Universal indicator is a mixture of dyes, so it changes colour gradually across the whole range. A single indicator only flips once, over a narrow band - compare them in the chart below.
The pH at which a single indicator changes is its transition range. Outside that range the colour tells you nothing more - phenolphthalein looks identical at pH 1 and pH 7, which is exactly why universal indicator exists.
Every reaction below is examinable in IGCSE 0620 and CBSE Class 10. Each one is shown with the observation and the gas test that identifies the product.
Neutralisation. The H⁺ from the acid and the OH⁻ from the alkali combine to make water - this is why the pH moves towards 7. The reaction is exothermic.
Universal indicator runs green at the neutral point. In a titration you would use phenolphthalein or methyl orange instead, because they give one sharp change.
Only metals above hydrogen in the reactivity series react. Copper does not.
Test: a lighted splint gives a squeaky pop - hydrogen.
Effervescence - brisk fizzing as CO₂ is released.
Test: the gas turns lime water milky. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The reaction behind baking soda fizzing in vinegar, and behind an antacid tablet.
Test: lime water turns milky, as above.
A metal oxide is a base. Copper(II) oxide is a base but not an alkali, because it does not dissolve in water - an IGCSE definition point.
Black CuO disappears and a blue copper(II) sulfate solution forms.
Warming is needed. This is the standard test for an ammonium ion.
Test: the gas turns damp red litmus blue - ammonia is the only common alkaline gas.
The single arrow shows a strong acid - it dissociates completely. The reversible arrow shows a weak acid - only a small fraction of molecules release H⁺ at any moment.
Acids show acidic behaviour only in water - H⁺ ions cannot form without it. Dry HCl gas does not turn dry litmus red.
Dilution is strongly exothermic. Adding water to concentrated acid makes the mixture spit and can crack the container.
Diluting lowers the concentration of H⁺ ions per unit volume, so the pH rises towards 7.
pH is a measure of how many hydrogen ions are in a solution. The scale runs from 0 to 14 and it is logarithmic, so a solution of pH 3 has ten times as many hydrogen ions as one of pH 4. Universal indicator is a mixture of dyes that gives a different colour at each part of the range - useful for a rough value, but a pH meter is what you use when a number matters.
pH 7 neutral · below 7 acidic · above 7 alkaline
Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.