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Chapter 5 - Heat

Chapter 5 : Heat

Textbook Exercise Solutions

Q1) Complete the following passage by choosing the correct words given in brackets:
Answer :

During a change of state of matter, the temperature of the substance remains constant. The heat energy absorbed during phase transition is called latent heat. The constant temperature at which a solid transforms into a liquid state is known as its melting point. The heat energy absorbed at constant temperature during transformation of liquid into gas is called latent heat of vaporization. When ice is heated, its temperature rises until 0°C. Further heating melts the ice into water at 0°C without any rise in temperature. The heat absorbed is used to overcome the attractive forces between molecules.

Q2) Read the paragraph and answer the following questions:
1. Which apparatus is used to study the anomalous behaviour of water?
Answer : Hope’s apparatus is used to study the anomalous expansion of water in the laboratory.
2. In Hope’s apparatus, where is the freezing mixture placed?
Answer : The freezing mixture (ice and common salt) is placed in the cylindrical trough attached to the middle part of Hope’s cylinder.
3. What is the reading of thermometer T₁ and T₂ when water attains maximum density?
Answer : The lower thermometer T₁ reads 4°C (where water achieves maximum density and settles at the bottom), while the upper thermometer T₂ gradually cools down to 0°C and freezes into ice at the top.
Q3) Explain the concept of Latent Heat with the help of a Temperature-Time graph:
Answer :

When ice at 0°C is heated, heat energy is continuously supplied, but the temperature of the ice-water mixture remains steady at 0°C until the entire quantity of ice has melted. This heat absorbed without any temperature change is called Latent Heat of Fusion.

Upon further heating, the temperature rises steadily from 0°C to 100°C. At 100°C (the boiling point), the temperature again remains constant while water converts into steam. This absorbed energy is called Latent Heat of Vaporization.

Time (Minutes) ➔ Temperature (°C) ➔ 0°C 100°C Melting (Ice + Water) Water (0°C to 100°C) Boiling (Water + Steam)
Fig 5.1: Temperature vs Time Phase Transition Curve
Q4) Write scientific reasons:
1. A metal wire loaded with heavy weights passes right through an ice slab without splitting it into two pieces.
Answer :
  • Under the weighted thin wire, intense pressure is exerted on the ice immediately below it.
  • High pressure lowers the melting point of ice below 0°C, causing the ice beneath the wire to melt into water at sub-zero temperatures.
  • As the ice melts, the wire sinks slightly deeper. The water above the wire is immediately relieved of the high pressure.
  • Relieved of pressure, the melting point reverts to 0°C, and the water refreezes into solid ice above the wire.
  • This phenomenon of melting under pressure and refreezing when pressure is released is called regelation. The wire passes completely through while the slab remains one solid intact block.
2. Water droplets appear on the outer surface of a chilled bottle taken out of a refrigerator.
Answer :
  • Ambient air contains invisible water vapor.
  • When the cold bottle is kept in room air, air in direct contact with the bottle surface is chilled below its dew point temperature.
  • At this reduced temperature, the air becomes saturated with moisture and cannot hold all its water vapor.
  • As a result, extra water vapor condenses into liquid water droplets on the outer surface of the bottle.
3. In cold regions in winter, aquatic life survives even when the surface of lakes freezes.
Answer :
  • This survival is made possible by the anomalous expansion of water.
  • As lake water cools below 4°C, it expands and its density decreases rather than increasing.
  • The colder water (0°C to 3°C) remains at the surface and freezes into a floating sheet of ice at 0°C.
  • Ice is a poor conductor of heat, which insulates the lower water layer from the freezing sub-zero atmospheric temperatures.
  • The denser water at 4°C remains stable at the bottom, providing a safe, liquid habitat for fish and aquatic plants throughout winter.
4. Burns caused by steam at 100°C are more severe than burns caused by boiling water at 100°C.
Answer :
  • Both steam and boiling water share the same temperature of 100°C.
  • However, 1 gram of steam contains an additional 540 calories (2260 J) of heat in the form of latent heat of vaporization.
  • When steam comes into contact with skin, it first condenses into water at 100°C, releasing this extra latent heat directly onto the tissues before cooling further.
  • Hence, steam imparts far more thermal energy to the skin, causing much deeper and more severe burns.
Q5) What is meant by the anomalous behaviour of water? Explain with the help of a volume-temperature graph.
Answer :

The abnormal expansion of water when cooled between 4°C and 0°C, or its contraction when heated from 0°C to 4°C, is called the anomalous behaviour of water.

