Change of state in a substance is a physical change in a matter. These changes accommodate reversible changes, that is the changes that can be changed from one physical state to another. It does not involve any chemical changes. The changes occur due to variations observed in the temperature or pressure of a substance. For instance, on increasing the temperature the intra-molecular interaction among the particles rise, therefore, the substance move more freely. On decreasing the temperature, the substances settle down in a fixed and rigid structure. Governed by the three states of matter, the different processes encountered in the cycle are melting, freezing, sublimation, deposition, condensation, and vaporization.
The various processes description is as follows:
| Melting | Freezing | Evaporation | Condensation | Sublimation |
| The transition occurs from the solid phase to the liquid phase. | The transition occurs from the liquid phase to the solid phase. | The transition occurs from the liquid phase to the gaseous phase. | The transition occurs from the gaseous phase to the liquid phase. | The transition of the solid phase to the gaseous phase without undergoing through the intermediate liquid phase. |
What happens when a liquid becomes a gas?
When a liquid becomes a gas, the particles separate from each other, with each particle going its own way in space. This is how gases tend to fill their containers. Indeed, in the gas phase, most of the volume is empty space; only about 1/1,000th of the volume is actually taken up by matter.
Vaporization
The transition in phase observed from the liquid phase to the gaseous state of liquid. There are two types of vaporization: Evaporation and Boiling.
Evaporation
Evaporation is the change of the liquid state into the gaseous state occurring at temperatures below the boiling point. Evaporation takes place at the surface of the liquid. Molecules of a liquid evaporate if they are situated near the surface and transmitted or moving in a specific direction. They must possess enough kinetic energy to overcome the intermolecular forces present in the liquid phase.
Factors That Affect the Evaporation
- Temperature –An increase in the temperature of a substance results in an increased rate of evaporation.
- Surface area – An increase in the surface area of a substance results in an increased rate of evaporation.
- Humidity – An increase in atmospheric humidity results in a decreased rate of evaporation.
- Wind speed – An increase in the speed of wind in a specific area results in an increased rate of evaporation.
Boiling
Boiling is the process in which the liquid changes to gas very quickly upon heating. The particles in water are bound together by strong forces of attraction between them. When we supply energy and heat the water particles, their corresponding kinetic energy increases. Some of these particles gain enough energy to break through the intramolecular forces of attraction and transform into vapours. They escape the surface and this process is known as boiling. The liquid thus makes a transition to the gaseous state. The process of boiling is an instance of rapid vaporization which occurs either at or above the boiling temperature or at or below the liquid’s surface.
Boiling point
The boiling point of a liquid is the temperature at which a liquid boils and then further changes rapidly into the gas state at atmospheric pressure. It is basically a measure of the force of attraction between the particles. The greater boiling point of a solvent is an indicator of greater forces of attraction between the particles. The boiling points of different liquids are different.
Example:
Boiling point of water = 100°C
Boiling point of alcohol = 78°C
Boiling point of mercury = 357°C
Factors affecting the boiling point of a liquid:
- Intermolecular forces – The forces of attraction between the molecules of the liquid increase on increasing the boiling point of the liquid.
- Pressure – As we increase the pressure applied on material, its volume reduces. This makes the molecule approach each other more closely, thereby, leading to a greater intermolecular force. When we pressurize gas, it transforms to liquid and then further to solid. It is also observed that liquids at high pressure have a high boiling point.
- Solutions – Solutions have a higher boiling point than pure solvents.
- Mixtures – On mixing two or more liquids, the obtained mixture will have a boiling point lying in a temperature range between the boiling points of the two liquids.
Factors affecting rate of vaporization
- Surface Area – Since the increase in surface area of an object leads to more number of particles getting exposure the temperature change, therefore, the surface area is directly proportional to the rate of vaporization.
- Pressure – As we decrease the pressure, the particles quickly gain the kinetic energy required to escape from the surface of the container. Therefore, pressure is inversely proportional to the rate of vaporization.
- Temperature –An increase in temperature results in an increase of the kinetic energy of the particles required to escape from the surface of the container. This results in the decrease of the force of interaction between the particles. Therefore, the temperature is directly proportional to the rate of vaporization.
- Speed of wind – Since the increase in wind speed leads to more number of particles getting being driven away by the wind, therefore, speed of wind is directly proportional to the rate of vaporization.
Latent Heat of Vaporisation
The amount of heat required to simulate the conversion of 1kg of liquid to the gaseous state at its corresponding boiling point at a specific temperature. Latent heat of vaporisation is different for different liquids.
Daily Life examples of vaporization
- Extraction of salt from seawater. The seawater is subjected to evaporation, which leaves the excess salt behind.
- Drying up wet clothes.
- Isolation of the components of a mixture in various kinds of manufacturing processes. For instance, the separation of certain components of petroleum.
Sample Questions
Question 1: Water vapours at 100o C causes more burns than liquid water at the same temperature. Why?
Answer:
Water vapours possess more energy at 100°C than the corresponding particles of liquid. This is because water vapours absorb greater energy than liquid in the form of latent heat of vaporisation.
Question 2: Why is there no water in Rajasthan whereas Mumbai is surrounded with water?
Answer:
The temperature of Rajasthan is indefinitely greater than that of Mumbai, therefore, with the increase in temperature, the kinetic energy of the particles increase. Therefore, the water evaporates quicker in Rajasthan.
Question 3: Differentiate between Evaporation and Boiling processes
Answer:
| Basis of Comparison | Evaporation | Boiling |
| Definition | The transition of liquid into a vapour | Steaming of a liquid under the influence of heat |
| Occurrence | Occurs at the surface of the liquid | Occurs throughout the liquid, since the heat is supplied. |
| Temperature | Minimal temperature change needed | A temperature is greater than the boiling point needed |
| Nature | Natural process | Unnatural process |
| Time | More | Less |
| Energy | Little or no energy required | Lot of energy required |
Question 4. Explain why the temperature of the boiling water at 100o C does not change on supplying heat.
Answer:
Water absorbs energy upon heating. This causes the intermolecular attractive forces to break and result in the phase transition of the liquid to the gaseous state. Until, water completes the process of boiling, the temperature of the system remains at 100 °C. The temperature of water rises only when the amount of liquid water rises until disappearance.
Question 5. How are the intermolecular forces in a liquid and its vapour pressure related to each other?
Answer:
The vapour pressure of a liquid is inversely proportional to the vapour pressure, that is it decreases as the strength of its intermolecular forces increases.
Question 6. Let us assume two liquids to have latent heat of evaporation A and B to be 100 J/kg and 350 J/kg respectively. Which one causes more cooling effect?
Answer:
Since, the latent heat of evaporation of liquid B is greater, therefore, it will absorb more heat from surroundings for evaporation. Hence, it will result in the production of a greater cooling effect.
Question 7. Explain why the temperature of the ice at 0 °C does not change upon heating the substance further.
Answer:
Ice absorbs energy upon heating. This causes the intermolecular attractive forces to break and result in phase transition of the solid to the liquid state. Until, water completes the process of freezing, the temperature of the system remains at 0 °C. The temperature of water rises only when the amount of water rises until disappearance.