Question

What happens inside water molecules if heat is added when water boils?
The energy is stored in the form of potential energy; therefore, the temperature doesn’t increase.
A.The energy is added as kinetic energy; therefore, the molecules move faster.
B.The energy is stored as potential energy; therefore, the molecules move faster.
C.The energy is added as kinetic energy; therefore, the temperature increases.
D.All options are correct

Answer 1

We can usually warm something by adding energy. When first added to a substance, energy is concentrated at one atom but will soon bump into others and spread the energy. Due to this, molecules in a substance will move faster.

Complete step by step answer:
We cannot measure heat directly but we can detect its effect on a substance by measuring the change in temperature. Generally, when we add energy to a bunch of atoms they start moving faster and get hotter. Similarly, if we remove energy from a bunch of atoms they usually move less and get cooler.
Adding heat energy results in a temperature rise in a usual manner but it’s not always true. For instance, when water is boiling, adding heat does not increase the temperature of water. This happens at the boiling temperature of every substance so that they can vaporize. At the boiling temperature, adding heat energy converts the liquid into a gas without raising the temperature of the system.
To this general rule, adding heat to a boiling liquid is an exception as more heat makes a higher temperature. The same thing happens when a solid changes into liquid. Once water reaches the boiling point, extra energy is required to change the phase and increase in the potential energy instead of kinetic energy.
As we add heat constantly to a substance in a solid phase, kinetic energy increases but the potential energy remains the same. As it reaches melting point, kinetic energy remains the same but potential energy increases. After a while, the substance changes completely into a liquid and heat is still being added constantly resulting in an increase in kinetic energy while potential energy remains the same. Finally, it reaches a boiling point i.e., changes into gas phase, the kinetic energy remains the same but potential energy increases with temperature being constant at the same time.

Hence, the correct option is (A).

Note:
A heating curve represents the phase transitions graphically which a substance undergoes as heat is added to it. Water has a high boiling point because of the presence of strong hydrogen bonds between the water molecules and we also know that water is both a strong hydrogen bond donor as well as an acceptor and it can exist in all the three phases: solid, liquid and gas.