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3. Phase change : 

Heat required for the change of phase or state,

Q = mL, L = latent heat.

(a) Latent heat (L) : The heat supplied to a substance which changes its state at constant temperature is called latent heat of the body.

(b) Latent heat of Fusion (Lf) : The heat supplied to a substance which changes it from solid to liquid state at its melting point and 1 atm. pressure is called latent heat of fusion.

(c) Latent heat of vaporisation (Lv) : The heat supplied to a substance which changes it from liquid to vapour state at its boiling point and 1 atm. pressure is called latent heat of vaporization.

If in question latent heat of water are not mentioned and to solve the problem it require to assume that we should consider following values.

Latent heat of ice : L = 80 cal/gm = 80 Kcal/kg = 4200 × 80 J/kg 

Latent heat of steam : L = 540 cal/gm = 540 Kcal/kg = 4200 × 540 J/kg

The given figure, represents the change of state by different lines

Change of Phase | Physics Class 11 - NEET

Note : If we increases the temperature of liquid (phase) Change of Phase | Physics Class 11 - NEET ­ but at a later time K .E. stop increasing and the phase of the liquid starts changing.

Ex.5 Find amount of heat released if 100 g ice at - 10°C is converted into 120°C, 100 g steam.

Change of Phase | Physics Class 11 - NEET

Qnet = 73.5 Kcal.

Ex.6 500 gm of water at 80°C is mixed with 100 gm steam at 120°C. Find out the final mixture. 

Sol. 

120°C steam Change of Phase | Physics Class 11 - NEET 100°C steam

Req. heat = 100 × Change of Phase | Physics Class 11 - NEET ×20 = 1 kcal

80°C water Change of Phase | Physics Class 11 - NEET 100°C water

Req. heat = 500 × 1 × 20 = 10 kcal

100gm steam Change of Phase | Physics Class 11 - NEET 100 gm water at 100°C

Req. heat = 100 × 540 = 54 kcal

Total heat = 55 kcal.

Remaining heat = 55 - 10 = 45 kcal

Now we have 600 gm water at 100°C ⇒ 4500 = m × 540 ⇒ Change of Phase | Physics Class 11 - NEET

So at last we have Change of Phase | Physics Class 11 - NEET gm steam and Change of Phase | Physics Class 11 - NEET of water

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FAQs on Change of Phase - Physics Class 11 - NEET

1. What is a change of phase?
Ans. A change of phase refers to the transition of matter from one state to another, such as from a solid to a liquid or from a liquid to a gas. It occurs when energy is added or removed from the substance, causing its particles to rearrange and change their arrangement.
2. What are the different phases of matter?
Ans. The three main phases of matter are solid, liquid, and gas. In the solid phase, particles are closely packed together and have a fixed shape and volume. In the liquid phase, particles are still close together but can move more freely, taking on the shape of their container. In the gas phase, particles are far apart and move rapidly, filling the entire space available.
3. How does a change of phase occur?
Ans. A change of phase occurs when a substance gains or loses energy in the form of heat. When energy is added, such as by heating, the particles of the substance gain kinetic energy and move more rapidly, causing the substance to change from a solid to a liquid or from a liquid to a gas. Conversely, when energy is removed, the particles lose kinetic energy, leading to a change from a gas to a liquid or from a liquid to a solid.
4. What is the melting point and boiling point of a substance?
Ans. The melting point of a substance is the temperature at which it changes from a solid to a liquid. At this temperature, the particles have gained enough energy to overcome the forces holding them in a fixed position. The boiling point, on the other hand, is the temperature at which a substance changes from a liquid to a gas. At this temperature, the particles have gained enough energy to break free from the liquid and enter the gaseous state.
5. How does a change of phase affect the properties of a substance?
Ans. A change of phase can significantly affect the properties of a substance. For example, during a change from a solid to a liquid, the substance's density typically decreases as the particles become less tightly packed. Additionally, the melting and boiling points can provide valuable information about the substance's identity and purity. Understanding the changes in properties during a change of phase is crucial in various fields, such as materials science, chemistry, and engineering.
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