Chemguide: Core Chemistry 14 - 16


This page explains what a salt is, and looks at the solubilities of the various salts in water.

What is a salt

Each of the following is a salt:

sodium chloride, NaCl

copper(II) sulfate, CuSO4

potassium sulfate, K2SO4

calcium nitrate, Ca(NO3)2

ammonium chloride, NH4Cl

Look at the relationship beween the salts above and the three common acids:

sodium chloride, NaCl, and hydrochloric acid, HCl

copper(II) sulfate, CuSO4, and sulfuric acid, H2SO4

potassium sulfate, K2SO4, and sulfuric acid, H2SO4

calcium nitrate, Ca(NO3)2, and nitric acid, HNO3

ammonium chloride, NH4Cl, and hydrochloric acid, HCl

In each case, the hydrogen ions in the acid have been replaced by metal ions or, in the final example, ammonium ions , NH4+.

The ionic charges have to balance in the salts, of course. That's why you need two K+ ions in potassium sulfate, but only one Cu2+ ion in copper(II) sulfate . . .

. . . and similarly, why you need two nitrate ions NO3- to balance the two charges on the calcium ion in calcium nitrate.

So, a salt is what you get when you replace the hydrogen in an acid by a metal or an ammonium group.

You can't necessarily do that replacement directly. For example, copper won't react with dilute sulfuric acid, and so you have to use a different method of producing the salt, but the end result is the same - the hydrogen in the acid will have been replaced by copper.

The solubilities of salts in water

I am also including hydroxides in this table. Hydroxides aren't salts, but when we are talking about salts, you may well need to know about the solubilities of hydroxides. Carbonates can be considered as the salts of carbonic acid, H2CO3.


soluble in water

insoluble in water

sparingly soluble in water

You need to know all this! But don't panic - if you tease out the patterns, there isn't that much to learn.

Concentrate on the things which are entirely soluble to start with:

  • All sodium, potassium and ammonium compounds are soluble.

  • All nitrates are soluble.

Now look at the things which are mostly soluble, or sparingly soluble:

  • Chlorides are soluble except lead and silver chlorides.

  • Sulfates are soluble except barium and lead. Calcium and silver sulfates are only slightly soluble.

You are very unlikely to come across silver sulfate at this (or any) level, so I wouldn't worry too much about that. You will come across calcium sulfate now and then.

Then there are the things which are mainly or entirely insoluble:

  • Lead compounds tend to be insoluble (apart from the nitrate, but all nitrates are soluble).

  • Carbonates are insoluble (apart from sodium, potassium and ammonium carbonates, but all sodium, potassium and ammonium compounds are soluble).

  • Hydroxides are insoluble (apart from sodium, potassium, ammonium and barium hydroxides. Calcium hydroxide is slightly soluble).

You aren't likely to come across barium hydroxide at this stage. Barium and silver are included in the table mainly because their compounds are involved with testing for sulfates and chlorides. You will meet these tests later.

Note:  I need to point out that some of the generalisations in what you have read aren't strictly accurate. They have been simplified to match what you are likely to meet at this level. For example, rubidium and caesium in Group 1 of the periodic table also have soluble carbonates and hydroxides, but you aren't likely to come across them.

Don't spend too long learning the detail of this now. You will find solubility patterns come up reasonably often during this stage of the course, and you will start to remember quite a lot of this just by repetition. Learn what you need to know as you go along.

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© Jim Clark 2020