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.

Edexcel iGCSE chemistry: the tests

Edexcel iGCSE Chemistry testsOn the iGCSE course, at various points along the syllabus, you’re meant to know tests for some important chemicals. Here’s a list of them, guides to doing some of them, and a look at past paper questions.

The guides

You can download simple guides to the tests as pdf files: simple, one-page summaries that put everything you need to know on a single page. Here’s one showing all the colours, and here’s one in black and white.

But before we start: precipitation

Many of these reactions create precipitates, and it’s probably worth covering what that means.

It means that when you mix two chemicals dissolved in water (that is, two solutions), a solid powder appears in the test tube, so the water looks cloudy.

How does precipitation happen?

Well, it’s down to the way chemicals dissolve in water. Let’s start with an ionic chemical, like sodium chloride (salt, formula NaCl). It’s made of sodium ions, Na+, and chloride ions, Cl.

Solid saltWhen dissolves in water, it splits into those ions. They are now free to move around in solution, separately from one another.


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Going to Extremes in GCSE Chemistry

Potassium chlorate & sulphuric acid Deradrian CCASA2Here’s a classic exam technique mistake people make: not specifically answering exactly what a question is asking for. Often, people have basic, works-in-any-situation answers memorised, but they’re not specific enough for the problem. But it’s easy to fix: you can quickly change what you’re doing and get higher marks and grades. Let’s look at how.

Bad Answer:

Going to extremes 1 That answer looks good! Francium’s outermost electron is easy to remove. Nothing about that answer is scientifically incorrect. Why didn’t we get any marks? (There’s a discussion of the science of what this question means below.)

Simple: it’s a stock answer, and it’s not detailed enough.

Francium isn’t just a reactive group 1 element, it’s the most reactive group 1 element. We need an extreme answer that emphasises that.

Going to extremes 4Maybe not like that.

We also need a bit more detail. What exactly makes it easy to pull the outermost electron off of francium?

Perfect Answer:

Here’s a perfect answer, emphasising the ways it’s tailored to give exactly what they want. Not many more facts, but it’s clear about the question it’s answering.

Going to extremes 2

Hint: Think if your answer has enough points in it to get all the marks on the mark scheme. Three marks means three points.

Of course, that’s a perfect answer. You might not be able to say all that much, or have time. Let’s strip that down to a minimal perfect answer: the shortest thing we can write that gets all the marks on this (AQA) paper. You don’t need anything inside the brackets.

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What’s the difference between ammonia and ammonium?

Let’s summarise:

Ammonia ammonium comparison

The main problem with ammonia is you can't put it in a sack.
The main problem with ammonia is you can’t put it in a sack.
  • Ammonia is a chemical, formula NH3. It’s made by the Haber process. At room temperature, it’s a gas, which makes it a real nightmare to carry around: you need a huge tanker. So fertiliser companies turn it into:
  • Ammonium compounds. Ammonium is an ion: a bit of another chemical with a +1 charge and the formula NH4+. Ammonium chemicals have names like ammonium nitrate, ammonium sulphate, and so on. There’s no such thing as ‘pure ammonium’ on its own: it’s always part of another chemical.

So why bother with ammonium chemicals? Because they’re solid powders at room temperature. Much easier to handle: you can put them in sacks and load them up into a lorry. At room temperature, all common ammonium chemicals are water-soluble white powders.
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