Specific heat capacity and changes of state

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Lesson notes

Big idea: specific heat capacity and changes of state

Key concept: Change. Energy transferred equals mass times specific heat capacity times temperature change. During a phase change, energy changes particle arrangements without necessarily raising temperature.
Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
Global context: Scientific and technical innovation. Thermal properties help choose materials for heating and cooling systems.

Thermal Expansion

When a material is heated at constant pressure, its temperature increases, volume expands, and density decreases.
Expansion occurs because molecules gain kinetic energy, vibrate/move faster, and push each other apart.
Solids expand the least, gases expand the most, and liquids are in between.
Useful applications: liquid-in-glass thermometers (liquid expands in capillary tube) and bimetallic strips (two metals expand at different rates, bending to close a circuit).
Undesirable consequences: buckling of railway tracks, road surfaces, and bridges; gaps are built in to allow expansion.

Thermal expansion diagram

Specific Heat Capacity

Internal energy is the total energy stored in a system due to particle motion (kinetic) and positions (potential).
Temperature is a measure of the average kinetic energy of particles.
Specific heat capacity (c) is the energy required to raise the temperature of 1 kg of a substance by 1 °C.
Low c → heats up/cools down quickly; high c → heats up/cools down slowly.
Equation: \Delta E = m c \Delta \θ , where \Delta E in J, m in kg, c in J/kg°C, \Delta \θ in °C.

Kinetic and potential energy of particles in a liquid

Investigating Specific Heat Capacity

Aim: determine specific heat capacity by measuring energy supplied (E = IVt) and temperature change.
Independent variable: time; dependent variable: temperature; control: material, current, voltage.
Equipment: thermometer, immersion heater, beaker/block, ammeter, voltmeter, power supply, digital balance, stopwatch.
Method: record mass, initial temperature, then heat while recording current, voltage, and temperature every 60 s for 10 min.
Calculate \Delta E = IV\Delta t and c = \Delta E / (m \Delta \θ).
Systematic errors: zero balance, account for evaporation; random errors: stir water, be consistent with fluctuating readings.

Apparatus for investigating specific heat capacity

Melting & Boiling

Fixed points of pure water: melting point = 0 °C, boiling point = 100 °C at atmospheric pressure.
During a change of state (melting/boiling), temperature remains constant even though energy is still being transferred.
Energy goes into overcoming intermolecular forces (potential energy), not increasing kinetic energy.
Boiling occurs at the boiling point throughout the liquid; melting occurs at the melting point as solid becomes liquid.
Condensation (gas→liquid) and solidification (liquid→solid) also occur at constant temperature as energy is removed.

Evaporation

Evaporation is a change from liquid to gas that occurs at any temperature and only from the surface.
More energetic molecules near the surface escape, reducing the average kinetic energy → liquid cools.
Rate of evaporation increases with: higher temperature, larger surface area, and greater air movement.
Evaporation vs boiling: evaporation at any temperature, only at surface; boiling at boiling point, throughout liquid.

Think like a scientist

Use a teacher-provided heating dataset to compare temperature changes for equal masses of different materials given equal energy inputs.
Comparison: the material heated. Outcome: the temperature rise in degrees Celsius.
Control: keep the sample mass and energy input constant. Explain why this makes the comparison fairer.
Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

Thermal properties help choose materials for heating and cooling systems.
Heat absorbed by the container and lost to surroundings must be considered when estimating specific heat capacity.
Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Slides

