Chemguide: Support for CIE A level Chemistry

These statements are about the trends in volatility and colour of the halogens as you go down the Group.

Before you go on, you should find and read the statements in your copy of the syllabus.

Volatility - statements 9.4(a) and 9.4(b)

Statement 9.4(a) asks you to describe the trend in volatility for the three halogens chlorine, bromine and iodine; statement 9.4(b) asks you to explain it. It might be safer to include fluorine as well in case CIE ask you to make any predictions about it.

You can consider melting points and boiling points as good indicators of volatility. If these are low, then the element will turn easily to a vapour - and that is all that is meant by volatility. If they are higher, then the element is less volatile.

You will find this discussed (along with lots of other atomic properties) on the page about the atomic and physical properties of the halogens.

According to the syllabus, everything you need for these statements is included in the section of the page headed "Trends in Melting Point and Boiling Point".

If you have the time, it would make sense to read the rest of the page as well, though. It would all be good revision.

Colour - statement 9.4(a)

This doesn't need explaining - it is just a matter of fact.

• Fluorine is a pale yellow gas.

• Chlorine is a greenish-yellow gas.

• Bromine is a dark red liquid, forming a reddish-brown gas even at room temperature.

• Iodine is a dark grey solid, forming a purple vapour on gentle heating.

The trend is for darker colours as you go down the Group.

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