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We are providing you with the best chemistry notes written by Sir Umair Khan. This is the first chapter of Class 10 Chemistry, States of Matter and Phase Changes. All notes for class 10 chemistry are here and All Video lectures are here. Watch and Subscribe, please.

All Important short questions.

Q1. What is the role/importance of kinetic particle theory? 

Kinetic particle theory not only explains all the laws that govern the behaviour of gases, it also explains the composition of liquid and solid states of matter and the interconversion of all three states.

Q2. What are the movement types in gas and liquid molecules? 

Molecules present in a gas and a liquid show all the three types of movements simultaneously namely, translational, rotational and vibrational.

Q3. Define internal energy.

The internal energy of a substance is the total energy it contains. It includes the kinetic energy of its particles and the potential energy due to bonding between them. Heat increases the internal energy of a system.

Q4. Define melting point. 

Heat increases the kinetic energy of the particles and they start vibrating at a higher frequency. At a particular temperature their vibrational motion becomes so fast that it overcomes the cohesive forces. As a result the solid starts melting and this temperature is called the melting point of the solid.

Q5. What is the effect of heat on solid?

 Heating a solid below its melting point increases its kinetic energy and temperature that weakens the force of attraction between its particles. Further heating the solid after it has started melting does not increase its temperature. Instead, all the heat energy provided at this moment is utilized to convert the solid into its liquid. 

Q6. Define evaporation. 

The molecules of a liquid keep on coming out from the surface of a liquid at all temperatures and such an escape of molecules from the surface is called Evaporation. 

Q7. Define boiling point. 

The temperature of a liquid at which its vapour pressure becomes equal to the atmospheric or external pressure is called its boiling point.

Q8. Why does the temperature of boiling liquid remain constant?

At the boiling point, the heat provided to the liquid is used to convert it  into gaseous form and during this its temperature remains constant. In other words, the heat provided is used only to break the forces of attraction between its particles. 

Q9. Define condensation. 

At a suitable lower temperature the increased attractions bring the  molecules so close that they are changed into a liquid form. This is called condensation. 

Q10. Define the heating-cooling curve. 

Interconversion of physical states can be understood with the help of a graph drawn between the internal energy and the temperature of a system. Such a graph is called a heating or cooling curve.

Q11. Define sublimation. 

The direct conversion of a solid to vapours without melting is called sublimation. For example, solid carbon dioxide, which is also called dry ice, changes directly to gaseous carbon dioxide at room temperature without first melting to a liquid state.

Q12. Where does the energy needed for sublimation come from?

Just like evaporation, the energy needed for sublimation also comes from within the substance, which then absorbs energy from the surroundings. This energy is sufficient to overcome the attractive forces of the neighbouring molecules, which then escape into the vapour phase. 

Q13. State Boyle’s law. 

The law states that the volume of a given mass of a gas is inversely proportional to its pressure at constant temperature.

Mathematically, Boyle’s law is written as:

V ∝ 1/p (V is the volume of a given mass of gas and P is its absolute pressure)

Q14. State Charles’ law?

The law states that the volume of a given mass of a gas varies directly with temperature when pressure is kept constant. The temperature must, however, be measured on the absolute or Kelvin scale.

 Mathematically, V ∝ T (P and mass of gas are kept constant)

Q15. State Avogadro’s law. 

This law states that equal volumes of different gases must contain an  equal number of molecules if the temperature and pressure are kept constant.

Mathematically,

V ∝ n (at constant temperature and pressure)

Q16. Define diffusion. 

Molecules present in gases are in a constant state of random motion. Due to this molecular motion, the gas particles spread out and intermix from an area of high concentration to an area of low concentration. This property of gases is called diffusion.

Q17. What is the role of the rate of diffusion in drug movement?

Rates of diffusion play a crucial role in the delivery and movement of drugs throughout the body. This process of diffusion ensures that essential substances reach their target locations quickly, which eventually helps in the treatment of diseases.

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All quick checks (Class 10 Chemistry chapter 14)

14.1 Quick Check

1. What is translational motion?
Translational motion is the motion in which particles move from one place to another in straight paths, but may be in random directions.

2. Is the average kinetic energy possessed by the particles of a gas and a liquid the same at the same temperature?
Yes, the average kinetic energy of particles of a gas and a liquid is the same at the same temperature. Kinetic energy is proportional to the absolute temperature (K.E ∝ T), not to the physical state. 

14.2 Quick Check

1. Why does water vapour at 100°C cause more severe burns as compared to liquid water at the same temperature?
Water vapours at 100°C contain extra latent heat of vaporization. When they condense on the skin, they release this heat, causing more severe burns.

2. Why do we feel comfortable wearing cotton clothes in summer?
Cotton clothes absorb sweat and allow it to evaporate easily. Evaporation produces a cooling effect, making us feel comfortable.

14.3 Quick Check

1. Does the change in temperature occur during the process of evaporation?
Yes, a cooling effect is produced during evaporation. During this process, the high kinetic energy particles escape from the surface of the liquid, leaving behind particles with lower kinetic energy. As a result, the average kinetic energy decreases and the liquid becomes cool. Heat is then absorbed from the surroundings.  

2. Does ice sublime?
Yes, ice can sublime slowly under suitable conditions by changing directly from solid into vapour.

3. Give one example each for sublimation and deposition other than those mentioned in the textbook.

• Sublimation: Ammonium chloride changes directly from solid to vapour on heating. 
• Deposition: Formation of snow from water vapour in clouds (vapour directly changes into solid ice crystals). 

14.4 Quick Check

(i) How does a gas exert pressure?

According to kinetic particle theory, gas particles move randomly in all directions. When these particles collide with the walls of the container, they exert force on them, producing pressure.

