- Define and explain The pH Scale in your own words
- Use key terms such as pH accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The pH Scale: use the 1–14 scale to measure how acidic or alkaline a solution is.
Use the 1–14 scale to measure how acidic or alkaline a solution is.
Key ideas
The pH scale measures hydrogen ions
Acids release hydrogen ions (H⁺) in water, and the more hydrogen ions, the lower the pH and the stronger the acid. A solution of pH 3 is more acidic than one of pH 5 — each whole step on the scale is a tenfold change. Pure water sits at pH 7, exactly neutral.
Neutralisation makes salt and water
When an acid meets an alkali, the hydrogen ions from the acid join the hydroxide ions from the alkali to make water: H⁺ + OH⁻ → H₂O. What is left behind is a salt — for example, hydrochloric acid plus sodium hydroxide gives sodium chloride (table salt) and water. Farmers use the same idea when they add lime (an alkali) to neutralise acidic soil.
Key term — pH: A scale from 0 to 14 measuring acidity or alkalinity: below 7 is acidic, 7 is neutral, above 7 is alkaline.
Four unlabelled solutions A, B, C and D give these results with universal indicator: A turns red, B turns green, C turns purple, D turns orange. Identify each as acidic, neutral or alkaline, and put the acids in order of strength.
Red means strongly acidic, so A is a strong acid. Orange means weakly acidic, so D is a weak acid. Green means neutral, so B is neutral. Purple means strongly alkaline, so C is a strong alkali. Lower pH means stronger acid, so A (red) is stronger than D (orange).
Answer: A: strong acid; D: weak acid; B: neutral; C: strong alkali. Acids in order of strength: A then D.
- Saying pH 7 is an acid pH 7 is neutral — neither acidic nor alkaline. Acids are below 7 and alkalis are above 7; pure water is the classic example of pH 7.
- Thinking all acids are dangerous Strength matters more than the word acid: citric acid in oranges and ethanoic acid in vinegar are weak acids we eat safely. It is concentrated strong acids, like concentrated sulfuric acid, that are hazardous.
Practice
(a) Red — vinegar is acidic. (b) It stays blue — soap solution is alkaline.
Lemon juice is more acidic — about ten thousand times more acidic (four pH steps of tenfold change each).
A solution of sodium chloride (salt) in water — the H⁺ and OH⁻ ions have combined to make water.
Litmus only tells you acid or alkali (red or blue), while universal indicator shows a range of colours matching each pH value, so you can estimate the actual pH.
Quick check
Which of these best defines "pH"?
A gardener's soil test shows pH 5 but the crop needs pH 7. What should be added, and why?
Write the word equation for the neutralisation of hydrochloric acid by sodium hydroxide.
- The pH Scale: use the 1–14 scale to measure how acidic or alkaline a solution is.
- The pH scale measures hydrogen ions: Acids release hydrogen ions (H⁺) in water, and the more hydrogen ions, the lower the pH and the stronger the acid.
- alkali: A soluble base with a pH above 7, such as soap solution; alkalis turn red litmus blue.
- Watch out for: saying pH 7 is an acid