What reactions occur in cation tests with sodium hydroxide?

In cation tests with sodium hydroxide, precipitates form due to reactions between the cation and the hydroxide ion.

When sodium hydroxide is added to a solution containing cations, it reacts with the cations to form a precipitate. This is a key part of cation testing, which is used to identify the presence of specific cations in a solution. The colour and solubility of the precipitate can help to identify the cation.

For example, if a solution contains aluminium ions (Al3+), adding sodium hydroxide will result in a white precipitate of aluminium hydroxide (Al(OH)3). This precipitate will dissolve in excess sodium hydroxide, forming a colourless solution. Similarly, calcium ions (Ca2+) will react with sodium hydroxide to form a white precipitate of calcium hydroxide (Ca(OH)2), which is insoluble in excess sodium hydroxide.

Copper(II) ions (Cu2+) will form a blue precipitate of copper(II) hydroxide (Cu(OH)2), while iron(II) ions (Fe2+) will form a green precipitate of iron(II) hydroxide (Fe(OH)2). Iron(III) ions (Fe3+), on the other hand, will form a brown precipitate of iron(III) hydroxide (Fe(OH)3). All these precipitates are insoluble in excess sodium hydroxide.

Zinc ions (Zn2+) will form a white precipitate of zinc hydroxide (Zn(OH)2) which will dissolve in excess sodium hydroxide to form a colourless solution. This is similar to aluminium ions but zinc can be distinguished from aluminium by adding excess ammonia solution. Zinc hydroxide will dissolve to form a clear solution, while aluminium hydroxide will not.

These reactions are important in analytical chemistry, as they allow chemists to identify the presence of specific cations in a solution. By observing the colour and solubility of the precipitate formed, it is possible to determine which cation is present.