Core practical: pH change with calcium hydroxide and calcium oxide Revision Notes | GCSE Edexcel Higher Chemistry

1. What is pH and Why Does it Matter?

You encounter acids and alkalis every day - from the citric acid in oranges to the sodium hydroxide in oven cleaner. But how do we measure just how acidic or alkaline something is? That's where the pH scale comes in.

The pH scale is like a thermometre for acidity. Just as temperature tells us how hot or cold something is, pH tells us how acidic or alkaline a solution is. This matters because pH affects everything from soil fertility to digestion to industrial processes.

2. The pH Scale - Your Chemical Ruler

Picture the pH scale as a ruler with very specific landmarks. At pH 7, you have neutral - pure water sits here. Below 7, solutions become increasingly acidic. Above 7, they become increasingly alkaline.

Here's the key pattern: pH 0-6 = acidic, pH 7 = neutral, pH 8-14 = alkaline. The further from 7, the more extreme the solution becomes.

3. Universal Indicator - The pH Detective

Universal indicator is your chemical detective for measuring pH. Unlike single indicators that only change at one pH, universal indicator is a clever mixture that shows different colours across the entire pH range.

The colour sequence follows a logical pattern: Red (pH 0-3) → Orange (pH 4-5) → Yellow (pH 6) → Green (pH 7) → Blue (pH 8-10) → Purple (pH 11-14). Remember it as 'Roy G. Biv' but with green in the middle for neutral.

Worked example: Using Universal Indicator to Measure pH

A student wants to find the pH of lemon juice using universal indicator solution.

1. Add 2-3 drops of universal indicator to 2 cm³ of lemon juice — Small amounts prevent dilution affecting the result
2. The solution turns red-orange — Universal indicator changes colour immediately on mixing
3. Red-orange corresponds to pH 2-3 on the standard chart — Always use the chart provided - don't guess colours
4. Lemon juice has pH approximately 2-3 (strongly acidic) — Universal indicator gives approximate pH, not exact values

Answer: Lemon juice pH ≈ 2-3 (strongly acidic)

4. Reading the Chemical Landscape

Now you can read the chemical landscape around you. Stomach acid (pH 1-2) is more acidic than lemon juice (pH 2-3). Soap (pH 9-10) is more alkaline than baking soda (pH 8-9). Pure water sits perfectly neutral at pH 7.

This knowledge helps explain everyday observations. Why does soap feel slippery? Because it's alkaline. Why do acids taste sour? Because your taste buds detect the excess hydrogen ions.

5. Exam Technique - Mastering pH Questions

pH questions appear regularly in GCSE exams. Examiners test whether you can identify acidic/alkaline solutions, use universal indicator correctly, and apply pH knowledge to real situations.

Master these fundamentals and you'll confidently tackle any pH question the exam throws at you. The pH scale isn't just numbers - it's your key to understanding the chemical world around you.

Exam precision note: In Understanding the pH Scale - The pH scale and neutralisation - Reactions of acids, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

The pH scale is useful because it combines numerical information with observation. Indicators give a colour that matches a pH range, while a pH meter gives a more precise value. In GCSE questions, you usually need to connect the colour evidence to whether the solution is acidic, neutral, or alkaline.