Question

When common salt is dissolved in water
A) Melting point of solution increases.
B) Boiling point of the solution increases.
C) Boiling point of the solution decreases.
D) Both Melting and Boiling point decrease.

Answer 1

We know that the boiling point is the temperature at which liquid is converted to its vapour form. Or we can also define it as the temperature at which vapour pressure of the liquid becomes equal to its atmospheric pressure.
Also, melting point is the temperature at which it changes from its liquid form to its solid form.

Complete answer:
So, here we have a pure solvent of water. To this pure solvent of water, we are adding the solute NaCl. Here the question is what happens when we do this.
To find out we have to understand what is happening in its molecules.
When there is pure solvent of water, its surfaces contain only water molecules. And while boiling the water, at some point the water molecules escape from its surface to its vapour form. This temperature is known as its boiling point. Now when we add some NaCl to it. The NaCl molecules also occupy its surfaces.
Now, when we boil the solutions the water molecules cannot escape as easily as before. Since they have NaCl molecules also in its surface. Hence we have to increase the temperature to get enough energy for ${H_2}O$ molecules to escape to its vapour form.
Therefore we understand that while adding a solute to its pure solvent the boiling point increases.
Now coming to melting point. Same logic applies here also, while converting the liquid molecules to its solid form we have to decrease the temperature more since the Nacl molecules are present on the surface. Therefore the freezing point is decreased.

Thus option (b) is correct.

Note: So, we understand when we add a solute to its pure solvent the boiling point increases. That is why, while making tea we add sugar after it is boiled. Otherwise we have to wait more time for the solution to reach its boiling point and consume more LPG gas.