# NCERT Solutions for Class 9 Science Chapter 1 Matter in our Surroundings

Q. Liquids generally have lower density as compared to solids, But you must have observed that ice floats on water. Find out why.

Ans. Ice and water both are the different states of matter of same substance but ice has greater volume than of the same amount of water. As density is inversely proportional to the volume, ice is less dense than water due to larger volume of ice than water. Objects having density less than that of water float on the surface of water.

Q. What is the physical state of water at the following temperatures?
(i) 25°C

(ii) 0°C
(iii) 100°C

Ans. (i) Liquid
(ii) Solid
(iii) Gas

Q. Why does the temperature of a substance remain constant during its melting point or boiling point?

Ans. When heat is supplied then on melting of a substance heat is absorbed for the conversion of solid state into the liquid state which further converts till it overcomes the forces of attraction between the particles and temperature remains constant till conversion is complete. This heat is called latent heat of fusion.

Q. How does the water kept in an earthen pot (matka) become cool during summer?

Ans. Earthen pot has pores on its surface from which water comes out on the outer surface causing cooling effect due to evaporation. This makes the water cool in the pot.

Q. (i) ‘Particles of matter are continuously moving’. Justify the statement with example.
(ii) Osmosis is a special kind of diffusion. Comment.

Ans. (i) The particles of matter possess kinetic energy with them which on increasing temperature start moving faster and continuously moving and colliding with each other and with the walls of the container. For example:

1. To demonstrate motion of particles in air: Place few lighted incense sticks in a corner of a room and move about the room and smell the fragrance of the incense sticks. The fragrance produced due to burning of incense sticks is due to movement of vapours produced rapidly in all directions.
2. To demonstrate motion of particles of solid matter: Drop a crystal of copper sulphate or potassium permanganate into a glass of hot water. Without disturbing the solution allow the crystals to settle down at the bottom. The colour of solid is seen spreading slowly. This is because solid particles diffuse in the water.
(ii) The phenomenon in which particles move from higher concentration to the lower concentration without the help of semipermeable membrane is called diffusion. Osmosis is the movement of solvent particles from lower concentration to the higher concentration when two solutions are separated by a semi-permeable membrane. Thus, osmosis is the special kind of diffusion involving movement of particles.

Q. (i) Suggest a method to liquefy atmospheric gases?
(ii) What properties are analysed to determine whether matter is a solid, liquid or gas?

Ans. (i) Atmospheric gases are taken in a closed cylinder with a piston fitted on it. Then by applying pressure and decreasing temperature, gas is easily liquefied.

(ii) Temperature and pressure are required to determine state of matter.

Q. (i) Give two reasons to justify—
(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.
(ii) Convert the following temperatures to the kelvin scale.

(a) 25°C
(b) 373°C
Ans. (i) (a) Water is liquid at room temperature i.e, 25°C because above its melting point i.e., 0°C and below its boiling point i.e., 100°C, water is in liquid state. (b) An iron almirah is a solid at room temperature because melting point of iron is higher than room temperature.
(ii) (a) °C + 273 = K
25°C + 273 = 298 K
(b) °C + 273 = K
373°C + 273 = 646 K

Q. Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Ans. Rigidity: The tendency of a substance to maintain their shape when any external force is applied on a substance. Solids are rigid but liquids and gases are not.

• Compressibility: When external force is applied on a substance the intermolecular space between molecules suppress them to fill up space and bring particles closer. Liquids and gases are compressible but solids are not.
• Fluidity: It is the ability of molecules to flow. Gases and liquids flow.
• Filling a gas container: Gas particles continuously vibrate randomly in the container in all directions. Thus, gas can fill a container and they take the shape of a container on filling.
• Shape: Solids have definite shape because intermolecular force of attraction between its molecules is maximum which gives it a definite shape. But liquids and gases takes shape of a container in which they are filled. Thus, shape defines a definite boundary
• Kinetic energy: It is the energy possessed by particles due to motion of particles.
• Density: Mass per unit volume is called density. Solids have highest density.

