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NCERT 6th Class (CBSE) Science: Sorting Materials into Groups

9th Class (CBSE) Science: Matter in Our Surroundings

Question: Why do people perspire a lot on a hot humid day?

Answer: On a hot, humid day, due to the heat our body starts sweating for the cooling mechanism i.e., by evaporation and gets cooling effect. But the air cannot hold any more water on a humid day and therefore the sweat or perspiration is seen.

Question: Why is it advisable to use pressure cooker at higher altitudes?

Answer: At higher altitudes, the atmospheric pressure is low and the water boils very fast and evaporates at faster rate therefore the pressure is required to increase the cooking process and this is done by using pressure cooker which increases the pressure inside the container and cooks food faster.

Question: What are fluids?

Answer: The states of matter that can flow due to less intermolecular force of attraction, are liquids and gases and are called as fluids.

Question: One kg cotton and one kg sand, which is more denser? Why?

Answer: One kg sand is more denser than 1 kg cotton because density = mass / volume

The volume required by cotton is more than the sand and density and volume are inversely proportional.

Question: Why is water liquid at room temperature?

Answer: At room temperature, the molecules of water have some intermolecular force of attraction and the room temperature cannot provide sufficient heat for these molecules to overcome their force of attraction and therefore remain in liquid phase.

Question: State the differences between solid, liquid and gas.


  1. Solid: Solid refers to a form of matter which has structural rigidity and has a firm shape which cannot be changed easily. For Eg: Fixed shape and volume.
  2. Liquid: Liquid is a substance, that flows freely, having a definite volume but no permanent shape. For Eg: No fixed shape but has volume.
  3. Gas: Gas refers to a state of matter, do not have any shape but conform to the shape of the container, completely, in which it is put in. For Eg: Neither definite shape nor volume.

Question: Cotton in solid but it floats on water. Why?

Answer: Cotton has large number of pores, in which air is trapped. Hence reducing its density and increasing the volume. Therefore cotton floats on water. But when these pores get filled with water it starts sinking.

Question: Why arc solids generally denser than liquids and gases?

Answer: Density of a substance is given by a formula= Mass / Volume In case of solids the molecules are tightly packed and hence large mass is concentrated in very small volume. Hence their density is more. But in case of liquids and gases, their molecules have inter molecular space and hence they don’t have large mass concentrated in small volume. So the density of solids is generally more than that of the liquids and gases.

Question: On a hot sunny day, why do people sprinkle water on the roof or open ground?

Answer: During hot sunny day, the surface of roof or ground absorbs large amount of heat and remains hot, on sprinkling water on these surfaces, the water absorbs large amount of heat from the surface due to its large latent heat of vaporization thereby allowing the hot surface to cool.

Question: On a hot sunny dug why do we feel pleasant sitting under a tree?

Answer: Tree has lot of leaves which constantly show transpiration. Transpiration is loss of water through small tiny pores of leaves called stomata. When this water comes on the surface of leaf the water evaporates thereby causing cooling effect. Therefore we feel pleasant sitting under the tree on a hot sunny day.

Question: The U’mpeuiUnc at which liquids change into vapours is very high, for example, water vaporises at 100°C then how is n possible for water to evaporate at room temperature or at are other temperature?

Answer: The molecules of water present on the surface of the exposed area which are in very small fraction, gains the energy from the surrounding. With this higher kinetic energy they are able to break the force of attraction between them and hence get converted into vapour state.
This phenomenon of change of a liquid into vapors that takes place at any temperature below its boiling point is called evaporation.

Question: Name the factors that affix l evaporation.

Answer: The rate of evaporation will increase with

  1. An increase of surface area,
  2. An increase of temperature,
  3. A decrease in humidity,
  4. An increase in wind speed.

Question: The melting point of ice is 273.16 K. What does this mean? Explain in detail.

Answer: Ice is solid at 0°C i.e., 273° K. The molecules of ice are tightly packed. These molecules have to overcome the force of attraction with which they are held and hence they gain this heat from the surrounding but the temperature remains the same as their energy is used to overcome the force of attraction between the particles. The particles have their state and starts vibrating freely and a stage reaches when the solid ice melts and is converted to liquid state at the same temperature i.e., 273 K.

Question: How is the high compressibility property of gas useful to us?

Answer: The gases have high compressibility. This property is used in the following situation:

  1. LPG (liquefied petroleum gas) is a fuel which is made up of petroleum gas. On compressing this petroleum gas it forms liquid.
  2. Oxygen cylinders in the hospitals have compressed gas filled in it.
  3.  CNG (compressed natural gas) is a natural gas, methane, which is compressed and used as a fuel in vehicles and at home.

Question: With the help of an example, explain how diffusion of gases in water is essential?

Answer: The gases from the atmosphere diffuse and dissolve in water. Gases like oxygen and carbon dioxide diffuse in water, are essential for the survival of aquatic animals and plants.

Animals breathe in this oxygen dissolved in water for their survival and plants can use carbon dioxide dissolved in water for photosynthesis.

Question: Pressure and temperature determine the state of a substance. Ex-plane this in detail.

Answer: (1) Any matter i.e., solid, liquid or gas when experiences an increase in temperature then they change their state.


Take ice cubes in a beaker or heat them slowly, the temperature increases and the ice melts to form liquid. Heat this liquid further it will become steam.

(2) On lowering down the temperature of any matter, show change in their state.


Take the steam that is coming out of a boiling water and allow it to cool down, it condenses to form water and on further cooling of this water we get ice.

(3) On applying pressure and reducing temperature we can liquefy gases or change them into solid.

Example: Take carbon-dioxide gas, reduce its temperature and apply lot of pressure on it so that it changes into solid carbon dioxide, called dry ice, which is used as refrigerant for cooling.

If the pressure on it is decreased it directly changes into gas. In LPG cylinders, the petroleum gas is cooled and with lot of pressure changes it into liquid state. While using this LPG, we release the pressure exerted on it and hence it comes out in the form of gas.

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