The pH scale (AQA GCSE Chemistry Combined Science): Revision Notes
The pH scale
What is the pH scale?
The pH scale runs from 0 to 14. It tells us how acidic or alkaline a solution is. Think of it as a measuring tool - just like a ruler measures length, the pH scale measures acidity.
pH Scale Colours: The scale has bright colours that help us see the pH value:
- Red/orange colours = very acidic (pH 0-3)
- Yellow colours = slightly acidic (pH 4-6)
- Green colour = neutral (pH 7)
- Blue colours = slightly alkaline (pH 8-10)
- Purple colours = very alkaline (pH 11-14)
Understanding pH values
Acidic solutions have a pH less than 7. The lower the number, the more acidic the solution. Examples include lemon juice and vinegar.
Neutral solutions have a pH of exactly 7. Pure water is neutral - it's neither acidic nor alkaline.
Alkaline solutions have a pH more than 7. The higher the number, more alkaline the solution. Examples include soap and bleach.
Critical pH Ranges to Remember:
- pH < 7 = Acidic
- pH = 7 = Neutral
- pH > 7 = Alkaline
The further from 7, the stronger the acid or alkali!
The science behind pH
Acids work because they release H+ ions (hydrogen ions) when dissolved in water. The more H+ ions present, the more acidic the solution becomes.
Alkalis work because they contain OH- ions (hydroxide ions). The more OH- ions present, the more alkaline the solution becomes.
This is why we can measure pH - we're actually measuring how many of these special ions are in the solution.
How to measure pH
There are two main ways to find the pH of a solution:
Universal indicator is a mixture of dyes that changes colour depending on pH. You add a few drops to your solution and compare the colour to a pH chart. It's easy to use but only gives you an approximate pH value.
pH probe is an electronic device that you place directly in the solution. It's more complicated to use but gives you an accurate pH reading to one or two decimal places.
Choosing Your pH Measurement Method:
- Use universal indicator when you need a quick, approximate result
- Use a pH probe when you need precise, accurate measurements for experiments
Neutralisation reactions
When you mix an acid with an alkali, something special happens. The H+ ions from the acid react with the OH- ions from the alkali.
Worked Example: Neutralisation Reaction
When acids and alkalis react, the hydrogen ions and hydroxide ions combine:
Step 1: Acid releases H+ ions in solution
Step 2: Alkali provides OH- ions in solution
Step 3: H+ and OH- ions react together
Step 4: Water is formed as the product
This means water is always made when acids and alkalis react together.
This reaction is called neutralisation because the acid and alkali cancel each other out, making a neutral solution.
Key Points to Remember:
- The pH scale goes from 0 to 14 and measures how acidic or alkaline solutions are
- pH < 7 = acidic, pH = 7 is neutral, pH > 7 = alkaline
- Acids produce H+ ions, alkalis contain OH- ions
- You can measure pH using universal indicator (easy but approximate) or a pH probe (accurate but more complex)
- When acids and alkalis react, they make water through neutralisation