Solid Liquid Gas Part -02
- Latent heat does not raise the temperature.
- But the latent heat has always to be supplied to change the state of a substance.
- It is called latent heat because it becomes hidden in the substance undergoing the change of state, and does not show its presence by raising the temperature. to change the state of a substance.
- The latent heat which we supply is used up in overcoming the forces of attraction between the particles of a substance during the change of state.
Latent Heat of fusion:
- The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
- So, particles in water at 273 K have more energy as compared to particles in ice at the same temperature.
- Latent heat of fusion of ice is 3.34 x 105 J/Kg.
- Atmospheric Pressure – Pressure exerted by the weight of the atmosphere.
- Just like in solids, the kinetic energy of particles of liquid increases, the force of attraction among them decreases and they start moving free and liquid starts changing into gas.
• The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Or Boiling point is the temperature where vapour pressure is equal to the atmospheric pressure.
• Boiling point of a liquid is a measure of the force of attraction between the particles. Higher the boiling point of a liquid, greater will be the force of attraction between its particles.
Factors affecting the boiling point -
- Boiling point increases on increases pressure over liquid that is why food cooks comparatively faster in pressure cooker.
- Boiling is a bulk phenomenon.
- Boiling point will increase on addition of soluble salts.
- Evaporation starts below the boiling point.
- The boiling point of water is 100 degree Celsius.
Latent Heat of vaporization :
- The amount of heat energy that is required to change 1 kg of a liquid into vapours at atmospheric pressure at its boiling point is known as the latent heat of vaporization.
- Latent heat of vapourization of water is 22.5 x 105 J/Kg.
- Particles in steam, that is, water vapour at 373 K have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.
What happens when we decrease the temperature?
- Condensation / Liquefaction – The process of converting a gas into a liquid by cooling down its temperature.
- Sublimation – change of state of a gas directly into solid and vice-versa is known as sublimation. For Example, Camphor is a solid that directly evaporates into the air without changing to a liquid state.
- Therefore, by increasing or decreasing the temperature we can change the states of matter into one another. Here is a diagram that sums this up.
Effect of change of Pressure
- By applying pressure, increasing the force of attraction among the particles ,the gas changes into a liquid.
Solid carbon dioxide is also known as dry ice.
Properties of dry ice
• It is stored under high pressure.
• Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state.
Uses of dry ice
• Use dry ice to keep freezer contents frozen during breakdowns.
• Use dry ice to keep refrigerator contents cool during breakdowns.
• Dry ice provides the most effective and inexpensive means to keep perishables chilled or frozen.
• Dry ice is used as a refrigerant for ice-cream and frozen food.
Water is the only substance on Earth that naturally occurs in three physical states:
Solid, Liquid, and Gas
Depending on temperature and atmospheric pressure, water can change from one state to another, a process called physical phase change.
- The temperature at which air is saturated with water vapour is known as dew point.
- The dew point temperature tells us the absolute quantity of moisture that is in the air, thereby indicating how humid it will feel outside to our bodies.
- When air has reached the dew point temperature at a particular pressure, the water vapour condense at the same rate at which liquid water evaporates.
- Phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.
Factors affecting evaporation :
- Temperature : As the temperature increases, the rate of evaporation also increases.
- Surface area : As the surface area increases, the rate of evaporation increases.
- Humidity : The rate of evaporation decreases with an increase in humidity.
- Wind speed : Increase in wind speed results in increased evaporation.
- A fan produces a feeling of comfort during the hot weather because our perspiration evaporates rapidly. Our body tries to cool ourselves by releasing sweat through pores of our skin. And when we sit under the fan, the sweat of our body evaporates faster (increase in wind speed results in increased evaporation) due to directed air upon us making our body cool.
- People sprinkle water on a roof after a hot summer day. This is because water has a large heat of vaporization. In a hot summer day, the roof is quite hot. Water absorbs a large amount of heat from the roof for its vaporization. The roof gets sufficiently cool and one can sleep comfortably.
- The water kept in an earthen pot remains cool even in summer because of evaporation.
- This is because an earthen pot has a large number of extremely small pores on its wall. Some of the water, continuously keeps seeping through these pores. This water evaporates continuously and absorbs its latent heat, required for vaporization from the earthen pot and the remaining water in the pot. In this way, the remaining water loses heat and gets cooled.
- When we pour some acetone (nail polish remover) on our palm .The particles gain energy from our palm or surroundings and evaporate causing the palm to feel cool.
- Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation. This helps us to feel cool.
Characteristics of evaporation:
- Evaporation is a surface phenomenon.
- Particles from the surface gain enough energy to overcome the intermolecular forces of attraction present in the liquid and change into the vapour state
- Evaporation starts below the boiling point.
Evaporation cause cooling
- In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.
It takes place at all temperatures.
Boiling occurs at a particular temperature.
Evaporation takes place from the surface. Therefore, it is a surface phenomenon.
In boiling, the entire liquid boils. Therefore, it is bulk phenomenon.
It can occur using the internal energy of the system.
Boiling requires an external source of heat.
Evaporation produces cooling.
Boiling does not produce cooling.
It is a slow process.
Boiling is a rapid process.