Question

When we add ice to water it becomes cold. Explain.

Answer 1

Latent heat of fusion:

1. Latent heat of fusion, also known as enthalpy of fusion, is the amount of energy that must be supplied to a solid substance in order to trigger a change in its physical state and convert it into a liquid.
2. For instance, the latent heat of fusion of one kilogram of water is the amount of heat energy that must be supplied to convert $1\mathrm{kg}$ of ice without changing the temperature of the environment (which is kept at $0°\mathrm{C}$) is $333.55\mathrm{kilojoules}$.
3. The latent heat of fusion of a substance also accounts for the energy required to accommodate any increase in the volume of the substance post the change of its physical state. The temperature at which the substance undergoes the phase transition is called the melting point of the substance.

Explanation:

1. The molecules of ice are much colder (relatively) than that of water.
2. Heat always moves from an area of greater heat to an area of lesser heat.
3. The second law of thermodynamics states that the entropy of any isolated system always increases.
4. Isolated systems spontaneously evolve towards thermal equilibrium- the state of maximum entropy of the system.
5. Atoms and molecules move quicker when they are hot and slower when they are cold. When the faster-moving H₂O molecules in the water collide with the slower-moving ice molecules, they jostle and speed up some of the ice molecules while losing some of their speed/energy.
6. Also, some of the ice will undergo a phase change, from solid to liquid.
7. This melting phase transition is an endothermic process that requires and absorbs energy from the temperature of the water.
8. As water and ice have different temperatures, if we add them together then there will be an exchange of heat, due to which ice starts melting and water becomes cold.
9. The heat required to melt the molecules of ice is provided by the molecules of water and this heat is the latent heat of fusion.