Melting, boiling and the kelvin scale

Heating a solid

Put ice in a pan and heat it. At first the ice warms up. Then it begins to melt. When all the ice has melted, we have water. If we go on heating, the water gets hotter, and at last it boils and becomes steam.

Melting

When we heat a solid, its particles take in energy, and they vibrate faster. At a certain temperature, the particles have enough energy to break out of their fixed places. The solid melts and becomes a liquid.

The temperature at which a solid melts to become a liquid, at normal pressure, is its melting point. The melting point of ice is $0\ ^\circ\text{C}$, which is $273\ \text{K}$.

The melting point tells us how strong the force between the particles is. The stronger the force, the higher the melting point. Ice melts at $0\ ^\circ\text{C}$. Iron melts at about $1538\ ^\circ\text{C}$.

Freezing

If we cool a liquid, the particles slow down. At the same temperature as the melting point, the liquid becomes a solid again. This is freezing. The melting point and the freezing point of a pure substance are the same.

Boiling

When we heat a liquid, the particles move faster. At a certain temperature, the liquid turns to gas quickly throughout the liquid, with bubbles of gas. This is boiling. The temperature at which a liquid boils, at normal pressure, is the boiling point. For water it is $100\ ^\circ\text{C}$, which is $373\ \text{K}$.

When a gas is cooled, it turns back to a liquid. This is condensation.

The kelvin scale

In science we often measure temperature in kelvin (K), the SI unit. The kelvin scale starts at the lowest possible temperature, $0\ \text{K}$, where the particles have the least energy. A change of $1\ \text{K}$ is the same as a change of $1\ ^\circ\text{C}$. So the two scales differ only by a number.

Do not write the degree sign with kelvin. We write $300\ \text{K}$, not $300\ ^\circ\text{K}$.

You should find that $0\ ^\circ\text{C}$ and $273\ \text{K}$ sit at the same mark, and that a rise of $10\ ^\circ\text{C}$ is also a rise of $10\ \text{K}$.

Practise with new numbers

Try it in Python

This program changes some temperatures from celsius to kelvin.

Remember these

• Melting point: a solid becomes a liquid. Boiling point: a liquid becomes a gas. Both are at normal pressure.
• Ice melts at $273\ \text{K}$, and water boils at $373\ \text{K}$.
• $\text{K} = {}^\circ\text{C} + 273$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Change $36\ ^\circ\text{C}$ and $-50\ ^\circ\text{C}$ to kelvin.

2. Change $425\ \text{K}$ to degrees celsius.

3. Ethanol melts at $-114\ ^\circ\text{C}$ and boils at $78\ ^\circ\text{C}$. Is it a solid, a liquid or a gas at $300\ \text{K}$?

4. What is the difference between melting and boiling? How is freezing related to melting?