Changing States Of Matter : Liquid To Gas : Boiling Point

We all have seen the phenomenon of changing states of matter, boiling and boiling point in action in our daily lives. Let us understand the science behind it. In the previous article  change of state: Melting on changing states of matter we learnt about the process of conversion of solid to liquid. We called this melting, and the temperature at which the substance melts is the melting point. In this article, we will focus on how to convert a liquid into a gas. It will involve us touching on topics like boiling point, vaporisation and sublimation.

States Of Matter

Starting from where we left off in the previous article; the ice cubes that we took in the beaker and heated have completely melted now. You will remember that as long as there is a phase change occurring there will not be any rise in the temperature. The temperature of the liquid water in the beaker starts to rise once all the ice has melted. So let’s continue heating this and find out what happens. On continued heating, the water first begins to let out all the air dissolved in it and you will see it coming out of the water as bubbles.

Once all the air is out, the bubbling ceases and the water continues to heat. It has been mentioned earlier that addition of heat to a body increases the vibrational energy of the atoms of the body. If we give it enough energy, the atom will eventually gain enough energy to vibrate out of the body. This is the next change of state. The molecules of water gain enough energy to escape from the surface of water. This change of state from liquid to gas is called Vaporisation.

Similar to phase change from solid to liquid, it is observed here too that the temperature of the water remains constant until the entire liquid in the beaker is converted to gas. The temperature at which the substance turns from its liquid state to gaseous state is known as its Boiling Point. One of the most commonly known boiling point is the boiling point of water which is at 100oC. We know that pressure effects the melting point of substances as seen in our use of ice skates, but does it affect the boiling point just the same?

The answer is yes. If we were to cover the beaker to prevent anymore gaseous water from escaping, we would notice that the boiling in the beaker would stop. As the water vapour generated is not allowed to go out, the accumulation of the gas increases the pressure inside the beaker which causes the boiling point of water to rise. This is what stops the boiling. Now if you removed the cover, the water would start boiling again. The temperature at which a substance boils at standard pressure is known as the normal boiling point of that substance.

It will interest you know that every object in existence undergoes state change. It is only a question of the amount of heat supplied to the substance. If you supply enough heat, everything on this planet can be made to change its state. The thing is though not every substance has to follow the solid-liquid-gas path. Some substances can naturally from their solid state to their gaseous state without entering the liquid state. This phenomenon is known as Sublimation. Examples of sublimation are, the element Iodine, Dry ice (solid CO2) and high quality coal which at a high temperature burns and sublimates into vapour.

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Practise This Question

A glass contains some water at room temperature 20C. Refrigerated water is added to it slowly.  When the temperature of the glass reaches 10C, small droplets condense on the outer surface.  Calculate the relative humidity in the room.  The boiling point of water at a pressure of 17.5 mm of mercury is 20C and at 8.9 mm of mercury it is 10C.