Class 9 Science Chapter 1 Question Answers - Matter in Our Surroundings

Q1: Discuss the factors which affect evaporation. 
Ans: There are four factors which affect evaporation. 

• Surface area : Escaping of particles from liquid state to vapour state depends on surface area. Therefore, the rate of evaporation increases with surface area. 
• Temperature : Rise in temperature, rise the kinetic energy of substance and therefore, chance of escaping of particles is great from liquid to vapour state.
  Wind : The rate of evaporation increases with speed of wind. 
• Humidity : Humidity is the amount of vapour present in the air. At fixed temperature air can’t hold more than fixed amount of water vapour. Therefore, the evaporation rate decreases if humidity increases in air.

Q2: Explain the inter-conversion of three states in terms of force of attraction and kinetic energy of the molecules.
Ans: The force working between the particles of a matter is called intermolecular force. Intermolecular forces are strong in solids and the particles are close to each other and thus make the substance rigid. In liquids, intermolecular force is less than solids and more than gases. So, they cannot have rigid shape and kinetic energy of the molecules is not enough to hold gas in open container.
Figure: States of Solid

Q3: How is the high compressibility property of gas useful to us?
Ans: The gases have high compressibility. This property is used in the following situation : 

• LPG (Liquefied Petroleum Gas) is a fuel which is made up of petroleum gas. On compressing this petroleum gas it forms liquid. 
• Oxygen cylinders in the hospitals have compressed gas filled in it. 
• CNG (Compressed Natural Gas) is a natural gas i.e. methane, which is compressed and used as a fuel in vehicles and at home.

Q4: Pressure and temperature determine the state of a substance. Explain this in detail.
Ans: Any matter, i.e. solid, liquid or gas when experiences an increase in temperature then they change their state.
Example:

When we take ice cubes in a beaker or heat them slowly, the temperature increases and ice melts to form liquid. We heat this liquid further it will become steam.
On lowering down the temperature of any matter, show change in their state.
Example:

We take the steam that is coming out of boiling water and allow it to cool down, it condenses to form water and on further cooling of this water we get ice.
On applying pressure and reducing temperature we can liquefy gases or change them into solid.
Example : We 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 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.

Q5: Give difference between Evaporation and Boiling.
Ans:

Q6: The melting point of ice is 273.16 K. What does this mean? Explain in detail.
Ans: 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 of ice start 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.

Q7: With the help of an example, explain how diffusion of gases in water is essential?
Ans:

• The gases from the atmosphere diffuse and dissolve in water. Diffused gases like oxygen and carbon dioxide 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.