Heating and cooling curves

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Lesson notes

Heating and Cooling Curves

A **heating curve** shows how temperature changes as a substance is heated.
A **cooling curve** shows how temperature changes as a substance cools.
For a **pure substance**, temperature remains constant during a state change (melting or boiling).
For an **impure substance**, melting and boiling occur over a **range of temperatures**.

Pure vs Impure Substances

A **pure substance** is made from only **one element or compound**.
An **impure substance** contains **more than one element or compound** (has impurities).
Pure substances have a single, sharp melting/boiling point; impure substances melt/boil over a range.

Internal Energy

**Internal energy** is the total energy stored in the particles of a substance.
It consists of **kinetic energy** (movement) and **potential energy** (bonds/forces) stores.
As thermal energy is transferred to a substance, its internal energy increases.

Changes During Heating

When temperature rises, the **kinetic energy** of particles increases.
In a solid, particles **vibrate more** as temperature increases.
During melting, bonds break and temperature stays constant; **potential energy** increases.
The thermal energy absorbed during a state change is called **latent heat** (from Latin for 'hidden').
In liquids and gases, particles **move faster** as temperature rises.

Changes During Cooling

As a gas cools, particles **slow down** and kinetic energy decreases.
During condensation, bonds form and temperature stays constant; potential energy decreases.
During freezing, bonds form and temperature stays constant; potential energy decreases.

Key Points

Temperature remains constant during **state changes** (melting, boiling, freezing, condensing).
The **kinetic energy** of particles changes only when temperature changes.
The **potential energy** of particles changes only during state changes.
Internal energy always increases when heating and decreases when cooling.

Particle arrangement in solid, liquid, and gas. In solids, particles vibrate in fixed positions; in liquids, they slide past each other; in gases, they move rapidly and are far apart.

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Practice questions

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1.What is a pure substance?
Easy
- AA substance made from only one element or compound
- BA substance that contains no impurities
- CA substance that has a fixed melting point
- DAll of the above
2.What is internal energy?
Easy
- AThe total kinetic energy of particles
- BThe total potential energy of particles
- CThe sum of the kinetic and potential energy stores of particles
- DThe energy transferred to a substance when heated
3.During which process does the temperature of a pure substance remain constant?
Easy
- AHeating a solid
- BMelting and boiling
- CCooling a gas
- DHeating a liquid
4.What happens to the kinetic energy of particles when a solid is heated and its temperature rises?
Easy
- AIt decreases
- BIt stays the same
- CIt increases
- DIt is converted to potential energy
5.Which statement about the cooling curve of a pure substance is correct?
Medium
- ATemperature decreases at a constant rate throughout
- BTemperature remains constant during condensation and freezing
- CThe gas phase has the highest temperature at the start
- DThe curve is identical to the heating curve
6.An impure substance melts over a range of temperatures. What does this indicate?
Medium
- AThe substance has a high specific heat capacity
- BThe substance contains more than one element or compound
- CThe substance is a pure compound
- DThe substance is in the gas phase
7.What happens to the potential energy of particles during melting?
Medium
- AIt decreases
- BIt remains constant
- CIt increases
- DIt is converted to kinetic energy
8.On a heating curve for a pure substance, which section shows an increase in kinetic energy without a change in state?
Hard
- AThe horizontal plateau
- BThe sloping line before a plateau
- CThe sloping line after a plateau
- DBoth sloping sections

Revision notes

## Heating and Cooling Curves - **Pure substance**: made from only one element or compound. It has a single melting/boiling point. - **Heating curve**: temperature rises until a state change, then stays constant during the change (e.g., melting, boiling). - **Cooling curve**: temperature falls until a state change, then stays constant during the change (e.g., condensing, freezing). - **Internal energy**: sum of kinetic energy and potential energy of particles. - As temperature rises, kinetic energy increases (particles move/vibrate faster). - During a state change, temperature is constant; bonds break/form, potential energy changes (latent heat). - **Impure substance**: contains more than one element/compound; melts/boils over a range of temperatures. ### Key Points - **Melting**: solid → liquid; bonds break, potential energy increases, temperature constant. - **Boiling**: liquid → gas; bonds break, potential energy increases, temperature constant. - **Latent heat**: thermal energy supplied during state change, 'hidden' because temperature doesn't change. - For impure substances, the heating/cooling curve shows a sloped region instead of a flat plateau.

Pure substance: A substance made from only one element or compound.
Impure substance: A substance containing more than one element or compound; melts/boils over a range of temperatures.
Internal energy: The total kinetic energy and potential energy stores of the particles in a substance.
Latent heat: Thermal energy supplied to a material during a state change, where temperature remains constant; from Latin for 'hidden'.
Heating curve: A graph showing temperature change as a substance is heated; plateaus indicate state changes.
Cooling curve: A graph showing temperature change as a substance is cooled; plateaus indicate state changes.

Flashcards

What is a pure substance?

A pure substance is made from only one element or compound.

What happens to temperature during a state change for a pure substance?

The temperature remains constant during a state change.

What is internal energy?

Internal energy consists of the kinetic energy and potential energy stores of the particles.

What happens to the kinetic energy of particles when a substance increases in temperature?

The kinetic energy store of the particles increases.

What happens to particles in a solid as it increases in temperature?

The vibrations of the particles in the solid increase.

What happens to bonds during melting?

Some bonds are broken during melting.

What happens to the potential energy of particles during melting?

The potential energy store of the particles increases.

What is latent heat?

Latent heat is the thermal energy supplied to a material during a state change, where temperature remains constant. 'Latent' comes from Latin for 'hidden'.

What happens to particles in a liquid or gas as temperature increases?

Particles move more quickly and their kinetic energy store increases.

What happens to bonds during boiling?

Bonds are broken during boiling.

What is an impure substance?

An impure substance contains more than one element or compound (i.e., contains impurities).

How does the melting/boiling of an impure substance differ from a pure substance?

For an impure substance, melting and boiling happen over a range of temperatures, not at a single temperature.

What happens to gas particles as they cool?

Gas particles slow down and their kinetic energy decreases.

What happens to the internal energy of a substance as it increases in temperature?

The internal energy increases.

Why does temperature stay constant during melting of a pure substance?

Because the thermal energy supplied is used to break bonds and increase potential energy, not to increase kinetic energy.

What are the three states of matter in order of increasing temperature on a heating curve?

Solid, liquid, gas.

On a cooling curve, what state changes occur as temperature decreases?

Gas condenses to liquid, then liquid freezes to solid.

What is the difference between a heating curve and a cooling curve?

A heating curve shows temperature increase over time as energy is added; a cooling curve shows temperature decrease over time as energy is removed.

What does a flat section on a heating curve represent?

A state change (melting or boiling) where temperature remains constant.

What does a flat section on a cooling curve represent?

A state change (condensing or freezing) where temperature remains constant.

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