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Practice questions

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1.What are the upper and lower fixed points on a Celsius scale thermometer?
Easy
- AThe boiling point of pure water and the freezing point of pure water
- BThe boiling point of saturated salt solution and the freezing point of salt-water
- CThe normal temperature of the human body and the freezing point of salt-water
- DThe boiling point of pure water and absolute zero
2.Which statement best describes a metal experiencing thermal expansion?
Easy
- AThe molecules increase in size.
- BThe molecules vibrate more and move further apart.
- CThe molecules expand and become larger.
- DThe molecules decrease in density.
3.When a substance is melting, what happens to its temperature and internal energy?
Medium
- ATemperature increases, internal energy increases.
- BTemperature remains constant, internal energy increases.
- CTemperature remains constant, internal energy decreases.
- DTemperature increases, internal energy remains constant.
4.Which of the following is a correct unit for specific heat capacity?
Easy
- AJ / kg
- BJ / °C
- CJ / (kg °C)
- DJ kg °C
5.The specific heat capacity of water is 4200 J/(kg °C). How much energy is required to raise the temperature of 2.0 kg of water by 5.0 °C?
Medium
- A42000 J
- B21000 J
- C8400 J
- D16800 J
6.In an experiment to determine the specific heat capacity of a metal block, a student uses an immersion heater. Which of the following is the correct equation to calculate the energy supplied to the block?
Easy
- AE = IVt
- BE = IRt
- CE = V/It
- DE = I²Vt
7.A student investigates the cooling of water. The results show that the rate of cooling is greater when the water is hotter. Which statement explains this?
Medium
- AHotter water has a higher specific heat capacity.
- BHotter water evaporates more quickly, removing more thermal energy.
- CHotter water expands, increasing its surface area.
- DHotter water has a lower density, so it rises and cools faster.
8.Which of the following factors increases the rate of evaporation of a liquid?
Easy
- ADecreasing the temperature of the liquid
- BDecreasing the surface area of the liquid
- CIncreasing the movement of air above the liquid
- DIncreasing the humidity of the air above the liquid

