Making salts from neutralisation and the insoluble base rule

A common GCSE chemistry question is how you can make a soluble salt. You can make it by mixing up a base and an acid, with the acid supplying the negative ion like chloride or sulphate, and the base supplying the positive (metal) ion. (See our guide to working out the formula of ionic chemicals for a list of common ions.

That gives you this kind of reaction:
And here’s an example of that, making regular salt:
Neutralisation exampleMaking sure there’s no reactant in the final mix

The problem with doing all that is that the starting chemicals are all water soluble. When you’ve finished, there might be reactant left in the mix if you don’t mix everything in the right amounts:
Problem with soluble reactants A way to deal with that is to make the salt using an acid (which is soluble), and an insoluble base. This is an example with calcium carbonate, a base which doesn’t dissolve in water. Look at the state symbols in red:
Insoluble base The only chemical left dissolved in the solution at the end will be the product.

To make sure that happens, you use an excess of base: more than you need to finish off the reaction. That doesn’t matter, because it’s powder: you can just fish it out of the water with filter paper, then evaporate all the salt off:
The insoluble base rule finalSeparating the salt out at the end:

At the end, you’ll need to separate the salt out from the water. You’ll need to say that you:

  1. Filter that excess unreacted base out.
  2. Heat the mix gently to remove some of the water.
  3. Leave the water to evaporate in an evaporating basin.

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