- The mixing method uses a calorimeter to measure the specific heat of a solid by observing the heat exchange until thermal equilibrium is reached.
- Heat lost by solid = Heat gained by water + Heat gained by calorimeter;
Q = Q1 + Q2 - The specific heat of the solid is calculated using known values of mass, temperature change, the specific heat of water, and the calorimeter.
Definitions [13]
Definition: Latent Heat
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.
OR
Latent heat is the quantity of heat energy required to change the state of unit mass of a substance from one phase to another, at constant temperature and constant pressure.
OR
The quantity of heat absorbed or given out by unit mass of a substance during change of state of the substance at a constant temperature is called the latent heat of the substance.
Definition: Latent Heat of Fusion
The quantity of heat required to convert unit mass of a substance from its solid state to the liquid state, at its melting point, without any change in its temperature, is called its latent heat of fusion (Lf).
OR
The heat energy absorbed at constant temperature during the transformation of solid into liquid is called the latent heat of fusion. 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.
Definition: Latent Heat of Vaporization
The quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point without any change in its temperature is called its latent heat of vapourization (Lv).
Definition: Regelation
The phenomenon in which the ice converts to liquid due to applied pressure and then re-converts to ice once the pressure is removed is called regelation.
Definition: Anomalous behaviour of water
The behaviour of water between its temperature from 0 °C to 4 °C is called anomalous behaviour of water.
Definition: Dew Point Temperature
Suppose unsaturated air at a certain temperature is taken and its temperature is decreased, a temperature is reached at which the air becomes saturated with vapour. This temperature is called the dew point temperature.
Definition: Humidity
The presence of water vapour in the air makes it moist. The moisture in the atmosphere is called humidity.
Definition: Absolute Humidity
The vapour content in the air is measured using a physical quantity called absolute humidity. The mass of vapour present in a unit volume of air is called absolute humidity. Generally absolute humidity is measured in kg/m3.
Definition: Relative Humidity
The ratio of actual mass of vapour content in the air for a given volume and temperature to that required to make the air saturated with vapour at that temperature is called the relative humidity.
% Relative humidity = \[\frac{\text{actual mass of water vapour content in the air in a given volume}}{\text{mass of vapour needed to make the air saturated in that volume}}\] × 100
Definition: Heat Capacity
The quantity of heat needed to raise the temperature of the whole body by 1°C (or 1 K) is called heat capacity.
Definition: Specific Heat Capacity
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.
OR
The amount of heat per unit mass absorbed or given out by a substance to change its temperature by one unit (one degree), i.e., 1°C or 1 K, is called specific heat capacity.
OR
The quantity of heat required to raise the temperature of a unit mass of a gas by one degree, whose exact value depends upon the mode of heating the gas and can range from zero to infinity or even be negative, is called the specific heat capacity of a gas.
Define heat capacity.
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.
Definition: Principle of Heat Exchange
Heat energy lost by the hot object = Heat energy gained by the cold object. This is called as ‘Principle of heat exchange’
Formulae [2]
Formula: Latent Heat
Q = m × L
where,
Q = Heat energy absorbed or released during phase change
m = Mass of the substance undergoing phase change
L = Specific Latent Heat (characteristic of the substance & process)
SI Units = J kg⁻¹
Formula: Specific Heat Capacity
Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]
or
Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]
Key Points
Key Points: Latent Heat
- Formula: Q = mL. Specific latent heat L has SI unit J kg⁻¹.
- Temperature stays constant during any phase change. Heat energy goes into breaking or forming intermolecular bonds, not into raising kinetic energy.
- Latent Heat of Fusion (water): Lf = 3.33 × 10⁵ J kg⁻¹ = 80 cal/g. Heat needed to melt 1 kg of ice at 0°C.
- Latent Heat of Vaporisation (water): Lv = 22.6 × 10⁵ J kg⁻¹ = 540 cal/g. Heat is needed to convert 1 kg of water to steam at 100°C.
- Lv ≫ Lf because vaporisation requires complete molecular separation and work against atmospheric pressure during expansion.
- All latent heat values depend on atmospheric pressure. Standard values quoted at 1 atm. Increasing pressure raises the boiling point (pressure cooker effect).
Key Points: Anomalous behaviour of water
- Anomalous behaviour of water refers to its contraction when heated from 0°C to 4°C, unlike most liquids that expand on heating.
- At 4°C, water attains its minimum volume and hence its maximum density.
- This behaviour is studied using Hope’s apparatus, in which temperature changes at the upper (T₂) and lower (T₁) parts of the water are recorded over time.
- As cooling begins, denser water at 4°C sinks, causing T₁ to fall rapidly, while T₂ falls more slowly.
- Once water cools below 4°C, its density decreases, causing it to rise, which in turn makes T₂ drop faster, while T₁ remains stable at 4°C briefly before decreasing.
Key Points: Unit of Heat
- SI unit of heat is Joule (J); CGS unit is calorie (cal).
- 1 cal = heat to raise 1 g of water by 1°C (14.5°C to 15.5°C).
- 1 kcal = heat to raise 1 kg of water by 1°C; 1 kcal = 1000 cal.
- The required heat may vary slightly outside the defined temperature range.
- 1 cal = 4.18 J; named after James Prescott Joule, who linked heat to energy.
Key Points: Specific Heat Capacity
- 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.
Key Points: Measurement of Specific Heat
Important Questions [7]
- The vapour content in the air is measured using a physical quantity called ____________.
- How do we feel about air in each of the following conditions? Relative humidity is more than 60%. Relative humidity is less than 60%.
- Observe the following diagram and answer the questions given below: Specific heat capacity of metals Which element has maximum specific heat capacity? Justify. Which element has minimum specific heat
- How much heat energy is necessary to raise the temperature of 5 kg of water from 20°C to 100°C?
- Name and state the principle used to measure the specific heat capacity of a substance.
- Read the following paragraph and answer the questions based on it. If heat is exchanged between a hot and cold object, the temperature of the cold object goes on increasing due to gain of energy
- Write the use of ‘Calorimeter’.
