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

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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 inter-conversion of the three states of matter—solid, liquid, and gas—depends on the forces of attraction and the kinetic energy of the molecules:

• Solids: Molecules are tightly packed, resulting in strong intermolecular forces. This makes solids rigid and stable.
• Liquids: Molecules are less tightly packed than in solids. The intermolecular forces are weaker, allowing molecules to slide past each other, giving liquids a definite volume but no fixed shape.
• Gases: Molecules are far apart with very weak intermolecular forces. They move freely and randomly, resulting in no definite shape or volume.

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: The melting point of ice is 273.15 K (or 0°C). This means:

• At this temperature, ice is in a solid state.
• The molecules in ice are tightly packed and held together by strong forces of attraction.
• As heat is added, the temperature remains constant until all the ice has melted.
• This heat energy is used to overcome the forces holding the molecules together, allowing them to move more freely.
• Once the ice melts, it transforms into liquid water at the same temperature, 273 K.

This process of melting is also known as fusion. During melting, the energy supplied does not increase the temperature but is absorbed to change the state from solid to liquid.

Q7: With the help of an example, explain how the 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.