Learn what makes a substance acidic or alkaline, how the pH scale works, what happens during neutralisation, and how indicators help you identify acids and bases — with fully worked exam examples throughout.
An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. Acids taste sour, react with metals to produce hydrogen gas, and turn blue litmus paper red. Common acids you encounter in everyday life include vinegar (ethanoic acid), lemon juice (citric acid), and stomach acid (hydrochloric acid).
A base is a substance that can neutralise an acid. Bases feel soapy, turn red litmus paper blue, and release hydroxide ions (OH⁻) when dissolved in water. When a base dissolves in water, it is called an alkali. Sodium hydroxide, potassium hydroxide, and ammonia solution are common alkalis you will meet in your chemistry course.
The key distinction to remember: all alkalis are bases, but not all bases are alkalis. A base only becomes an alkali if it actually dissolves in water.
The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14. A pH below 7 means the solution is acidic, a pH of exactly 7 means neutral, and a pH above 7 means alkaline.
The scale is logarithmic, which means each step represents a tenfold change in concentration. pH 3 is ten times more acidic than pH 4, and one hundred times more acidic than pH 5. You do not need to calculate this at O Level, but understanding it helps you answer comparison questions correctly.
An indicator is a substance that changes colour depending on whether the solution is acidic, neutral, or alkaline. The two most common indicators you need to know are litmus and universal indicator.
Litmus only tells you whether a solution is acidic or alkaline — it cannot tell you the exact pH value. For that, you need universal indicator.
Universal indicator gives a range of colours from red (strongly acidic) through green (neutral) to dark purple (strongly alkaline). Match the colour against the printed colour chart to read off the approximate pH. In the lab, universal indicator paper or solution both work.
When an acid reacts with a base, the two substances cancel each other out and produce a salt and water. This is called a neutralisation reaction.
Acid + Base → Salt + Water
The salt produced depends on which acid and base you use. The name of the salt comes from the base (first part) and the acid (second part). For example, hydrochloric acid always produces chloride salts, sulfuric acid always produces sulfate salts, and nitric acid always produces nitrate salts.
Question: Write the word equation for the reaction between hydrochloric acid and sodium hydroxide. Name the salt produced.
Word equation:
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
Salt produced: Sodium chloride (NaCl) — common table salt.
Explanation: Sodium comes from the base (sodium hydroxide). Chloride comes from the acid (hydrochloric acid). The H⁺ from the acid and the OH⁻ from the base combine to form water.
Question: Write the balanced symbol equation for sulfuric acid reacting with sodium hydroxide.
Unbalanced: H₂SO₄ + NaOH → Na₂SO₄ + H₂O
Step 1 — Count atoms on each side:
Step 2 — Balance sodium first: Put a 2 in front of NaOH on the left.
H₂SO₄ + 2NaOH → Na₂SO₄ + H₂O
Step 3 — Check hydrogen: Left = 2 + 2 = 4. Right = 2. Put a 2 in front of H₂O.
Balanced equation: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Question: A student adds sodium hydroxide solution drop by drop to hydrochloric acid. The initial pH is 2. What happens to the pH as more alkali is added? What pH is reached at the end point?
Answer: The pH starts at 2 (strongly acidic). As alkali is added, H⁺ ions are progressively neutralised by OH⁻ ions, so the pH rises. When exactly enough alkali has been added to neutralise all the acid, the pH reaches 7 (neutral). If excess alkali is added beyond this point, the pH rises above 7 into the alkaline range.
Question: A solution turns universal indicator orange-red. Is the solution acidic, neutral, or alkaline? Estimate its pH.
Answer: Orange-red on a universal indicator chart corresponds to a pH of approximately 3–4. The solution is acidic.
Question: Name the salt produced when nitric acid reacts with potassium hydroxide. Write the word equation.
Word equation: Nitric acid + Potassium hydroxide → Potassium nitrate + Water
Salt: Potassium nitrate — potassium from the base, nitrate from nitric acid.
Once you can name salts confidently and write balanced neutralisation equations, this topic becomes one of the most straightforward in O Level chemistry. Practise with different acid-base combinations until naming salts feels automatic.