Infographic on acids, acid strength and concentration. A selection of acids are shown from strong to weak. A strong acid is one which ionises almost completely in water. A weak acid ionises very little. The acid dissociation constant is a measure of the strength of an acid, and can also be expressed logarithmically to make comparisons between acids easier. Concentration is a measure of the amount of acid in a particular volume, and is distinct from strength.
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Even if you’re not a chemist, you’ll doubtless remember learning about acids back in school. They’re routinely described as strong or weak, concentrated or dilute. But what’s the difference between a strong acid and a concentrated acid? Explaining that is a little trickier than it sounds; in this graphic, we give it a go!

It’s easy to forget that a large number of the acids we use are solutions – the acid dissolved in water. Though we’ll often use them in this form, acids can come as solids (such as citric acid and tartaric acid), liquids (such as ethanoic acid, the acid in vinegar), and gases. One of the acids we’re most familiar with, hydrochloric acid, is a solution of hydrogen chloride gas in water.

Acid strength is related to how acids behave when they’re in water. When added to water, the acid will react with the water molecules to form ions. The presence of hydrogen ions produced by this reaction (well, technically hydronium ions, H3O+) is what causes acidity. Acid strength is related to the degree to which this reaction occurs.

A strong acid will react with water and end up essentially 100% ionised in the solution. If you could peer into the solution at the molecular level, you’d see next to no acid molecules remaining; they’d all have split up and formed ions. A weak acid, on the other hand, hardly ionises at all, and you’d still see the acid molecules in their original form if you could look closely enough.

So that chemists can compare the strength of one acid to another, they use the concentrations of the acid and the ions produced by its reaction with water to calculate the acid dissociation constant, Ka. This constant then gives a numerical value to the degree to which the acid ionises. The higher the value of Ka, the more the acid ionises in water, and the stronger the acid.

Because the Kscale can wind up with some fairly small numbers, it can be tricky to compare acid strengths easily using it. For this reason, we can also convert the number to another logarithmic scale, referred to as pKa. This scale churns out much more convenient numbers, though its relation to acid strength is the other way around to that of Ka. In pKa‘s case, the lower its value, the stronger the acid.

How, then, does this relate to the concentration of an acid? Is a strong acid the same as a concentrated one? Well, not quite. In fact, it’s possible to have a concentrated strong acid – but it’s also possible to have a concentrated weak acid. This is because concentration refers simply to how much of the acid is present in a given volume of water, and how much the acid ionises in water is largely irrelevant to this. A concentrated acid contains a large amount of acid in a given volume; a dilute acid contains a small amount of acid in a given volume.

You might recall that the pH scale can be used to gauge the concentration of an acid. This scale is actually directly linked to the amount of hydrogen ions in a solution. The greater the number of hydrogen ions in a given volume, the lower the pH will be (the pH scale typically runs from 0 to 14). Because the scale is logarithmic, for every drop of one whole unit in pH, the hydrogen ion concentration increases tenfold.

In terms of acid safety, both strength and concentration are important, but concentration has greater significance. Strange though it may seem, a concentrated weak acid can actually pose a greater risk than a dilute strong acid, which is why it’s important to be aware of the difference between the two!

If you want to find out more about acids, there’s an older post on the pH scale here, as well as a closer look at some of the acids found in fruits here. Additionally, for chemistry students, there’s also a titrations guide!

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