(c) Change of state
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
58P Heating and energy storesExplain why heating a system will change the energy stored within the system and raise its temperature or produce changes of state59P Melting, evaporation and boilingDescribe the changes that occur when a solid melts to form a liquid, and when a liquid evaporates or boils to form a gas510P Particle arrangement and motionDescribe the arrangement and motion of particles in solids, liquids and gases511P Change of state practicalPractical: obtain a temperature–time graph to show the constant temperature during a change of state512P Specific heat capacityKnow that specific heat capacity is the energy required to change the temperature of an object by one degree Celsius per kilogram of mass (J/kg °C)513P Thermal energy equationUse change in thermal energy = mass × specific heat capacity × change in temperature, ΔQ = mcΔT.514P Specific heat capacity practicalPractical: investigate the specific heat capacity of materials including water and some solids
Track energy when a system is heated
Heating transfers energy into a system, increasing the energy stored within it. The observable result depends on whether the substance remains in one state or is changing state.
| What is happening? | Where the transferred energy goes | Temperature response |
|---|---|---|
| substance stays in one state | the particles' average kinetic energy increases | temperature rises because temperature is linked to average kinetic energy |
| substance changes state | energy is used to overcome attractive forces and change particle separation and arrangement | temperature remains constant until the change of state is complete |
Once a change of state has finished, further heating again increases average kinetic energy, so the temperature of the new state rises. Throughout, energy is conserved: it has been transferred into the system even when the thermometer does not rise.
A constant temperature during melting or boiling does not mean no energy is transferred. It means the transferred energy is changing the state rather than increasing the particles' average kinetic energy at that time.
Distinguish melting, evaporation and boiling
A change of state rearranges particles without changing the substance's chemical identity. Melting changes a solid to a liquid; evaporation and boiling both change a liquid to a gas.
| Process | Where and when it occurs | Particle change |
|---|---|---|
| melting | throughout a solid at its melting point | particles leave fixed positions and begin to move past one another while remaining close together |
| evaporation | only at the liquid surface; it can occur below the boiling point | higher-energy surface particles escape from the liquid into the gas state |
| boiling | throughout the liquid at its boiling point | gas forms within the liquid as particles separate widely, producing bubbles that rise |
During each change, energy transferred to the substance weakens or overcomes attractive forces between particles. During melting and boiling, the temperature stays constant until all of the sample has changed state.
Evaporation and boiling are not the same process. Evaporation is a surface process that can happen at many temperatures; boiling occurs throughout the liquid at a fixed boiling point for the given pressure.
Compare particles in solids, liquids and gases
The state of a substance is described by how its particles are arranged and how they move. The particles themselves remain particles of the same substance during a physical change of state.
| State | Arrangement and spacing | Motion |
|---|---|---|
| solid | closely packed in a regular arrangement | vibrate about fixed positions |
| liquid | closely packed but arranged irregularly | move and slide past one another |
| gas | widely spaced in an irregular arrangement | move rapidly and randomly in all directions |
Because solid particles keep fixed neighbours, a solid keeps its shape. Liquid particles remain close but can change neighbours, so a liquid flows while keeping nearly the same volume. Widely separated gas particles move through the available space, so a gas fills its container.
Heating does not make individual particles expand. The spacing, arrangement and average motion change. In diagrams, particle size should therefore stay the same while the gaps and positions show the state.
Obtain a temperature-time graph for a change of state
A temperature-time investigation shows a change of state as a horizontal or nearly horizontal section: time passes and energy continues to transfer, but the temperature stays constant while the state changes.
Method: 1. Place a suitable pure sample with a temperature probe in a heat-safe container. 2. Heat gently at steady power, starting below its melting point. 3. Start a stopwatch and record temperature at equal time intervals. 4. Stir gently once the sample is liquid, where safe, so the measured temperature is representative. 5. Continue until the state change is complete and the new state warms. 6. Plot temperature on the vertical axis against time on the horizontal axis.
Before the plateau the solid warms; at the plateau solid and liquid coexist and the temperature gives the melting point; after the plateau the liquid warms. A cooling experiment gives the corresponding constant-temperature section while the liquid freezes.
Keep sample mass and heater power fixed, keep the probe immersed without touching the container, and repeat to identify anomalies. Wear eye protection and handle the heater and hot container with care. Real data may slope slightly because of energy loss or uneven temperature.
Interpret specific heat capacity
Specific heat capacity is the energy required to change the temperature of 1 kg of a substance by 1 °C. Its unit is joules per kilogram degree Celsius, J/(kg °C).
A larger specific heat capacity means more energy is required for the same mass and temperature rise. Therefore, when equal masses receive the same energy, the substance with the larger specific heat capacity has the smaller temperature increase.
A value of 900 J/(kg °C) means 900 J must be transferred to 1 kg of the substance to raise its temperature by 1 °C. Raising 1 kg by 2 °C would require twice that energy if the state does not change.
Specific heat capacity is a property of the substance, not the total energy stored in one object. The energy needed also depends on the object's mass and the size of its temperature change; the definition applies while the substance remains in the same state.
Use the thermal energy equation
The change in thermal energy depends on the mass, the substance's specific heat capacity and the temperature change.
\Delta Q=mc\Delta T
ΔQ is the change in thermal energy in joules (J), m is mass in kilograms (kg), c is specific heat capacity in J/(kg °C), and ΔT is the temperature change in °C. Rearrangements include c=ΔQ/(mΔT) and m=ΔQ/(cΔT).
Example: a 0.80 kg block with c=900 J/(kg °C) warms from 20 °C to 35 °C. ΔT=35−20=15 °C, so ΔQ=0.80×900×15=10800 J.
Use the temperature change, not the final temperature, and convert grams to kilograms before substituting. This equation describes heating or cooling within one state; energy transferred during a change of state is not calculated with mcΔT because the temperature is constant.
Investigate the specific heat capacity of water and solids
To measure specific heat capacity, transfer a measured amount of electrical energy to a known mass and measure its temperature rise. Then calculate c=E/(mΔT).
| Sample | Preparation and temperature measurement | Energy measurement |
|---|---|---|
| water | measure the water's mass in an insulated container; immerse the heater and probe; use a lid and stir gently | record heater power P and heating time t, so E=Pt, or measure voltage and current so E=VIt |
| solid block | measure the block's mass; fit the heater and probe into separate holes with good thermal contact; insulate the block | use the same Pt or VIt method while recording the block's temperature rise |
Record the initial temperature, heat for a measured time, then record the highest uniform temperature and calculate ΔT. Repeat the run and compare values. For comparisons between materials, keep mass, input power, heating time, insulation and probe placement consistent where possible.
Not all electrical energy reaches the sample: the heater, container and surroundings also gain energy. Insulation, a lid, rapid readings, stirring water and good thermal contact reduce this error. Keep a water heater submerged, use a low-voltage supply and do not touch hot equipment.