AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The three states of matter: predict the states of substances at different temperatures given appropriate data
Every substance has a melting point and a boiling point at a stated pressure.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every substance has a melting point and a boiling point at a stated pressure. Given data, compare the temperature with these values. Below the melting point the substance is solid; at or above the melting point but below the boiling point it is liquid; at or above the boiling point it is gas. For example, if a metal melts at 660 °C and boils at 2470 °C, then at 25 °C it is solid, at 1000 °C it is liquid and at 3000 °C it is gas. At exactly the melting point, solid and liquid coexist; at exactly the boiling point, liquid and gas coexist. Pressure matters, so check whether the data apply to the pressure in the question.
What earns the marks
- Identify the melting point and boiling point of the substance from the data provided.
- Compare the given temperature with the melting point to decide whether the substance is solid or has melted.
- Compare the given temperature with the boiling point to decide whether the substance is liquid or has boiled.
- State the predicted state clearly, using the correct term solid, liquid or gas.
- Recognise that at the melting point solid and liquid are both present, and at the boiling point liquid and gas are both present.
- Check that the pressure stated in the data matches the pressure in the question before predicting.
Common mistakes and how to put them right
Using the wrong inequality.
Put it right by writing the order solid < melting point ≤ liquid < boiling point ≤ gas.
Ignoring the pressure at which the melting and boiling points were measured.
Put it right by checking the data label and only predicting for that pressure.
Treating a temperature exactly equal to the melting point as only liquid.
Put it right by stating that solid and liquid coexist at that temperature.
Exam tips
- Underline the melting point and boiling point in the data before you start.
- Write the comparison as an inequality, for example 25 °C < 660 °C, so your reasoning is visible.
- If the question gives several temperatures, answer each one separately and name the state for each.
Try the lesson
The start of the The three states of matter: predict the states of substances at different temperatures given appropriate data lesson, made from checked teaching notes for this specification point.
Have a go first
Substance X melts at 20 °C and boils at 90 °C. What is the state of substance X at 50 °C?
Listen
Every substance has a melting point and a boiling point at a stated pressure. Given data, compare the temperature with these values. Below the melting point the substance is solid; at or above the melting point but below the boiling point it is liquid; at or above the boiling point it is gas. For example, if a metal melts at 660 °C and boils at 2470 °C, then at 25 °C it is solid, at 1000 °C it is liquid and at 3000 °C it is gas. At exactly the melting point, solid and liquid coexist; at exactly the boiling point, liquid and gas coexist. Pressure matters, so check whether the data apply to the pressure in the question. A strong answer earns marks for these points. Identify the melting point and boiling point of the substance from the data provided. Compare the given temperature with the melting point to decide whether the substance is solid or has melted. Compare the given temperature with the boiling point to decide whether the substance is liquid or has boiled. State the predicted state clearly, using the correct term solid, liquid or gas. A common slip: Using the wrong inequality: a temperature below the melting point is solid, not liquid. Instead, writing the order solid < melting point ≤ liquid < boiling point ≤ gas. Exam tip: Underline the melting point and boiling point in the data before you start.
Key words
Say what each word means before you turn the card.
Melting point
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Boiling point
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Comparing temperature with fixed points
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Effect of pressure on change of state
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See every definition
- Melting point
- The fixed point at which a substance changes between solid and liquid, with solid and liquid coexisting at exactly that temperature.
- Boiling point
- The fixed point at which a substance changes between liquid and gas, with liquid and gas coexisting at exactly that temperature.
- Comparing temperature with fixed points
- Checking a given temperature against the melting point and boiling point to decide whether a substance is solid, liquid or gas.
- Effect of pressure on change of state
- Melting and boiling points are stated at a pressure, so the data must apply to the pressure in the question before predicting.
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