Changing the State of Matter: Quick Revision

A brief note on Changing the State of Matter. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Heating makes particles move faster. At the melting point a solid becomes a liquid; at the boiling point a liquid becomes a gas.
• $\text{K} = {}^\circ\text{C} + 273$. Nothing is colder than $0\ \text{K}$.
• While the state changes, the temperature stays the same: the heat goes into breaking the pull between the particles. This is latent heat.
• Sublimation is solid to gas directly (camphor, naphthalene, iodine, dry ice). The reverse is deposition.
• A gas becomes a liquid when it is compressed and cooled, as LPG is in a cylinder.

The numbers at a glance

A worked example

How much heat turns $0.35\ \text{kg}$ of ice at $0\ ^\circ\text{C}$ into water at $25\ ^\circ\text{C}$?

1. Melt the ice. $Q_1 = 0.35\ \text{kg} \times 3.34 \times 10^{5}\ \text{J/kg} = 116\,900\ \text{J}$.
2. Warm the water. $Q_2 = 0.35\ \text{kg} \times 4180\ \text{J/(kg}\,^\circ\text{C)} \times 25\ ^\circ\text{C} = 36\,575\ \text{J}$.
3. Add them. $Q = 116\,900\ \text{J} + 36\,575\ \text{J} = 153\,475\ \text{J} \approx 1.53 \times 10^{5}\ \text{J}$.

Practice with new numbers

A program to try

The program works out the worked example, step by step.

Mistakes to avoid

• Taking $273$ away to get kelvin. Add it. A kelvin temperature is never negative.
• Adding $273$ to a temperature change. A rise of $20\ ^\circ\text{C}$ is a rise of $20\ \text{K}$.
• Saying the temperature rises while ice melts. It stays at $0\ ^\circ\text{C}$ until all the ice has melted.
• Using grams with J/kg. Change grams to kilograms first.

Check yourself

1. Change $-23\ ^\circ\text{C}$ to kelvin, and $410\ \text{K}$ to degrees celsius.

2. How much heat melts $0.9\ \text{kg}$ of ice at $0\ ^\circ\text{C}$?

3. Why does a burn from steam hurt more than a burn from boiling water?

4. How can sand and camphor be separated?