What is happening when water is heated from \(25^{\circ} \mathrm{C}\) to \(155^{\circ} \mathrm{C}\) ?

When you heat water, you’re adding energy to it. As water warms up from its initial temperature, it absorbs this energy and its own temperature rises. This process of heating is continuous, with the water temperature steadily increasing until it hits a specific critical point, known as the boiling point.

During the heating journey, the physical properties of water don't change initially. It remains in its liquid form, just at a higher temperature. If you've ever warmed water on a stove, you have observed how it starts out calm, then as it gets warmer, you might notice small bubbles and the water moves around more vigorously. This is due to the increased energy levels of the water molecules.

The boiling point is a unique temperature where a substance changes from a liquid to a gas. For pure water at sea level, this point is at a temperature of exactly 100 degrees Celsius (212 degrees Fahrenheit). At this moment, something interesting happens – the temperature of the water doesn’t increase despite heat continuing to be added. This is because the added heat is now used for the phase transition, specifically breaking the hydrogen bonds between water molecules, which takes a lot of energy. This period of consistent temperature during boiling is an important property of pure substances and is used to define their boiling point.

A phase transition is a change from one state of matter to another. In our water example, this transition is from liquid to gas. This transition occurs at the boiling point where the evaporation process accelerates dramatically, generating bubbles of steam that rise to the surface and escape into the air. This is the vigorous boiling you see.

Importantly, temperature remains constant during this transformation. Only when all the liquid has turned into gas will the temperature of the vapor start to rise again, if heat continues to be supplied.

Heat energy plays a crucial role in the water heating process. In the phase leading up to the boiling point, heat raises the water’s temperature. However, once the boiling point is reached, heat energy is then used to convert the water from a liquid to a gas, which involves breaking molecular bonds. This is known as the latent heat of vaporization.

In the case of water being heated above its boiling point, the steam (water in its gaseous form) will absorb additional heat and its temperature will continue to climb as long as heating persists. The ability of water to store and transfer heat is one of its most vital characteristics, affecting climates, weather patterns, and even the operation of our homes and industrial processes.