q5. List the three states of matter. Which state of matter is rigid? Why?

Solution

(a)Solid, liquid and gas are the three states of matter. (b)Solid state is rigid since its atoms are closely packed and intermolecular spaces between them are the minimum among the three states of matter.

Q6. The rate of evaporation of a liquid increases on heating. Explain.

Solution

When the temperature of a liquid is increased by heating it, more particles of the liquid get enough kinetic energy to go into the vapour state. This increases the rate of evaporation.

Q7. At what temperature does solid ice and liquid water co-exist together?

Solution

At the melting point of ice which is 0oC, solid ice and liquid water co-exists.

Q8. The boiling point of water is 100o C under normal atmospheric pressure. Convert this temperature in SI units.

Solution

We know that: T (in K) = T(oC) + 273

= 100 + 273 = 373 K

Q9. “The particles of matter are continuously moving”. Explain this statement with an example.

Solution

Add a small amount of red ink in a glass full of water. The red ink slowly mixes with water and finally the colour of the water becomes red. This activity shows that particles of matter are moving.

Q10. Why is dry ice stored under high pressure?

Solution

Dry ice is stored under high pressure because on decreasing the pressure on dry ice, it gets converted directly into carbon dioxide gas.

Q11. How does the perspiration or sweating help keep our body cool on a hot day?

Solution

On a hot day, when our body temperature tends to rise too much, our sweat glands give out moisture (sweat) on our skin. When this sweat evaporates, it takes the latent heat of vaporisation from our body hence making our body cool.

Q12. A rubber band changes its shape on stretching, then why is it called a solid?

Solution

A rubber band is considered a solid because it changes shape under the action of force and when the force is removed, it regains its original shape.

Q13. How does evaporation cause cooling?

Solution

When evaporation occurs, the particles of the liquid absorb heat from the surroundings to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surrounding cool.

Q14. How will you demonstrate that water vapour is present in air?

Solution

The presence of water vapour in air can be demonstrated by the following experiment: We take a steel tumbler and put some well crushed ice in it. Allow the steel tumbler to stand undisturbed for about 5 minutes with the ice in it. We would observe that a large number of tiny drops of water appear on the outer surface of the steel tumbler. This happens because the air around the steel tumbler contains water vapour in it. When these water vapour come in contact with the cold, outside surface of steel tumbler, they condense to form tiny droplets of water.

Q15. When is the air said to be damp or dry? What happens to the humidity in such conditions?

Solution

When the amount of water vapours present in the air is small, the air appears to be ‘dry’ and the humidity is said to be ‘low’. On the other hand, when the amount of water vapours in the air is large, the air appears to be ‘damp’ and humidity is said to be ‘high’.

Q16. Give two examples each of solid, liquid and gas mixed in liquid.

Solution

1. Solid mixed in liquid: (i) Salt water (ii) solution of potassium permanganate 2. Liquid mixed in liquid: (i) Lemon juice (ii) Dettol in water 3. Gas mixed in liquid: (i) Soft drink/Soda water (ii) Mist

Q17. List two differences between boiling and evaporation.

Solution

1. Boiling occurs only when the liquid is heated whereas evaporation takes place on its own. 2. Boiling takes place at a specific temperature called boiling point whereas evaporation takes place at all temperatures.

Q18. What are the basic characteristics of liquids?

Solution

Characteristics of liquids: 1) They have fixed volume but no fixed shape. 2) They cannot be compressed much. 3) They flow easily. 3) The positions of particles are not fixed.

Q19.   Differentiate between evaporation and boiling.

Solution

Difference between Evaporation and Boiling Evaporation Boiling It is a surface phenomenon. It is a bulk phenomenon. It is a slow process. It is a rapid process. It takes place at all temperatures but below the boiling point. It takes place at a definite and constant temperature.

Q20. What is the elementary constituent of matter?

Solution

Matter is made up of small invisible particles called atoms or group of atoms known as molecules which can be same or different.

Q21. A gas jar containing air is upside down on a gas jar of bromine vapour. It is observed that after some time, the gas jar containing air also become completely reddish brown. (a) Explain why this happens. (b) Name the process involved.

Solution

(a) Both air and bromine vapours are in a gaseous state and has a random motion. The particles of a gas have maximum kinetic energy. They move with high speed in all directions and can exert pressure on the walls of its container. The moving particles of bromine vapour and air collide with each other and bounce about in all directions due to which they get mixed uniformly. (b) This process is called diffusion.

Q22. Why should we wear cotton clothes in summer?

Solution

In summers, we perspire more. Cotton being a good absorber of water helps in absorbing the sweat and exposes it to the atmosphere for evaporation. When sweat evaporates from our body, it takes heat from our body. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.

Q23. What is meant by relative density of a substance? The relative density of mercury is 13.6. What is its density in SI units? Given that the density of water is 1000 kg/m3.

Solution

Relative density is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.  The density of mercury in SI units will be 13.6 x 103 kg/m3

Q24. A rubber band changes its shape on stretching, then why is it called a solid?

Solution

A rubber band is considered a solid because it changes shape under the action of force and when the force is removed, it regains its original shape.

Q25. (a) Differentiate between three states of matter on the basis of following properties. (i) Intermolecular forces. (ii) Arrangement of molecules. (b) Liquids generally have lower density compared to solids. But you must have observed that ice floats on water. Give reason.

Solution

(a) Solids: (i) Have maximum intermolecular forces of attraction.           (ii) The molecules are closely packed. Liquids: (i) Have lesser inter molecular forces of attraction.             (ii) The molecules are less closely packed. Gases: (i) Have least intermolecular forces of attraction.           (ii) The molecules are far away from each other. (b) When water freezes to form ice, its volume increases and hence its density decreases. As ice has lower density than water, it floats on water.

Q26. Explain the movement of molecules in frozen milk and milk at room temperature.

Solution

Frozen milk is a solid so its particles will be closely packed and as a result their movement will be restricted. At room temperature, frozen milk gets converted to liquid milk so its particles will not be very closely packed and hence will be constantly moving and sliding over each other.

Q27. Which of the two diffuses faster: a liquid or a gas?

Solution

Gases diffuse faster than liquids. This is because the particles of gases are much farther apart from one another as compared to the solids and liquids. Also, the force of attraction between the particles is negligible, hence particles of a gas move freely in all the directions. Gases thus can mix or diffuse into other gases.

Q28. Ice floats on water whereas a needle sinks. Explain.

Solution

The density of ice is greater than that of water. In ice, the molecules are arranged in a manner that it has an open structure something like a cage. Due to this unusual arrangement, ice floats on water. Similarly, the density of needle is more than that of water. A needle sinks in water due to closely packed arrangement of molecules.

Q29. Account for the following: (a) An iron rod is solid at room temperature. (b) When sugar crystals dissolve in water, the level of water does not rise appreciably.

Solution

(a) Iron rod is solid at room temperature because of stronger forces of attraction between its particles.  (b) The sugar crystals occupy the intermolecular spaces present between the water molecules. Hence, there is no appreciable increase in the water level.

Q30. State four characteristics of solids.

Solution

Characteristics of solids are: (i)They have fixed shape and volume. (ii)They can not be compressed much. (iii)They are rigid. (iv)They have high density. (v) Some solids may change their shape when an external force is applied but when that force is removed they can regain their original shape. This shows that some solids are elastic.

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