Aims of this Episode:
- Describe the pH scale,
- Explain why certain substances have certain pH values,
- Describe and explain a neutralisation reaction involving an acid and a base,
- Interpret basic reactions and name the products.
The pH Scale
pH is a measure of how acidic or alkali a substance is. Simply, it can help us to decide if a substance is dangerous or not.
Like many things in science, it’s best if we can measure pH accurately. To do this we use the pH scale; this scale runs from one to 14, but also has colours: One (Highly Acidic), Seven (Neutral), 14 (Highly Alkali).
But what does highly acidic actually mean? Well, a substance is classified as an acid if it has Hydrogen Ions (H+) in it; the more Hydrogen Ions there are in a certain amount, the more acidic the substance is, so it will have a lower pH number.
Alkalies work in a similar way except instead of H+ ions, they are called alkalies because of the Hydroxide Ions (OH-) in the substance. The more of these there is, the higher the pH.
Acids will cancel out alkalies, so if you mix an acid and an alkali in the correct amounts, you will get a neutral substance (pH 7).
There are a couple of different ways to measure pH, some give you a colour (which you can then convert to a number) whilst others measure the pH number itself:
- Universal Indicator – this chemical changes colour depending on the pH.
- Litmus Paper – like UI (above) Litmus Paper changes colour.
- Red Cabbage Juice – basically a homemade version of UI.
- pH Meter – a device that gives a very accurate pH number.
Most of the time you will see acids and alkalis in liquid form. As well as the normal liquids there is something called a Base. A Base is a solid (usually a powder) that dissolves (is soluble) to form an alkali.
This is the start of a major practical in GCSE Chemistry:
- Dissolve a base in water to form an alkali,
- Add Universal Indicator (this should turn purple),
- Gradually add acid until the UI turns green,
- Stir in some charcoal to remove the UI,
- Pour the mixture through some filter paper to remove the charcoal,
- Leave the substance overnight to evaporate the water – you will be left with a salt.
In this reaction the acid reacts with the alkali to form water and a salt.
Often you will be asked to name the salt that is produced. This is simply a case of using the name of the metal oxide used in the reaction and the name of the acid. Nitric Acid will form a Nitrate salt etc.