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 different states of matter of the same substance but ice has a greater volume than the same amount of water. As density is inversely proportional to the volume, ice is less dense than water due to a larger volume of ice than water. Objects having a 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 the melting of a substance heat is absorbed for the conversion of the solid state into the liquid state which further converts till it overcomes the forces of attraction between the particles and the temperature remains constant till the conversion is complete. This heat is called the 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 a 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 an 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:
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 the 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 the melting point of iron is higher than at 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 its shape when any external force is applied to a substance. Solids are rigid but liquids and gases are not.
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 the windows of the cooler due to evaporation of water on hot dry days causes cooling inside the cooler and we feel the cool air in the room when air blows.
Q. (i) Define matter.
(ii) What are the characteristics of the particles of matter?
(iii) Arrange the following by mentioning the 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: Chairs, air, perfume, mountains, snow, trees, buildings, etc.
(ii)Characteristics of the particles of matter are:
(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 the same temperature?
(ii) (a) What happens 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 the water gets converted into ice and the latent heat of solidification is given off.
(b) Steam will produce a more severe burn as it consists of hidden heat called latent heat of vaporization.
(iii) The 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 the 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 the temperature remains constant. This happens because during
Q. Alka was making tea in a kettle. Suddenly she felt intense heat from a 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 the latent heat of vaporization.
Q. Give a reason for the following:
(i) Sugar crystals dissolve faster in hot water than in 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 an 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 the 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 a 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.
FAQ: NCERT Solutions for Class 9 Science - Chapter 1: Matter in Our Surroundings
Matter is anything that has mass and occupies space. It is made up of particles and exists in three primary states: solid, liquid, and gas. All substances in the universe are considered matter.
Solid: Molecules are closely packed and have a fixed shape and volume.
Liquid: Molecules are less tightly packed than in solids and can move past each other, allowing liquids to flow and take the shape of their container.
Gas: Molecules are far apart and move freely, so gases do not have a fixed shape or volume and expand to fill the entire container.
Physical Change: A change in which the form or appearance of matter is altered but no new substance is formed (e.g., melting of ice, tearing paper).
Chemical Change: A change in which one or more substances are transformed into new substances with different properties (e.g., rusting of iron, burning wood).
The characteristics of matter include:
Mass: The amount of matter present in an object.
Volume: The space occupied by an object.
Density: The mass per unit volume of a substance.
Intermolecular Forces: The forces that hold the particles of matter together, which vary in different states.
The state of matter is determined by the arrangement, movement, and energy of its particles. Factors such as temperature and pressure can cause matter to change states, such as from solid to liquid (melting) or from liquid to gas (evaporation).
Evaporation occurs at the surface of a liquid and can happen at any temperature. It is a slow process.
Boiling occurs throughout the liquid at a specific temperature, known as the boiling point, and is a faster process.
Temperature plays a significant role in changing the state of matter. When matter is heated, its particles gain energy and move more vigorously, which can lead to a change in state, such as from solid to liquid (melting) or liquid to gas (evaporation). Conversely, when matter is cooled, the particles lose energy, and a state change like condensation or freezing can occur.
Intermolecular forces are the forces of attraction or repulsion between the molecules of a substance. These forces are responsible for determining the state of matter and its properties, such as density and melting/boiling points.
Ice melts at 0°C and water boils at 100°C because these are the melting point and boiling point, respectively, of water at standard atmospheric pressure (1 atm). These points are characteristic of water and depend on the temperature at which the intermolecular forces in the substance are overcome.