(ii) How does the volume of a gas change when its temperature is decreased? (Pressure and mass constant)

When the temperature decreases, the kinetic energy of gas particles decreases. The particles move more slowly and come closer together, so the volume of the gas decreases.

14.5 Quick Check

1. Which gas among the following will diffuse at the fastest speed, and which will diffuse at the slowest speed under similar conditions of temperature and pressure?
Gases given: NO₂, SO₂, H₂S

• Fastest diffusion: H₂S
• Slowest diffusion: SO₂

This is because lighter gases diffuse faster than heavier gases.

2. Explain with the help of the kinetic particle theory why ammonia gas diffuses faster than hydrogen chloride gas?
According to kinetic particle theory, lighter gas particles move faster at the same temperature. Ammonia (NH₃) has a lower molecular mass (17g/mol) than hydrogen chloride (HCl), which is 36.5g/mol, so ammonia diffuses faster.

EXERCISE Class 10 Chemistry Chapter 14

A. Multiple Choice Questions

Choose and tick the correct answer from the given choices.

1. According to kinetic theory, the basic difference between solid, liquid, and

gas, is due to:

(a) The difference in the movements of the particles. 

(b) the chemical properties of the particles.

(c) the size of the particles.

(d) the shape of the particles.

2. Upon heating, the rate of evaporation:

(a) decreases

(b) increases

(c) remains the same

(d) initially decreases and then increases upon further heating

3. The inter-particle attractions are the strongest in:

(a) Solids 

(b) Liquids

 (c) Plasma

 (d) Gas

4. Cooling the vapours of some gases change them directly into solid state.

This phenomenon is called:

(a) evaporation  

(b) condensation

(c) sublimation

(d) deposition

5. Physical state in which the particles possess the maximum energy:

(a) Solid 

(b) Liquid

(c) Gaseous 

(d) Vapour

6. How does the process of evaporation depend on the force of attraction

present among the molecules of a liquid?

(a) It decreases with the increasing strength of attraction.

(b) It increases with the increasing strength of attraction.

(c) It is independent of the strength of the force of attraction.

(d) It first increases and then decreases with the increasing strength of the force of attraction.

7. Which gas will diffuse at the fastest rate?

(a) HCl  

(b) SO₂

(c) H₂S

(d) CO₂

8. The phase changes A and B in 

gas→A→ liquid→B→ solid are:

(a) Melting, evaporating 

(b) Condensation, melting

(c) Condensation, freezing 

(d) Boiling, freezing

9. A student noticed frost falling on the ground at 6.30 am and disappearing

by 8.30 am. Which phase change was responsible for changing the frost into water vapour?

(a) Melting  

(b) Evaporation

(c) Sublimation 

(d) Deposition

B. Short Answer Questions

14.1 From where does the energy come when a liquid evaporates?
The energy required for evaporation comes from the inner high kinetic energy molecules and their surroundings. The liquid particles absorb heat energy and escape into the air as vapours.

14.2 Is condensation an endothermic process?
No, condensation is an exothermic process. During condensation, gas particles lose heat energy and change into liquid.

14.3 Why do naphthalene balls (used to repel insects) disappear after some time?
Naphthalene balls disappear because they undergo sublimation. They directly change from solid state into gas without becoming liquid.

14.4 Why does the temperature remain constant during a phase change?
During a phase change, the heat energy is used to change the state of matter instead of increasing temperature. Therefore, the temperature remains constant until the change is complete.

14.5 Is it possible to compress a liquid like a gas?
No, liquids cannot be compressed easily like gases because the particles in liquids are already closely packed together.

14.6 Can you change the temperature at which water boils in an experiment in your lab?
Yes, the boiling point of water can be changed by changing pressure. Lower pressure decreases the boiling point, while higher pressure increases it.

14.7 In which season do wet clothes get dry after a relatively longer time?
Wet clothes dry more slowly in the winter or rainy season because the temperature is lower and the air contains more moisture.

14.8 When a solid is heated, what happens to its particles which are vibrating?
When a solid is heated, its particles gain kinetic energy and vibrate more rapidly about their fixed positions.

14.9 Do solids and liquids also diffuse just like gases?
Yes, solids and liquids also diffuse, but much more slowly than gases because their particles move less freely.

14.10 Why is dew formed in the early hours of the morning?
Dew is formed because water vapours in the air cool down and condense into tiny water droplets on cool surfaces during early morning.

C. Constructed Response Questions

14.1 Differentiate between evaporation and boiling.

14.2 Describe how boiling is related to the external pressure?

Boiling occurs when the vapour pressure of a liquid becomes equal to the external pressure. If the external pressure decreases, the liquid boils at a lower temperature. If the external pressure increases, the boiling point increases.

14.3 Why does the process of evaporation not need the input of energy?

Evaporation does not need external heating because liquid particles absorb energy from the surroundings. The high-energy particles escape from the surface into the air.

14.4 Why do the heating and cooling curves for phase changes adopt the same path?

The heating and cooling curves follow the same path because the amount of heat absorbed during melting is equal to the heat released during freezing. Similarly, the heat absorbed during boiling is equal to the heat released during condensation.

14.5 Why does ice melt when pressure is exerted on it?

When pressure is applied on ice, its melting point decreases. As a result, some ice melts into water even at temperatures near 0°C.

14.6 Why do phase changes occur?

Phase changes occur because matter gains or loses heat energy. This changes the motion(K.E) and arrangement of molecules, causing matter to change from one state to another.

D. Descriptive Questions

14.1 Explain the difference in the rates of diffusion of two gases based on kinetic theory.

14.2 How does the process of sublimation helpful in printing and in the usage of air freshener? 

14.3 Draw heating curve for the physical changes water undergoes with change in temperature.