9.03.1 pH & neutralisation

Syllabus

  • Acids produce Hydrogen ions (H+) in aqueous solutions.
  • Aqueous solutions of alkalis contain Hydroxide ions (OH– ).
  • The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.
  • A solution with pH 7 is neutral.
  • Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7.
  • In neutralisation reactions between an acid and an alkali, Hydrogen ions react with Hydroxide ions to produce water.
  • This reaction can be represented by the equation: H+ (aq) + OH- (aq) --> H2O (l)

Students should be able to:

  • describe the use of universal indicator or a wide range indicator to measure the pH of a solution
  • use the pH scale to identify acidic or alkaline solutions.

What does this mean?

pH scale

In Years 7 & 8 we learn that acids are sharp-tasting but not always unpleasant.

And that alkalis feel soapy to the touch and neutralise (cancel out) acids if added in the correct amount.

You will also have used Universal Indicator and probably made an indicator from elderberries, red cabbage or beetroot.

And you will have learned something about the pH scale.

This runs from 1 (Strongly Acidic) to 14 (Strongly Alkaline) with neutral at 0.

The further away from pH7, the stronger the acid (or alkali)

The scale to the right is what you would expect to see for each pH with Universal Indicator.


Notice that it is in "rainbow-order" - but the colours vary since Universal Indicator is a mixture of simpler indicators and each mixture may be a little different.

If we wanted to get a very accurate pH value for a solution then more accurate versions of Universal Indicator exists, usually in the form of sticks with several bands of colour.

Or you could simply invest in a pH probe - these are fiddly and quite expensive.

What are acids?

Acids are substances that produce Hydrogen ions (H+) in water.

An ion is simply a particle with a positive or negative charge.

The stronger the acid, the better it is at producing H+ ions.

HCl is the formula of Hydrochloric acid.

Dissolved in water it all breaks down into H+ and Cl- ions

HCl --> H+ + Cl-

For this reason, Hydrochloric acid is very strong

It has a low pH.

What are Bases? What are alkalis?

Any substance that neutralises an Acid is a Base.

Some bases dissolve in water to produce Hydroxide ions (OH-).

These bases are called Alkalis.

The stronger the alkali, the better it is at producing OH- ions.

NaOH is the formula of Sodium Hydroxide.

Dissolved in water it all breaks down into Na+ and OH- ions

NaOH --> Na+ + OH-

For this reason, Sodium Hydroxide is a very strong alkali.

It has a high pH

Why is water neutral?

When water breaks down into ions it forms equal amounts of H+ (acid) and OH- (alkali).

So, it is impossible for pure water not to be neutral.

H2O --> H+ + OH-

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