Q. (i) Name A, B, C, D, E and F in the following diagram showing change in its state.

(ii) Why does a desert cooler cool better on a hot dry day?

Ans. (i) A: Melting
B: Vaporization
C: Condensation
D: Solidification
E: Sublimation
F : Deposition

(ii) When water is constantly flowing over windows of cooler due to evaporation of water in hot dry day causes cooling inside cooler and we feel cool air in room when air blows.

Q. (i) Define matter?
(ii) What are the characteristics of the particles of matter?
(iii) Arrange the following by mentioning state of matter according to forces of attraction.

 Substances Force of attraction between particles Physical state of matter Water, table salt, Ice, wax, oxygen, chalk, milk, LPG, petrol, match sticks, perfume, incense fumes Weak/strong/moderate Solid/liquid/gas

Ans. (i) Anything that occupies space and has mass is called matter. It exists in the form of five basic elements such as air, earth, fire, sky and water.
For example: Chair, air, perfume, mountain, snow, tree, building, etc.

(ii)Characteristics of the particles of matter are:

1. Matter is made up of small particles called atoms.
2. These particles are too small to be observed with naked eye.
3. These particles are constantly moving.
4. These particles have spaces between them.
5. Particles of matter attract each other because of the force of attraction.

(iii)

 Substances Force of attraction between particles Physical state of matter Water, milk, petrol Moderate Liquid Table salt, ice, wax, chalk, match sticks Strong Solid LPG, perfume, incense fumes Weak Gas

Q. (i) Why does ice at 0°C appear colder than water at same temperature?

(ii) (a) What does happen to the molecular motion and energy of 1 kg of water at 273 K when it is converted into ice at the same temperature?
(b) What produces more severe burns, boiling water or steam?

(iii) How is the latent heat of fusion related to the energy exchange that takes place during this change of state?

Ans. (i) Particles in water at 0°C (273K) have more energy as compared to the particles in ice at the same temperature. It is due to the latent heat of fusion.
(ii) (a) Molecular motion decreases as water gets converted into ice and latent heat of solidification is given off.
(b) Steam will produce more severe burn as it consist of hidden heat called as latent heat of vaporization.
(iii) Latent heat of solidification is equal to the latent heat of fusion during this change of state.

Q. (i) Define the term ‘latent heat of fusion’ of a solid. How much is the latent heat of fusion of ice?
(ii) A student heats a beaker containing ice and water. He measures the temperature of the content of the beaker as a function of time. Which of the following would correctly represent the result? Justify your choice.

Ans. (i) The latent heat of fusion of a solid is the quantity of heat in joules required to convert 1 kg of the solid (at its melting point) to liquid, without any change in temperature. The latent heat of fusion of ice is 3.34 × 105 J/kg.
(ii) On heating the mixture, ice melts but temperature remains constant. This happens because during

Q. Alka was making tea in a kettle. Suddenly she felt intense heat from puff of steam gushing out of the spout of the kettle. She wondered whether the temperature of the steam was higher than that of the water boiling in the kettle. Comment.

Ans. The temperature of both boiling water and steam is 100°C, but steam has more energy because of latent heat of vaporization.

Q. Give reason for the following:
(i) Sugar crystals dissolve faster in hot water than cold water.
(ii) Ice is solid at 0°C, while water is liquid at room temperature.
(iii) Sponge though compressible is a solid.
(iv) Evaporation causes cooling.
(v) Rate of evaporation of an aqueous solution decreases with increase in humidity.
(vi) Water stored in an earthen vessel becomes cool.

Q. (i) (a) Which of the two diffuses faster: a liquid or a gas?
(b) Gases completely fill the vessel in which they are kept. Explain.
(c) Under what conditions gases can be liquefied? In which form LPG is filled in gas cylinder that we use at home?
(d) Why are we able to sip hot tea or milk faster from a saucer rather than from a cup?

Q. (ii) Give reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.