Change of State
Change of State :- Matter typically exists in three states: solid, liquid and gas. When heat energy is supplied to a substance it changes from one state to another and the process is known as a change of state. Two normal changes of states are solid to liquid and liquid to gas (as well as the other way around).
Let us take some cubes of ice in a beaker to study the change of state on heating or cooling.
Slowly heat it on a constant heat source and continuously stir the mixture of water and ice. Note the temperature at regular interval of times & draw a graph between temperature and time. You will get a graph as shown in figure below :
Initially you will observe that there is no change in the temperature until the entire ice is not melted in the beaker. At this point both the solid and liquid states of the substance coexist in thermal equilibrium with each other. The heat supplied is being utilized in changing the state from solid (ice) to liquid (water). This temperature is called melting point.
∴ Melting point is the temperature at which the solid and the liquid states of a substance coexist in thermal equilibrium with each other.
Melting point depends on (i) the nature of the substance & (ii) pressure. The melting point of a substance at 1 atmospheric pressure is called its normal melting point.
To study how melting point depends on pressure, let us perform an activity. Take an ice slab and put a metallic wire over the slab by hanging two blocks, say 5 kg each, at the ends of the wire as shown in figure below :-
You will observe that the wire passes through the ice slab and the slab does not split. This happens due to the fact that just below the wire, pressure is high, due to which melting point decreases and ice melts at lower temperature. When the wire has passed, pressure reduced and melting point again increases, so water above the wire freezes again. This phenomenon of refreezing is called regelation.
Regelation is defined as the phenomenon in which the ice melts to the water below 0°C on the application of pressure and refreezes back to ice on the removal of pressure.
Skating is possible on snow because of regelation. Water is formed below the skates due to the increase of pressure and it acts as a lubricant.
After the whole of ice gets converted into water(point P) and if we continue further heating, we shall see that temperature begins to rise(P→Q). The temperature keeps on rising till it reaches nearly 100 °C when it again becomes steady(point Q). The heat supplied is now being utilized in change of state from water(liquid state) to vapours(gaseous state). The change of state from liquid to vapour (or gas) is called vaporization.
At this point both the liquid and gaseous states of the substance coexist in thermal equilibrium with each other. This temperature is called boiling point.
∴ Boiling point is the temperature at which the liquid and the vapour states of a substance coexist in thermal equilibrium with each other.
Boiling point depends on (i) the nature of the substance & (ii) pressure. The boiling of a substance at 1 atmospheric pressure is called its normal boiling point.
To study how boiling point depends on pressure, let us perform an activity. Take a flask and fill more than half of it with water. Keep it over a burner and fix a thermometer and steam outlet through the cork of the flask as shown in figure below :-
As water gets heated, the air which was dissolved in the water, will come out as small bubbles. As these air bubbles(formed at the bottom) rise to the cooler water near the top, they condense and disappear. Gradually, as the temperature of the entire mass of the water reaches 100 °C, bubbles of steam reach the surface and boiling is said to occur. The steam formed in the flask becomes visible as it comes out of the flask. It condenses as tiny droplets of water, giving a foggy appearance.
Now if the steam outlet is closed for some time then the pressure inside the flask increases and the boiling stops. After some time when more heat is absorbed by the water, it again starts boiling at a higher temperature(depending upon the increase in pressure). Thus boiling point increases with increase in pressure.
Let us now remove the burner. Allow water to cool to about 80°C. Remove the thermometer and steam outlet. Close the flask with the airtight cork. Keep the flask turned upside down on the stand & pour ice-cold water on the flask.
Water vapours in the flask condense reducing the pressure on the water surface inside the flask. Water begins to boil again, now at a lower temperature. Thus boiling point decreases with decrease in pressure.
This is the reason why cooking is difficult on hills. At high altitudes, atmospheric pressure is low, due to which the boiling point of water reduces as compared to that at sea level. On the other hand, boiling point is increased inside a pressure cooker by increasing the pressure. Hence cooking is faster in pressure cooker.
However, all substances do not pass through the three states: solid-liquid-gas. There are certain substances which changes from the solid to the vapour state directly and vice versa. The change of state from solid to vapour without passing through the liquid state is called sublimation, and the substance is said to sublime. During sublimation both the solid and vapour states of a substance coexist in thermal equilibrium. Examples of sublimes : Dry ice (solid CO2), naphthalene, iodine, camphor.