5.5 - Changes of State

Heating or cooling changes temperature or state

• Heating transfers energy to the particles in a substance; cooling transfers energy away.
• Heating or cooling leads to a temperature change or a change of state.

Heating and cooling

• Temperature change - Heating makes particles move or vibrate faster, raising the temperature. Cooling slows particles down, lowering the temperature.
• Change of state - If heated enough, the energy breaks the bonds between particles rather than raising their temperature. When cooled enough, bonds form between particles.

Temperature stays constant during a change of state

• While a substance is changing state, its temperature does not change.
• During melting or boiling, energy is absorbed to break bonds rather than raise the temperature.
• During condensing or freezing, bonds form and release energy, so the temperature does not fall until the change of state is complete.
• Melting point is the temperature at which a substance melts; boiling point is the temperature at which it boils.
• For pure water at normal atmospheric pressure: melting point 0 °C, boiling point 100 °C.

Practical - Temperature-time graph for melting ice

Method

1. Fill a beaker with crushed ice and place a thermometer in it. Record the starting temperature.
2. Heat the beaker steadily with a Bunsen burner.
3. Record the temperature and state of the contents (e.g. solid, partly melted, liquid) every 20 seconds until the water boils.
4. Plot a graph of temperature against time.

What the graph shows

• The graph rises as the ice warms, has a flat section while it melts, rises again as the liquid warms, then flattens while the water boils.
• Each flat section is a change of state at constant temperature: the energy supplied breaks bonds between particles rather than raising the temperature.
• The flat sections occur at the melting point (0 °C for water) and the boiling point (100 °C for water).