Revision notes

## Big idea: specific heat capacity and changes of state - **Key concept: Change.** Energy transferred equals mass times specific heat capacity times temperature change. During a phase change, energy changes particle arrangements without necessarily raising temperature. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Thermal properties help choose materials for heating and cooling systems. ## Thermal Expansion - When a material is heated at constant pressure, its **temperature increases**, **volume expands**, and **density decreases**. - Expansion occurs because molecules gain kinetic energy, vibrate/move faster, and push each other apart. - **Solids expand the least**, **gases expand the most**, and **liquids** are in between. - Useful applications: **liquid-in-glass thermometers** (liquid expands in capillary tube) and **bimetallic strips** (two metals expand at different rates, bending to close a circuit). - Undesirable consequences: **buckling** of railway tracks, road surfaces, and bridges; gaps are built in to allow expansion. ## Specific Heat Capacity - **Internal energy** is the total energy stored in a system due to particle motion (kinetic) and positions (potential). - Temperature is a measure of the **average kinetic energy** of particles. - **Specific heat capacity (c)** is the energy required to raise the temperature of 1 kg of a substance by 1 °C. - Low c → heats up/cools down quickly; high c → heats up/cools down slowly. - Equation: \( \Delta E = m c \Delta \theta \), where \(\Delta E\) in J, m in kg, c in J/kg°C, \(\Delta \theta\) in °C. ## Investigating Specific Heat Capacity - Aim: determine specific heat capacity by measuring energy supplied (E = IVt) and temperature change. - Independent variable: **time**; dependent variable: **temperature**; control: material, current, voltage. - Equipment: thermometer, immersion heater, beaker/block, ammeter, voltmeter, power supply, digital balance, stopwatch. - Method: record mass, initial temperature, then heat while recording current, voltage, and temperature every 60 s for 10 min. - Calculate \(\Delta E = IV\Delta t\) and \(c = \Delta E / (m \Delta \theta)\). - Systematic errors: zero balance, account for evaporation; random errors: stir water, be consistent with fluctuating readings. ## Melting & Boiling - Fixed points of pure water: **melting point = 0 °C**, **boiling point = 100 °C** at atmospheric pressure. - During a change of state (melting/boiling), **temperature remains constant** even though energy is still being transferred. - Energy goes into **overcoming intermolecular forces** (potential energy), not increasing kinetic energy. - **Boiling** occurs at the boiling point throughout the liquid; **melting** occurs at the melting point as solid becomes liquid. - **Condensation** (gas→liquid) and **solidification** (liquid→solid) also occur at constant temperature as energy is removed. ## Evaporation - Evaporation is a change from **liquid to gas** that occurs **at any temperature** and **only from the surface**. - More energetic molecules near the surface escape, reducing the average kinetic energy → **liquid cools**. - Rate of evaporation increases with: **higher temperature**, **larger surface area**, and **greater air movement**. - Evaporation vs boiling: evaporation at any temperature, only at surface; boiling at boiling point, throughout liquid. ## Think like a scientist - Use a teacher-provided heating dataset to compare temperature changes for equal masses of different materials given equal energy inputs. - **Comparison:** the material heated. **Outcome:** the temperature rise in degrees Celsius. - **Control:** keep the sample mass and energy input constant. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Thermal properties help choose materials for heating and cooling systems. - Heat absorbed by the container and lost to surroundings must be considered when estimating specific heat capacity. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Melting & Boiling: Fixed points of pure water: **melting point = 0 °C**, **boiling point = 100 °C** at atmospheric pressure.
Thermal Expansion: When a material is heated at constant pressure, its **temperature increases**, **volume expands**, and **density decreases**.
Investigating Specific Heat Capacity: Aim: determine specific heat capacity by measuring energy supplied (E = IVt) and temperature change.
Specific Heat Capacity: **Internal energy** is the total energy stored in a system due to particle motion (kinetic) and positions (potential).
Evaporation: Evaporation is a change from **liquid to gas** that occurs **at any temperature** and **only from the surface**.
Designing scientific inquiries: Plan a fair comparison: Use a teacher-provided heating dataset to compare temperature changes for equal masses of different materials given equal energy inputs.
Evaluating evidence and impacts: Thermal properties help choose materials for heating and cooling systems. Heat absorbed by the container and lost to surroundings must be considered when estimating specific heat capacity.
temperature increases: When a material is heated at constant pressure, its **temperature increases**, **volume expands**, and **density decreases**.
volume expands: When a material is heated at constant pressure, its **temperature increases**, **volume expands**, and **density decreases**.
density decreases: When a material is heated at constant pressure, its **temperature increases**, **volume expands**, and **density decreases**.
Solids expand the least: **Solids expand the least**, **gases expand the most**, and **liquids** are in between.
gases expand the most: **Solids expand the least**, **gases expand the most**, and **liquids** are in between.
liquids: **Solids expand the least**, **gases expand the most**, and **liquids** are in between.
liquid-in-glass thermometers: Useful applications: **liquid-in-glass thermometers** (liquid expands in capillary tube) and **bimetallic strips** (two metals expand at different rates, bending to close a circuit).
bimetallic strips: Useful applications: **liquid-in-glass thermometers** (liquid expands in capillary tube) and **bimetallic strips** (two metals expand at different rates, bending to close a circuit).
buckling: Undesirable consequences: **buckling** of railway tracks, road surfaces, and bridges; gaps are built in to allow expansion.

Flashcards

What are the upper and lower fixed points on a Celsius scale thermometer?

The boiling point of pure water and the freezing point of pure water On the Celsius scale, the upper fixed point is the boiling point of pure water (100°C) and the lower fixed point is the freezing point of pure water (0°C).

Which statement best describes a metal experiencing thermal expansion?

The molecules vibrate more and move further apart. Thermal expansion occurs because molecules vibrate more as they gain kinetic energy, causing them to push each other apart. The molecules themselves do not increase in size.

When a substance is melting, what happens to its temperature and internal energy?

Temperature remains constant, internal energy increases. During melting, energy is used to overcome intermolecular forces, so temperature stays constant while internal energy increases because potential energy increases.

Which of the following is a correct unit for specific heat capacity?