  • When water at 0°C is heated, instead of expanding like ordinary liquids, its volume steadily decreases until it reaches 4°C.
  • At 4°C, water has minimum volume and therefore maximum density (1 g/cm³ or 1000 kg/m³).
  • Heating beyond 4°C causes water to expand normally with increasing volume.
Temperature (°C) ➔ Volume of 1 kg water ➔ 4°C (Minimum Volume) Max Density
Fig 5.2: Volume vs Temperature Curve Showing Anomalous Behaviour of Water
Q6) Explain the experiment to demonstrate the specific heat capacity of different metals using a wax tray:
Answer :
  1. Take three solid spheres of Iron (Fe), Copper (Cu), and Lead (Pb) having identical masses.
  2. Place all three spheres in a boiling water bath (100°C) for sufficient time so that all attain the same initial temperature of 100°C.
  3. Take out all three spheres at the same time and place them gently on the surface of a thick wax slab.
  4. Observation: The iron ball sinks deepest into the wax block, the copper ball sinks to an intermediate depth, and the lead ball sinks the least.
  5. Inference: Although all three spheres received the same temperature, iron absorbed the maximum heat and imparted it to the wax, while lead absorbed the least. This demonstrates that different substances have different specific heat capacities (c): ciron > ccopper > clead.
Q7) Explain the principle of heat exchange:
Answer :

When a hot object and a cold object are placed in thermal contact within an insulated (calorimetric) container where no heat escapes to the environment:

Heat lost by the hot object = Heat gained by the cold object

This is based on the Law of Conservation of Energy. Heat continues to transfer until both objects achieve a uniform thermal equilibrium temperature.

Q8) Solve the following numerical problems:
1. How much heat energy is necessary to raise the temperature of 5 kg of water from 20°C to 100°C?
Answer :
Given: Mass m = 5 kg, Initial temperature T₁ = 20°C, Final temperature T₂ = 100°C
Temperature difference ΔT = T₂ - T₁ = 100 - 20 = 80°C
Specific heat capacity of water c = 1 kcal / (kg·°C) = 4184 J / (kg·°C)
Formula: Q = m × c × ΔT
Q = 5 × 1 × 80 = 400 kcal
In Joules: 400 × 4184 J = 1.674 × 10⁶ J
Heat required = 400 kcal (or 1.67 × 10⁶ Joules).
2. A copper sphere of mass 100 g is heated to 100°C and then dropped into a copper calorimeter of mass 100 g containing 170 g of water at 20°C. Find the maximum temperature of the mixture. (Specific heat of copper = 0.1 cal/g·°C).
Answer :
Given:
Sphere: m₁ = 100 g, c₁ = 0.1 cal/g·°C, T₁ = 100°C
Water: m₂ = 170 g, c₂ = 1.0 cal/g·°C, T₂ = 20°C
Calorimeter: m₃ = 100 g, c₃ = 0.1 cal/g·°C, T₃ = 20°C
Let final equilibrium temperature = T
Heat lost by sphere = Heat gained by (water + calorimeter)
m₁ × c₁ × (100 - T) = m₂ × c₂ × (T - 20) + m₃ × c₃ × (T - 20)
100 × 0.1 × (100 - T) = 170 × 1 × (T - 20) + 100 × 0.1 × (T - 20)
10 × (100 - T) = (170 + 10) × (T - 20)
1000 - 10T = 180T - 3600
190T = 4600  ⇒  T = 4600 / 190 ≈ 24.21°C
The maximum equilibrium temperature of the mixture is 24.21°C.
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Chapter Overview: Heat

Class 10 Maharashtra State Board Science 1 Chapter 5 covers thermal physics principles including heat vs temperature, latent heat (fusion and vaporization), regelation, anomalous expansion of water between 0°C and 4°C, humidity, dew point, relative humidity, specific heat capacity (Q = mcΔT), and the principle of heat exchange measured via calorimetry.

Frequently Asked Questions (FAQs)

Q1. What is latent heat?

Latent heat is the heat energy absorbed or released by a substance during a change of physical state without any change in temperature.

Q2. Why is water considered anomalous between 0°C and 4°C?

Unlike ordinary liquids that expand on heating, water contracts between 0°C and 4°C, attaining maximum density at 4°C before expanding normally above 4°C.

Q3. What is the relation between relative humidity and dew point?

When relative humidity reaches 100%, air is completely saturated with moisture at that temperature, and this temperature is called the dew point.

Q4. What is the SI unit of specific heat capacity?

The SI unit is Joule per kilogram kelvin (J / kg·K) or J / (kg·°C); in CGS, it is cal / (g·°C).

Related Study Topics

Class 10 Heat Solutions Latent Heat of Fusion Anomalous Expansion of Water Hope's Apparatus Regelation Experiment Specific Heat Capacity Calorimetry Numericals SSC Board Science 1
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