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Melting point and boiling point
It is useful to know the temperatures at which a substance changes state. For example, a mercury thermometer does not work below -39°C because the mercury inside will be frozen solid, so it will not move up or down the thermometer.
Melting point
The temperature at which a substance melts, turning from a solid into a liquid, is called its melting point. Different substances have different melting points. For example, ice melts at 0°C but ethanol (alcohol) melts at -114°C.
The temperature at which a substance freezes, turning from a liquid to a solid, is the same as its melting point. So, if you want to freeze water into ice you have to cool it down to at least 0°C, but if you want to freeze ethanol you have to cool it down to at least -114°C.
The melting point of pure water is 0°C and its boiling point is 100°C
Boiling point
The temperature at which a substance evaporates as fast as it can, turning from a liquid into a gas, is called its boiling point. Different substances have different boiling points. For example, water boils at 100°C but ethanol boils at 78°C.
The temperature at which a substance condenses, turning from a gas to a liquid, is the same as its boiling point. So, if you want to condense steam into water you have to cool it down to at least 100°C, but if you want to condense ethanol vapour you have to cool it down to at least 78°C.
Which state?
You can easily work out the state of a substance if you know its temperature, its melting point and its boiling point:
- If the temperature is below the melting point, the substance will be a solid.
- If the temperature is between the melting point and the boiling point, the substance will be a liquid.
- If the temperature is above the boiling point, the substance will be a gas.
Have a go at the activity to check your understanding of this.
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