J / (kg °C) Specific heat capacity is energy per unit mass per unit temperature change, so the unit is J/(kg °C) or J/kg°C.

The specific heat capacity of water is 4200 J/(kg °C). How much energy is required to raise the temperature of 2.0 kg of water by 5.0 °C?

42000 J Using ΔE = mcΔθ = 2.0 × 4200 × 5.0 = 42000 J.

In an experiment to determine the specific heat capacity of a metal block, a student uses an immersion heater. Which of the following is the correct equation to calculate the energy supplied to the block?

E = IVt The electrical energy supplied is given by E = IVt, where I is current, V is potential difference, and t is time.

A student investigates the cooling of water. The results show that the rate of cooling is greater when the water is hotter. Which statement explains this?

Hotter water evaporates more quickly, removing more thermal energy. At higher temperature, more energetic molecules escape by evaporation, removing more thermal energy, so the rate of cooling is greater.

Which of the following factors increases the rate of evaporation of a liquid?

Increasing the movement of air above the liquid Increased air movement carries away water vapour, replacing it with drier air, which increases the rate of evaporation.

Which of the following is a difference between boiling and evaporation?

Boiling occurs throughout the liquid, evaporation only at the surface. Boiling happens throughout the liquid (bubbles form), while evaporation occurs only at the surface.

Which state of matter expands the most when heated at constant pressure?

Gas Gases expand the most because their particles have high energy and can completely overcome intermolecular forces, allowing them to move apart significantly.

A student measures the temperature of water as it is heated. The temperature rises steadily until it reaches 100°C, then remains constant even though heating continues. Which statement explains the constant temperature?

The energy supplied is being used to overcome intermolecular forces during boiling. At boiling point, additional thermal energy goes into overcoming intermolecular forces (potential energy), not increasing kinetic energy, so temperature remains constant.

A student investigates how the rate of cooling of water depends on the uncovered surface area. She uses two lids: lid A leaves half the surface uncovered, lid B leaves a quarter uncovered. She finds that the water cools faster with lid A than with lid B. Which lid gives a greater rate of cooling?

Lid A, because the temperature drops more quickly A larger uncovered surface area allows more evaporation, so lid A (half uncovered) cools the water faster than lid B (quarter uncovered).

In an experiment to determine the specific heat capacity of a metal block, a student uses an immersion heater. The energy supplied to the block is calculated using the equation E = IVt. Which additional measurements are needed to calculate the specific heat capacity?

Mass of the block and temperature change Specific heat capacity c = ΔE/(mΔθ). ΔE is found from IVt, so mass m and temperature change Δθ are required.

A student cools water from 70 °C and records temperature every 30 seconds. The results show that the temperature difference between the water and the room decreases over time. Which statement best explains why the rate of cooling decreases?

The temperature difference between the water and the surroundings decreases The rate of heat transfer depends on the temperature difference between the object and its surroundings. As the water cools, this difference decreases, so the rate of cooling decreases.

The fixed points on a Celsius thermometer are based on the properties of pure water. What are these fixed points?

The melting point of ice and the boiling point of water The upper fixed point is the boiling point of pure water (100 °C) and the lower fixed point is the melting point of pure ice (0 °C).

A pure substance is heated at a constant rate. Why does its temperature remain constant during the first plateau?

Energy overcomes attractions as the solid melts During melting, energy changes the particle arrangement by overcoming attractions rather than increasing average kinetic energy.

How would you design the inquiry for specific heat capacity and changes of state?

Use a teacher-provided heating dataset to compare temperature changes for equal masses of different materials given equal energy inputs. Compare the material heated; record the temperature rise in degrees Celsius; keep the sample mass and energy input constant.

What limitation matters when applying specific heat capacity and changes of state?

Heat absorbed by the container and lost to surroundings must be considered when estimating specific heat capacity.

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Time, t (s)	Temperature, θ (°C)
0	84
60	75