At standard pressure, the specific temperature at which a liquid boils is called the boiling point of that liquid.
It depends on several factors.
Factors on which the Boiling Point of a liquid depends
- Nature of the liquid: Different liquids have different boiling points. The more volatile the liquid, the lower its b.p.
- Pressure on a liquid: The boiling point of a liquid depends on the pressure. An increase in pressure on a liquid increases its boiling point and a decrease in pressure decreases its b.p.
- Presence of solute in a liquid: If a solid is dissolved in a pure liquid, the b.p. of the solution is higher than the b.p. of the pure liquid.
Determination of Boiling Point of Water
Take some amount of water in a glass flask and the mouth of the flask is closed with a rubber stopper. The shell has three holes. A thermometer is inserted through a hole. The thermometer bulb should be slightly above the surface of the water. A manometer is inserted through one of the other two holes and an exhaust tube is inserted through the other to allow steam to escape.
Now the flask is placed on a tarp and attached to a stand and heated with a Bunsen burner. First bubbles come out of the water, then the water starts to boil. During this time a lot of steam comes out through the exhaust pipe. Reading the thermometer at this time will show that the thermometer reading has stabilized around 100°C. This elevation is the b.p. of water. At this time the level of mercury in both arms of the U-tube of the manometer is equal because the vapour pressure at the time of boiling is equal to the atmospheric pressure.
Effect of Pressure on Boiling Point
Lowering the pressure on a liquid lowers its boiling point – this phenomenon is exploited in various industries. For example —
- To prepare condensed milk,
- To thicken an aqueous solution of hydrogen peroxide,
- To prepare sugar molasses from an aqueous solution of sugar.
Increasing the pressure on a liquid increases its boiling point – this theory is used in various industries. For example —
- In the preparation of artificial silk,
- To sterilize tin cans of foodstuffs,
- To sterilize operating instruments, bandages etc. in hospitals.
Effect of Altitude on Boiling Point of Liquids
Atmospheric pressure decreases as one rises above the sea level. We know that as pressure decreases, the boiling point of a liquid decreases.
Reasons for longer cooking time on mountains: Air pressure decreases due to the altitude of mountains. So the b.p. of water lowers over the mountains, i.e. water boils at less than 100°C. That’s why it takes more time to cook the food.
Also, Read
Differences between Evaporation and Boiling
Factors Affecting the Rate of Evaporation
