When the cubes of ice melt down from solid to liquid water or when a candle burns into flames, you must have observed and wondered how it happens? Well, this is because of the changing states of matter. But have you ever thought about why substances change form?
Actually, this process of changing states in matter occurs when a matter absorbs or loses energy. Atoms and molecules move more rapidly when a substance absorbs energy, as a result of which the kinetic energy increases that forces or pushes the particles to such an extent that they change their form. Generally, this energy is heat or thermal energy.
What are changes of state?
Generally, matter experiences two main types of changes: chemical change and physical change. In the process of physical change, there is no alteration in the natural identity of the matter, though the change in its state, size, and shape occurs. And as far as chemical change is concerned, there are no chances of reversing it back; you can only do it under some special conditions. Some common state changes include freezing, melting, condensation, vaporization etc.
What is the reason for the occurrence of phase changes?
Phase changes occur due to the change in pressure and temperature of a system. The contact between the molecules increases when the pressure or temperature increases. In the same way, when the temperature declines, the molecules and atoms settle into a more stiff or solid structure.
Freezing
In a liquid state solvent, the particles are loosely bonded with each other, which indicates a weak force of attraction between them as compared to the solid state. When the temperature of a liquid is dropped below the freezing point of solid, then liquid changes into solid ice, and this phenomenon is termed as freezing.
Freezing is basically a phase transition that happens when a liquid turns into a solid-state, and the temperature at which it occurs is known as the freezing point. It is an exothermic reaction since the heat is released during the freezing process. Freezing is also termed solidification.
To summarise all of this, the solidifying phase change of liquid is what we call as freezing, which primarily occurs due to the cooling of a substance.
Melting
The process of solid turning into liquid state is known as melting; it is also termed as fusion. When the cubes of ice from the freezer are placed in a room with normal temperature, the ice starts melting because it absorbs energy from the warmer air present in the surroundings. As a result of which, the ice overcomes the force of attraction between its particles that are holding it together, which permits them to slip out of the static position held by the ice. And the temperature at which solid changes into liquid is known as the melting point.
Vaporization
Vaporization is basically a transition phase in which the liquid state of a substance changes into the gaseous state. When liquid converts into gas, then particles separate from each other because the liquid particles completely overcome the force of attraction between them and change to the gaseous state. This happens because the particles of liquid gain enough energy when heated. The temperature at which a liquid boils is its boiling point.
Vaporization is of two types – Evaporation and Boiling.
Condensation
The process of transformation of the stage of water, where the vapour converts to the liquid state, is termed Condensation.
Condensation can be accomplished in the following two ways:
- If the air is cooled until its dew point.
- Saturation with water vapour to such an extent that it resists holding any further quantity of water.
Condensation can also be called the reverse reaction of evaporation, where the liquid water turns to a vapour state. Through the process of evaporation into the air moisture at several atmospheric temperatures and pressures, the rate of Condensation can be measured. The process of Condensation also includes the removal of heat from a system where the vapour is transformed into liquid.
Sublimation
Sublimation is the process in which solids directly transform into gases. When solids absorb sufficient energy to overcome the forces of attraction between their particles completely, then the process of sublimation occurs. One of the most common examples of solid that goes under sublimation is dry ice.
Important Questions on Changing States of Matter
1) What are the two basic types of changes that occur in the matter?
Generally, matter experiences two main types of changes: chemical change and physical change. In the process of physical change, there is no alteration in the natural identity of the matter, though the change in its state, size, and shape occurs. And as far as chemical change is concerned, there are no chances of reversing it back; you can only do it under some special conditions.
2) How many basic changes of states of matter are there?
There are basically six changing states of matter; they are as follows:
1. Melting: The process in which solid gets converted into liquid.
2. Freezing: It is basically a phase transition that happens when a liquid turns into a solid state.
3. Vaporization: The transition phase in which the liquid state of a substance changes into the gaseous state.
4. Condensation: The process of transformation of the stage of water where the vapour converts to the liquid state.
5. Sublimation: The process in which solids directly transform into gases.
6. Deposition: The process in which gas gets transformed into a solid form.
3) What are the factors affecting the states of matter change?
The change in the state happens due to the following factors:
- Change in intermolecular force of attraction
- Variation in temperature
- Variation in pressure
- Alteration in kinetic energy of the particle.
4) Define Melting point and Boiling point.
Melting Point: The temperature at which solid changes or melts into a liquid at atmospheric pressure is known as the melting point.
Boiling point: At atmospheric pressure, the temperature at which a liquid boils to form vapours is known as its boiling point. The boiling point of water is 373k.
5) Define Latent heat of fusion.
Latent Heat of Fusion: The amount of heat required to convert 1 kilogram of the solid into liquid without any change in temperature is called the latent heat of fusion.
6) Why is solid CO2 termed as dry ice?
On the decrease of pressure to 1 atm, solid CO2 gets transformed directly to the gaseous state without coming into a liquid state that’s why it is also identified as Dry ice. Because of this reason, it is stored under high pressure.
7) What is Latent heat of vaporization?
Latent Heat of Vaporization: When a material in the liquid state is given energy, it transforms from the liquid phase to the vapour phase without change in temperature, and the energy absorbed in this process is known as latent heat of vaporization.
The latent heat of vaporization of water is 2260kJ/Kg which is equal to 40.8kJ/mol.
8) What are the real-life examples of condensation?
Some of the real-life examples of condensation are –
- Formation of clouds and water cycle
- Morning dew
- Fog on the mirror
- Foggy breath during cold weathers
- Foggy windshield
9) Why do water vapours at 100o C cause more burns as compared to liquid water at the same temperature?
Water vapours hold more energy at 100° C than the equivalent particles of liquid. This happens because water vapours absorb larger energy in the form of latent heat of vaporisation, as compared to liquids.
10) What causes phase change?
Phase changes occur due to the change in pressure and temperature of a system. The contact between the molecules increases when the pressure or temperature increases. In the same way, when the temperature declines, the molecules and atoms settle into a more stiff or solid structure.
Practice Questions
- Why does evaporation result in cooling?
- Why do we wear cotton clothes in summer?
- Differentiate between melting and boiling.
- Why can gases be compressed easily as compared to liquids?
- What is Humidity?
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