The pH Scale
What is the pH scale?
- The pH scale goes from 1 – 14
- All acids have pH values of below 7, all alkalis have pH values of above 7
- The lower the pH then the more acidic the solution is
- pH 0-2 = strong acid
- Extremely acidic substances can have values of below 1
- pH 3-6 = weak acid
- pH 0-2 = strong acid
- The higher the pH then the more alkaline the solution is
- pH 8-11 = weak alkali
- pH 12-14 = strong alkali
- A solution of pH 7 is described as being neutral
The pH Scale
The pH scale showing acidity, neutrality and alkalinity
What are some common acids and alkalis?
Table of common acids and alkalis
| Common Acids | Common Alkalis |
| Sulfuric acid, H2SO4 | Sodium hydroxide, NaOH |
| Hydrochloric acid, HCl | Ammonia, NH3 |
| Vinegar (ethanoic acid), CH3COOH | Baking powder |
How do we measure pH?
- Universal indicator is a wide range indicator and can give only an approximate value for pH
- It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution
- A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours
- Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation
Colour range of universal indicator
Worked example
The following table shows the pH of some substances.
| Substance | pH |
| lemon juice | 2.2 |
| limewater | 10.5 |
| saliva | 6.3 |
| milk of magnesia | 10.1 |
- Name the strongest alkali
- Name the strongest acid
- Name the substances which is closest to neutral
Answer
i) Limewater has the highest pH in the table of 10.5 so is the strongest alkali
ii) Lemon juice has the lowest pH in the table of 2.2 so is the strongest acid
iii) Saliva is the closest substance to being neutral as its pH value of 6.3 is closest to pH 7
