The pH scale (AQA GCSE Chemistry): Revision Notes
The pH scale
What is the pH scale?
The pH scale is a way to measure how acidic or alkaline a solution is. It runs from 0 to 14, like a number line that tells us about different liquids.
Think of it as a scale that helps us understand what's in our solutions!
The pH scale is logarithmic, which means each number represents a 10-fold change in acidity. This makes it a powerful tool for comparing very different solutions on the same scale.
Understanding pH values
The pH scale is divided into three main sections:
Acidic solutions (pH less than 7)
- These solutions taste sour
- They contain lots of H+ ions
- Examples: lemon juice, vinegar
- The lower the number, the more acidic it is
Neutral solutions (pH equals 7)
- These are neither acidic nor alkaline
- Pure water is neutral
- They have equal amounts of H+ and OH- ions
Alkaline solutions (pH more than 7)
- These solutions feel slippery
- They contain OH- ions
- Examples: soap, bleach
- The higher the number, more alkaline it is
Remember that pH 0 is extremely acidic (like battery acid) and pH 14 is extremely alkaline (like household bleach). Most substances we encounter daily fall somewhere between pH 3 and pH 10.
How to measure pH
There are two main ways to find out the pH of a solution:
Universal indicator
- This is a special liquid that changes colour
- Different colours show different pH values
- Easy to use but only gives approximate results
- You compare the colour to a chart
pH probe
- This is an electronic device
- It gives exact pH numbers to one or two decimal places
- More accurate than universal indicator
- More complicated to use
When using universal indicator, always compare the colour immediately after adding it to your solution. The colour can fade or change over time, leading to inaccurate readings.
What happens when acids and alkalis meet?
When you mix an acid with an alkali, something special happens called a neutralisation reaction.
Worked Example: Neutralisation Reaction
The key reaction is:
This always makes water! The acid and alkali cancel each other out.
For example, when potassium hydroxide (an alkali) reacts with nitric acid:
They make potassium nitrate (a salt) plus water.
Why does this matter?
Understanding pH is important because:
- It helps us know if something is safe to touch or drink
- It's used in cooking, cleaning, and medicine
- It helps us understand chemical reactions
Many biological processes depend on maintaining the right pH. For example, our blood must stay at pH 7.4, and our stomach acid is around pH 1.5 to help digest food properly.
Key Points to Remember:
- The pH scale goes from 0 to 14
- Less than 7 = acidic, 7 = neutral, more than 7 = alkaline
- Acids contain H+ ions, alkalis contain OH- ions
- Universal indicator changes colour to show pH
- When acids and alkalis mix, they make water and a salt