Revision: Heat >> Calorimetry Physics (English Medium) ICSE Class 10 CISCE

The total internal energy of all the molecules of a substance is called its thermal energy.

The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.

The measurement of the quantity of heat is called calorimetry.

The sum of the potential energy and kinetic energy of a molecule is called its internal energy.

Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.

One kilo-calorie of heat is the heat energy required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C.

The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C.

Temperature is a parameter which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

The heat capacity of a body is the amount of heat energy required to raise its temperature by 1°C (or 1 K).

The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1°C (or 1 K).

OR

Heat capacity of a body when expressed for the unit mass is called the specific heat capacity of the substance of that body.

OR

The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat of that object.

The heat capacity of a body is the quantity of heat required to raise its temperature by 1°C. It depends upon the mass and the nature of the body.

A calorimeter is a cylindrical vessel which is used to measure the amount of heat gained (or lost) by a body when it is mixed with another body or substance.

The change from solid to liquid phase is known as melting, while the reverse change from liquid to solid is called freezing. 

The direct change from solid to vapour is called sublimation.

The change from vapour to solid is called solidification.

Sublimation is the process in which a solid changes directly into a gas on heating, without passing through the liquid state.

The triple point of water is that point where water in a solid, liquid and gas state co-exists in equilibrium and this occurs only at a unique temperature and a pressure.

The process of change from one state to another at a constant temperature is called the change of phase.

The constant temperature at which a solid changes to liquid is called the melting point of the solid.

The reverse change from liquid to solid phase with the liberation of heat at a constant temperature is called freezing and the temperature at which a liquid freezes to solid is called its freezing point. 

The change from solid to liquid phase by the absorption of heat at a constant temperature is called melting.

The change from liquid to gas (or vapour) phase on absorption of heat at a constant temperature, is called vaporisation.

The particular temperature at which vaporisation occurs is called the boiling point of liquid.

The change from vapour to liquid phase on liberation of heat at a constant temperature is called condensation (or liquefaction) and the particular temperature at which the condensation occurs is called the condensation point of vapour.

The heat energy absorbed (or liberated) in change of phase is not externally manifested by any rise or fall in temperature, it is called the latent heat.

Specific latent heat of a phase is the quantity of heat energy absorbed (or liberated) by the unit mass of the substance for the change in its phase at a constant temperature.

S.I. unit of specific latent heat is J kg⁻¹, and 1 cal g⁻¹ = 4.2 × 10³ J kg⁻¹.

The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid.

The constant temperature, at which the ice converts into water is called the melting point of ice.

The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vapourization.

The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion.

The heat energy absorbed at constant temperature during transformation of solid into liquid is called the latent heat of fusion.

Heat capacity C' = \[\frac{\text{amount of heat energy supplied (Q)}}{\text{rise in temperature (Δt)}}\]

or

Heat capacity of the body C' = \[\frac {Q}{Δt}\]

Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]

or

Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]

Heat energy lost by the hot body = Heat energy gained by the cold body

or

m₁ c₁ (t₁ - t) = m₂ c₂ (t - t₂)

L = \[\frac{\text{heat absorbed (or liberated) for the change of phase}}{\text{mass}}\]

or

L = \[\frac {Q}{m}\]

• Heat is the energy that flows from a hot body to a cold body when they are kept in contact.
• The S.I. unit of heat is joule (J), while calorie and kilocalorie are commonly used units.
• One calorie is approximately equal to 4.2 joule, and kilocalorie is used to measure the energy value of foods.

• If no heat flows between two bodies in contact, they are said to be at the same temperature, even though their thermal energies may be different.
• The S.I. unit of temperature is kelvin (K), and absolute zero (0 K) is the temperature at which molecular motion ceases.

• Heat capacity (C′) of a body is directly proportional to the heat energy (Q) supplied and inversely proportional to the rise in temperature (Δt), i.e., C'​ = \[\frac {Q}{Δt}\].
• S.I. unit of heat capacity is joule per kelvin (J K⁻¹), and 1 cal K⁻¹ = 4.2 J K⁻¹.

• Heat energy absorbed (Q) depends on: mass (m), rise in temperature (Δt), and specific heat capacity (c), i.e., Q ∝ m × Δt × c.
• Heat capacity (C') and specific heat capacity (c) are related by: C′ = m × c.

• The climate near the shoreline is moderate due to the high specific heat capacity of water, which causes slower heating and cooling than on land.
• Hot water bottles are used for fomentation because water does not cool quickly, thus providing more heat energy over time.
• Water is used as an effective coolant in machines because it removes more heat without a significant temperature rise.
• Farmers fill their fields with water to protect crops from frost, since water’s high specific heat capacity prevents the temperature from falling to 0°C.

• Most substances expand on melting, but some, like ice and bismuth, contract on melting.
• An increase in pressure decreases the melting point of substances that contract on melting (like ice), while it increases the melting point of substances that expand on melting.
• The presence of impurities lowers a substance's melting point, as seen when salt lowers the melting point of ice to form a freezing mixture.

• The boiling point of a liquid increases with pressure and decreases with a fall in pressure; this is why water boils at 120–125°C in a pressure cooker.
• At high altitudes, water boils at 100°C due to lower atmospheric pressure, making cooking difficult and slower in open vessels.
• The addition of impurities increases the boiling point of a liquid; e.g., adding salt to water raises its boiling point, aiding faster cooking.

• Snow on mountains does not melt all at once due to the high specific latent heat of fusion of ice (= 336000 J kg⁻¹).
• In cold countries, water in lakes and ponds does not freeze all at once because it must lose a large quantity of heat due to the high latent heat of fusion.
• Drinks cool more quickly when ice at 0°C is added than when ice-cold water is added, because ice absorbs 336 J g⁻¹ of heat energy to melt.
• When ice in a frozen lake starts melting, the surroundings become very cold because the melting ice absorbs heat from the atmosphere.
• It is colder after a hailstorm because melting ice absorbs heat from the surroundings, lowering the temperature further.