Change of State
Latent Heat
Sometimes when heat energy is given to a substance, it does not increase its temperature. Instead, it changes the state of the substance. This type of heat energy is known as latent heat.
There are two types of latent heat:
- Latent heat of fusion
- Latent heat of vaporization
Latent Heat of Fusion
The latent heat of fusion is the quantity of energy required to change a solid into a liquid without any change in temperature.
Latent Heat of Vaporization
The latent heat of vaporization is the quantity of energy required to change a liquid into a gas (vapor) or steam at a constant temperature.
Specific Latent Heat
The specific latent heat of a substance is the quantity of heat required to change a unit mass of the substance from one state to another without a change in temperature.
(a) Specific Latent Heat of Fusion (\( L_f \))
The specific latent heat of fusion is the quantity of heat required to change a unit mass of a solid into a liquid without a change in temperature.
For ice, the specific latent heat of fusion is:
\[ L_f = 33,600 \, \text{J/kg} \]The heat energy required is given by:
\[ Q = M L_f \](b) Specific Latent Heat of Vaporization (\( L_v \))
The specific latent heat of vaporization is the quantity of heat required to convert a unit mass of liquid into vapor (gaseous state) without any change in temperature.
For water, the specific latent heat of vaporization is:
\[ L_v = 2.26 \times 10^6 \, \text{J/kg} \]The heat energy required is given by:
\[ Q = M L_v \]Evaporation
Evaporation is the process where a liquid changes into a gas. It occurs at all temperatures and takes place from the surface of the liquid. Volatile liquids like Freon, ether, and alcohol evaporate easily. Evaporation causes cooling as latent heat is absorbed from the surroundings to convert the liquid into gas. For example, the human body cools through sweat evaporation after exercise.
Factors Affecting the Rate of Evaporation
- Temperature: Higher temperatures increase the rate of evaporation.
- Pressure: Higher pressure decreases the rate of evaporation.
- Surface Area: A larger exposed surface area results in faster evaporation.
- Nature of the Liquid: Liquids with lower boiling points evaporate faster.
- Wind and Humidity: Dry air and wind remove vapor, increasing the evaporation rate.
Applications of Evaporation and Latent Heat
- Refrigerators
- Air Conditioners
Application in Refrigerators
Refrigerators use the cooling effect of evaporation. A volatile liquid, such as ammonia or Freon, evaporates inside copper coils surrounding the freezer compartment. An electric pump reduces pressure, causing the liquid to evaporate, absorbing heat from the surroundings and cooling the refrigerator’s interior.
The vapor is then compressed in a condenser, where it releases heat and condenses back into a liquid. This heat is dissipated through cooling fins at the back of the refrigerator by convection, radiation, and conduction. A thermostat regulates the cooling process.
Boiling Point
Boiling occurs at a specific temperature and pressure, where the temperature remains constant during the process. It happens throughout the liquid, and external factors like wind do not affect it.
Definition of Boiling Point
The boiling point of a liquid is the temperature at which its saturated vapor pressure equals atmospheric pressure.
A saturated vapor is a vapor in equilibrium with its liquid in a confined space, and the pressure it exerts is called saturated vapor pressure.
Effects of Pressure and Dissolved Substances on Boiling and Freezing Points
Effect of Pressure on Boiling Point
- Decreasing Pressure: Lowers the boiling point. For example, cooling a flask of boiling water and then pouring cold water over it causes condensation, reducing pressure and making the liquid boil again.
- Increasing Pressure: Raises the boiling point. This principle is applied in pressure cookers, where increased pressure raises the boiling point of water, allowing food to cook faster and saving fuel.
Effect of Pressure on Freezing Point
- Increased Pressure: Lowers the melting point of substances that expand when freezing (e.g., water/ice).
- For substances that contract when freezing (e.g., paraffin), increased pressure raises the melting point.
Effect of Dissolved Impurities
- On Boiling Point: Dissolved substances raise the boiling point.
- On Freezing Point: Dissolved substances lower the freezing point.
Humidity, Mist, Cloud, and Dew Point
Humidity
Humidity refers to the amount of water vapor in the air. When the air contains a high amount of moisture, it is described as humid.
Relative Humidity
Relative humidity indicates how moist the air is. It is defined as the ratio of the actual water vapor present in a given volume of air to the maximum amount it can hold at the same temperature, expressed as a percentage. Extremely high or low humidity levels can cause discomfort.
It can be expressed mathematically as:
\[ \text{Relative Humidity} = \frac{\text{Mass of water vapor in air} (m)}{\text{Mass of water vapor required to saturate air} (M)} \times 100\% \]Alternatively, it can be written in terms of vapor pressure:
\[ \text{Relative Humidity} = \frac{\text{Vapor Pressure (V.P)}}{\text{Saturation Vapor Pressure (S.V.P) at air temperature}} \times 100\% \]At the dew point:
\[ \text{Relative Humidity} = \frac{\text{Saturation Vapor Pressure of water at dew point}}{\text{Saturation Vapor Pressure (S.V.P) of water at original air temperature}} \times 100\% \]Thus, the general formula is:
\[ \text{Relative Humidity} = \frac{m}{M} \times 100\% = \frac{\text{V.P}}{\text{S.V.P}} \times 100\% \]Measurement of Relative Humidity
A hygrometer is used to measure relative humidity. It consists of:
- A dry bulb thermometer that measures air temperature.
- A wet bulb thermometer, covered with a moist cloth, which records a lower temperature due to evaporative cooling.
The greater the difference between the dry and wet bulb temperatures, the lower the humidity. If humidity is high, less water evaporates from the wet bulb, resulting in a smaller temperature difference.
Mist
Mist consists of condensed water droplets suspended in the air near the Earth's surface, formed when air cools below the dew point. In extreme conditions, mist can develop into fog, reducing visibility.
Clouds
Clouds are masses of tiny water droplets floating in the atmosphere. They form when moist air rises, cools, and condenses into visible water droplets.
Dew and Dew Point
Dew is a natural form of water that forms when water vapor condenses. The dew point is the temperature at which the air becomes saturated with water vapor, causing condensation. At this temperature, the actual vapor pressure equals the saturated vapor pressure.
The dew point is measured using a dew point hygrometer.
How Dew Forms
Air contains water vapor, and the amount of vapor varies with temperature. During the day, when temperatures are high, water evaporates into the air. At night, as temperatures drop, the air can no longer hold as much moisture. If the temperature falls below the dew point, condensation occurs, forming dew.
Dew is common in deserts, where some animals rely on it as their main water source.