Chapter 23: HEAT


When a solid is heated the kinetic energy of the molecules is increased and it overcomes the force of attraction of molecules and the solid melts. When a solid melts the temperature doesn’t increase, the heat energy is used up and is hidden inside the solid and is used for changing its state so it’s called latent heat of fusion.

Open system is where there is an exchange of matter and energy between system and surroundings. Closed system has only exchange of energy. In isolated systems both are absent.

Law of Thermodynamics

First law of thermodynamics is the law of conservation of energy

“Energy can neither be created nor destroyed”.

Specific heat capacity is the amount of heat absorbed or reject by unit mass of a substance to raiseits temperature by 1°C. Water has higher specific heat than land and so it heats and cools slower. Skating is possible on snow as water is formed belowthe skates due to low temperature. This acts as a lubricant.

                        F = 9C/5 + 32; F= temp in Fahrenheit, C= temp in Celsius

Effect of Heat on substances

Solids like camphor, ammonium chloride, naphthalene, anthracene turn from solid to gas on heating. This is called sublimation.

Even on application of pressure the state of matter changes. Pressure exerted by gases is measures using unit atmosphere. 1 atm is pressure at sea level.It decreases with height.

Evaporation occurs as a few particles of a liquid on its surface have higher kinetic energy and they break away from the rest and turn into vapor.

In solids, heat is transferred from higher temperature to lower temperature by process called conduction [Copper is coated on the bottom of the utensils as it’s a good conductor and helps in spreading out the heat evenly].

Water and air are poor conductors of heat. In water, when its heated hot water at the bottom rises up and cold water moves down. This continues till the whole water is heated. This process is called convection.

Radiation is the process by which sun’s heat reaches us. Radiation doesn’t need a medium. All objects emit radiation when heated; the frequency of radiation goes from lower to higher as the temperature increases.

States of Matters

Plasma – fourth state of matter. This consists of super energized and excited ionized gaseous matter. The neon bulb has neon gas and CFL has argon or helium, this gets energized when electricity is passed through them. This creates plasma that glows depending on the nature of the gas. In sun and stars plasma is created due to high temperatures and produces light.

Bose-Einstein condensate – fifth state of matter. BEC is formed when a gas at very low density is cooled to super low temperatures.

Density measured as kilogram per meter cube.

Natural gas contains methane [50-90%], ethane, propane, butane and hydrogen sulphide [waste]. LPG uses propane and butane. when natural gas is heated we get carbon black used in tyre industries.

Nuclear Energy

Nuclear reactors work on nuclear fission [fission was discovered by Fermi]. Atom bombs also work on the same principle. In reactors, neutrons need to be slowed to ensure interaction with the uranium isotope this is done using heavy water [D2O] or graphite.

Cadmium rods are used to control the reaction. Energy produced by the sun and the working of the hydrogen bomb is based on nuclear fusion.

India has a three stage nuclear program.

In first stage, the natural uranium is used as fuel, heavy water as moderator [pressurized heavy water reactors] and we get plutonium.

In second stage, plutonium and uranium oxide is used to give uranium -233 in fast breeder reactors where no moderator is needed.

In final stage, thorium and uranium-233 is used to get energy [thorium based reactors].

Heavy water reactors need frequent refueling and produce less energy compared to light water reactors that use distilled water as coolant and fissile uranium -235 as fuel.


Anomalous behavior of water

Water exhibits anomalous behavior below 4°C, its volume increases and density decreases. Due to this the ponds and lakes freeze at the top during winters as at 4°C water becomes less dense and remains on top and solidifies. This leaves animal and plant life intact at bottom.

Effect of Wool and Cotton on Heat

Dark clothes or surfaces absorb more heat and hence dark clothes are preferred in winter. Light clothes reflect more heat and so such clothes are preferred in summer.

Wool is a bad conductor of heat, woolen fibers trap air and this prevents body heat from escaping. So woolen clothes are preferred in winters. We should use two blankets instead of one in winters as the trapped layer of air between the blankets prevents heat from escaping.

Cotton clothes are sweat absorbent, they expose the sweat to the atmosphere and when it evaporates it absorbs latent heat from the body, this heat is removed and the body cools down.

Effect of Air parcel on heat

Construction of houses is done using hollow bricks that have air trapped in them. This prevents heating of houses during summer and keeps flats cool. During winter the heat from flats doesn’t escape out keeping them warm.


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