Changes of State: Melting, Freezing, Boiling and Condensing
Have you ever watched the ice in your lemon tea disappear, or seen steam rush out of a rice cooker? These are changes of state. In this chapter you will learn the names of the changes, show that mass stays the same when ice melts, find out where the thermal energy goes, and read temperature–time graphs like a scientist. Let's get started!
How particles are arranged in solids, liquids and gases is covered in Particle theory and Particle model for the three states of matter. Here we focus on what happens during a change.
1. The Changes of State
A change of state is a physical change: the form changes, but it is still the same substance. Ice, liquid water and steam are all water.
Melting and freezing
Melting – a solid changes into a liquid when heated (an ice-cream cone on a hot day).
Freezing – a liquid changes into a solid when cooled (water becoming ice cubes in the freezer).
A solid melts at its melting point and a liquid freezes at its freezing point. For a pure substance these are the same temperature. For pure water, both are 0 °C.
Boiling, evaporation and condensation
Boiling – a liquid changes into a gas quickly, with bubbles forming all through the liquid, at a fixed boiling point. Pure water boils at 100 °C at normal air pressure.
Evaporation – a liquid changes into a gas slowly, only at its surface, at temperatures below the boiling point (wet clothes drying on a rack).
Condensation – a gas changes into a liquid when cooled (droplets on a cold can of drink on a humid Hong Kong day).
Sublimation: a quick look
A few substances change straight from a solid into a gas. This is sublimation. Dry ice (solid carbon dioxide), used to keep ice-cream mooncakes cold, does this.
Memory aid – "Heat IN, Heat OUT":
Energy taken IN: melting, boiling, evaporation, sublimation (particles are "set free").
Energy given OUT: freezing, condensation (particles "settle down").
Key takeaway: Melting, freezing, boiling, evaporation, condensation and sublimation are changes of state. They are physical changes.
2. Mass Stays the Same
Does melting ice gain or lose mass? Scientists do not guess. They test it.
Experiment: weighing ice before and after it melts
Hypothesis: The mass of ice will stay the same when it melts.
1. Put some ice cubes in a container and close the lid tightly.
2. Weigh it on an electronic balance.
3. Wait until all the ice has melted.
4. Wipe the outside dry and weigh it again.
Result: The readings are the same (apart from tiny errors), so the data support the hypothesis.
\(\text{mass before melting} = \text{mass after melting}\)
Why close the lid and wipe the container dry?
Without a lid, some water evaporates, so the mass seems to go down. A cold container also makes water vapour in the air condense on its outside, so the mass seems to go up. The lid and the wiping keep the test fair.
Experiment: freezing water in a sealed bottle
Weigh a capped plastic bottle of water, freeze it overnight, wipe it dry and weigh it again quickly. The mass is the same, but the bottle bulges because water expands when it freezes. The volume changes; the mass does not. (Never freeze a full glass bottle – it can crack.)
Why does the mass stay the same?
No particles are added or taken away; they only change how they are arranged and move. Same particles, same mass. (See Particle theory for diagrams.)
Key takeaway: When a substance melts or freezes, its mass stays the same. Its volume may change, but no matter is gained or lost.
3. Energy In, Energy Out – but the Temperature Stays the Same
Heat crushed ice gently while stirring. The thermometer stays at about 0 °C until all the ice has melted, even though you keep heating! Where does the energy go?
Absorbing energy: melting and boiling
During melting and boiling, the substance absorbs thermal energy. The energy is used to overcome the attractions between particles, not to speed them up. So the temperature stays constant until the change is complete. Evaporation also absorbs thermal energy, which is why sweating cools you down after PE.
Releasing energy: freezing and condensing
During freezing and condensation, the substance releases thermal energy to the surroundings, and again the temperature stays constant. In a freezer, the cold air carries this energy away.
Did you know? A steam burn is usually worse than a boiling-water burn, even though both are at 100 °C. Steam condenses on your skin and releases extra thermal energy. Take care when opening a rice cooker or a dim sum steamer! Also, the white "cloud" above a kettle is not steam. Steam is invisible; the cloud is tiny droplets of condensed water.
Key takeaway: Melting and boiling absorb thermal energy; freezing and condensation release it. During a change of state, the temperature stays constant.
4. Reading Temperature–Time Graphs
A temperature–time graph has time on the x-axis and temperature on the y-axis. A graph made while heating is a heating curve; one made while cooling is a cooling curve.
The golden rule:
Sloping part = temperature changing = one state only.
Flat part (plateau) = temperature constant = a change of state, with two states together.
The heating curve of water
Ice at -10 °C from a freezer is heated steadily until it boils:
A to B (sloping up): solid ice warms from -10 °C to 0 °C.
