CHEMISTRY CH 1

Short Answer Type Questions

1. Why do we see water droplets collected on the outer surface of a glass container, containing ice?

Answer: The water vapour present in air, comes in contact with the cold outer surface of the container thereby condensing it to form water droplets.

2. Explain why solids have fixed shape but liquids and gases do not have fixed shape.

Answer: Solids have fixed shape due to strong intermolecular force of attraction between them. The liquids and gases have molecules with less intermolecular force of attraction and hence they can flow and take shape of the container.

3. Liquids and gases can be compressed but it is difficult to compress solids. Why?

Answer: Liquids and gases have intermolecular space, on applying pressure externally on them the molecules can come closer thereby minimizing the space between them. But in case of solids there is no intermolecular space to do so.

4. A balloon when kept in sun, bursts after some time. Why?

Answer: The balloon has air filled in it. The balloon when kept in sun gets heated and the air inside it also gets heated. The molecules of air get energy, and vibrate faster thereby exerting large force on the walls of the balloon. Due to this expansion of gases the balloon bursts.

5. 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.

6. Distinguish between evaporation and boiling.

Answer:

7. 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.

8. 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.

9. 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.

10. 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.

11. State the differences between solid, liquid and gas.

Answer:

12. 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.

13. 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 intermolecular 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.

14. 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 vaporisation thereby allowing the hot surface to cool.

15. 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.

16. The temperature 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 vapours that takes place at any temperature below its boiling point is called evaporation.

17. Name the factors that affect 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.

18. 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.

19. 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.

20. 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.

Long Answer Type Questions

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

Answer: (i) 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.

(ii) 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.

(iii) 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 diy 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.

2. Explain giving examples the various factors on which rate of evaporation depends.

Answer: The rate of evaporation depends on the following factors:

(1) Surface area: If the surface area is increased the rate of evaporation also increases.

(a) To dry the clothes we spread them to dry faster.

(b) Tea in saucer cools faster than in a cup.

(2) Temperature: If the temperature is increased the rate of evaporation also increases. Due to increase in temperature the particles gain more kinetic energy and change their phase from liquid to gaseous. Water will evaporate faster in sun than in shade.

(3) Humidity: It is the amount of water vapour present in air. The air can hold definite amount of water vapour, at a given temperature. If the amount of water vapour is high in the air then the rate of evaporation decreases. On hot and humid day, desert coolers are not effective as the air cannot hold any more moisture to get the cooling effect.

(4) Wind speed: With the increase in wind speed, the rate of evaporation increases. The particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.

Value-Based Questions

1. Adil parked his bicycle on a sunny day in a parking stand of his school campus. When the school got over Adil saw his burst cycle type. Thereafter he kept less air in his cycle types and did not inflate them fully.

(a) Why did the type burst?

(b) Why is air compressible?

(c) What value of Adil is reflected in the above act?

Answer: (a) The tyre burst because the air inside the tyre got heated and therefore exerted pressure on the walls of the tyre.

(b) Air is compressible because it has large intermolecular space.

(c) Adil showed the value of intelligence, awareness and self responsibility.

2. Akshay’s friend visited his house in Mumbai and he was surprised to see air conditioners installed in all of his rooms. His friend advised Akshay to use water-coolers and save electricity. On this Akshay told, him that the water-cooler is not at all effective in coastal areas.

(a) Why are water-cooler not effective in coastal areas?

(b) What are the other two factors on which evaporation of water depends?

(c) What value of Akshay’s friend is seen in this act?

Answer: (a) Water coolers are not effective in coastal areas due to high rate of humidity.

(b) The other two factors on which evaporation of water depends are temperature and surface area.

(c) Akshay’s friend showed the value of concerned citizen, morally responsible and friendly in nature.

3. Sita lived in a village and could, not afford refrigerator in her house. She knew how to keep water cold and preserve all perishable items in her house. She kept wet cloth surrounding the earthen pot to keep water cool, she also kept vegetables fresh by keeping them in wet gunny bag and timely sprinkled water over it.

(a) Why did Sita keep wet cloth surrounding the earthen pot?

(b) Suggest one more method of keeping the house cool in summer.

(c) What value of Sita is reflected in the above case?

Answer: (a) The wet cloth gave the cooling effect to the pot, as the water in the cloth evaporated and evaporation causes cooling effect.

(b) By sprinkling some water on the lawn/veranda of the house can keep the house cool.

(c) Sita showed the value of responsible behaviour.

4. Shreya commutes in a CNG fitted van to school every day along with many other students. She told the van driver to get the CNG connection certified and timely checked it for any leakage or loose connection of pipes. She told the driver to be more careful during summers.

(a) What is CNG?

(b) Why should one be more careful with CNG cylinders during summer?

(c) What value of Shreya is seen in the above act?

Answer: (a) CNG is Compressed Natural Gas used as fuel.

(b) During summers, the CNG connections and cylinder need to be checked because the gas expands due to heat and if there would be any leakage then it would cause fire in the vehicle.

(c) Shreya showed the value of concerned citizen and morally responsible behaviour.