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.

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.

Going to extremes 3 Continue reading

Maths in Chemistry: Getting back to the whole thing


Say you know that you’ve drunk half a bottle of water, and you’ve drunk half a litre of water.
It’s not hard to work out that the bottle’s a one-litre bottle of water.

But what if the percentage is something a bit more complicated?
Say you’ve drunk 28% of a bottle of water, and you’ve so far drunk 421ml?

This is a potential exam question in chemistry.
The maths is a standard formula, and it’s simplest to just learn it and practice with a few examples. Continue reading