B to C (flat at 0 °C): ice is melting; ice and water are both present.
C to D (sloping up): liquid water warms from 0 °C to 100 °C.
D to E (flat at 100 °C): water is boiling; water and steam are both present.
After E: only steam is left, and it gets hotter.
D to E is much longer than B to C: boiling water away takes much more energy than melting the same mass of ice.
The cooling curve of stearic acid
Stearic acid is a white, waxy solid that melts at about 69 °C, so a hot water bath can melt it. Melted stearic acid is left to cool. Typical results:
Time (min): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Temperature (°C): 80, 75, 71, 69, 69, 69, 69, 69, 66, 62
0 to 3 min (sloping down): the liquid cools.
3 to 7 min (flat at 69 °C): it is freezing; liquid and solid are both present, and it releases thermal energy.
After 7 min (sloping down): the solid cools towards room temperature.
So its freezing point is 69 °C, and a heating curve would show its melting point at the same temperature.
How to read melting, freezing and boiling points
Step 1. Find the flat parts.
Step 2. Read across to the temperature axis.
Step 3. Name the change. On a heating curve, the lower flat part is the melting point and the higher one is the boiling point. On a cooling curve, the higher flat part is where the gas condenses and the lower one is the freezing point.
Pure substances and mixtures
A pure substance (pure water, stearic acid) melts and boils at fixed temperatures, so its graph has sharp flat parts. Candle wax (paraffin wax) is a mixture, so it softens and melts over a range of temperatures and its graph only bends gently. Impurities change these points too: salt water freezes below 0 °C and boils above 100 °C.
Key takeaway: A sloping part means one state warming or cooling. A flat part means a change of state at the melting (freezing) point or the boiling point.
5. Practical Work: Heating a Substance and Plotting the Graph
This is a Key Practical Task of this unit.
Steps
Hypothesis: While solid stearic acid melts, its temperature will stay constant.
1. Put a thermometer (or a temperature probe with a data logger) in the middle of solid stearic acid in a boiling tube.
2. Clamp the tube in a beaker of water and heat gently. This water bath heats the substance gently and evenly.
3. Record the temperature every 30 seconds until all the solid has melted and the temperature has risen a little more.
4. Once it melts, stir gently with a stirrer, not the thermometer. Wear goggles.
Conclusion: A flat part on your graph while the solid melts supports the hypothesis.
Variables and plotting the graph
Independent variable: time. Dependent variable: temperature. Controlled variables: e.g. the amount of substance, the heating rate and the thermometer position.
Plot time on the x-axis and temperature on the y-axis, label both axes with units (min, °C), and draw a smooth line. Ignore any point that is clearly out of place.
Sources of error and how to improve
Parallax error – Improve: read the thermometer at eye level.
Thermometer touching the glass (the glass can be hotter) – Improve: clamp the bulb in the middle of the substance.
Uneven or too-fast heating (the flat part becomes short and unclear) – Improve: use a water bath, a small flame and gentle stirring.
Energy lost to the surroundings – Improve: cover or insulate the beaker.
Late readings – Improve: use a data logger to record automatically.
Accuracy, precision and repeating the experiment
Accuracy is how close a result is to the true value: for boiling water, 99 °C is more accurate than 96 °C. Precision is how close repeated readings are to each other. Repeat the experiment and average the flat-part temperatures. If one reading is very different (an outlier), check it and repeat that measurement.
Key takeaway: Heat gently and evenly, read carefully at regular times, and repeat.
Common Mistakes
Common mistake 1: "Boiling water gets hotter if I keep heating it." Correct: It stays at the boiling point; the extra energy turns more water into steam.
Common mistake 2: "Water gets lighter when it freezes, because ice floats." Correct: The mass stays the same. Ice floats because the same mass of ice has a larger volume, so a lower density (see Density).
Common mistake 3: "The flat part means the heating stopped." Correct: Heating continues; the energy is changing the state.
Common mistake 4: "Sugar melts in hot tea." Correct: Sugar dissolves (see Dissolving). Melting is a solid turning into a liquid when heated.
Quick Review
1. Melting: solid → liquid. Freezing: liquid → solid. Boiling/evaporation: liquid → gas. Condensation: gas → liquid. Sublimation: solid → gas.
2. Mass stays the same on melting or freezing.
3. Melting and boiling absorb thermal energy; freezing and condensation release it.
4. During a change of state, the temperature stays constant.
5. Sloping part = one state; flat part = change of state.
6. Pure water melts at 0 °C and boils at 100 °C.
Well done! Keep practising those graphs and you will soon read them like a pro!