Acid and alkali — defined by their ions
Learn both mark-scheme definitions exactly — each names the ION, not just "acidic". An acid is a source of hydrogen ions, , in water; an alkali is a soluble base that is a source of hydroxide ions, , in aqueous solution. The ions are released only once the substance dissolves, which is why acidic or alkaline behaviour needs water. So when asked WHY a solution is acidic, state that it releases ions. means dissolved in water.
The pH scale — 0 to 14, and strength
The pH scale runs from 0 to 14: pH 0-6 is acidic (lower pH = more strongly acidic), pH 7 is neutral, pH 8-14 is alkaline (higher pH = more strongly alkaline). Rough guide: 0-2 strongly acidic, 3-6 weakly acidic, 7 neutral, 8-11 weakly alkaline, 12-14 strongly alkaline. A strong acid or alkali gives a pH nearer the ends of the scale (further from 7) than a weak one at the same concentration. pH measures acidity, not how much is dissolved.
Indicators and universal indicator
An indicator is a substance with a different colour in acid and alkali. The three single indicators give a two-way answer: litmus red in acid, blue in alkali; phenolphthalein colourless in acid, pink in alkali; methyl orange red in acid, yellow in alkali (orange in neutral). Universal indicator is a mixture of dyes showing a RANGE of colours, used to estimate the approximate pH by matching its colour to a chart: red (acidic) to green (pH 7) to purple (alkaline).
Drawn from real examiner reports.
Dry HCl gas is not acidic
Hydrogen chloride behaves as an acid only once it dissolves in water, where it releases hydrogen ions, . Dry HCl gas contains no water, so no free ions are released and dry litmus stays unchanged. Writing that the dry gas itself is acidic, or that it releases without water, loses the mark.
Phenolphthalein: pink means alkali
Phenolphthalein is colourLESS in acid and PINK in alkali (and colourless in neutral). Candidates reverse this and write "pink in acid", or muddle it with another indicator. Anchor it as "pink = alkali". An indicator only scores if BOTH its acid colour and its alkali colour are correct and the right way round — one right, one wrong scores nothing.
Methyl orange is yellow in alkali
Methyl orange is red in acid and yellow in alkali, with orange only as its neutral colour. The examiner-flagged error is giving the alkali colour as "orange" or "red" — only yellow scores. Do not let its neutral colour (orange) leak into the alkali answer. Anchor it as red = acid, yellow = alkali.
Digest: methyl orange in alkali is yellow, not orange or red (Jun23 Q5aii).
"Acidic" is not an explanation
When asked WHY a solution is acidic (e.g. why it turns litmus red), restating "it is an acid" or "it is acidic" is circular and scores zero. Name the cause: the solution contains / releases hydrogen ions, . The same applies to alkalis — a solution is alkaline because it contains hydroxide ions, . Name the ion, not the label.
Strong is not the same as concentrated
"Strong" and "concentrated" describe different things and must not be swapped. Strength is judged from the pH — a stronger acid or alkali has a pH nearer the ends of the scale (further from 7). Concentration is how much acid or alkali is dissolved per unit volume (the amount of solute). Classifying a solution from its pH describes its strength, not its concentration.
Match the indicator to the question
If the question asks only whether a solution is acidic or alkaline, use one indicator (litmus, phenolphthalein, methyl orange) and quote its colour. If it asks HOW acidic, use universal indicator and match its colour to the pH chart: 0-6 acidic, 7 neutral, 8-14 alkaline.
For neutralisation, come back to the ions
When explaining a neutralisation, come back to the ions: hydrogen ions from the acid react with hydroxide ions from the alkali to form water, . The full products are a salt plus water, but this ionic change is the core.
Name the ion, do not restate the observation
For any "explain why it is acidic / alkaline" mark, name the ion responsible — for acids, for alkalis. Restating the observation ("it is acidic", "it turns litmus red") describes the result but does not explain it, and scores nothing.
The core ionic neutralisation equation to memorise:
State symbols: solid, liquid, gas, aqueous (dissolved in water).
Define an acid in terms of ions.
A student has two colourless solutions. One is dilute hydrochloric acid (an acid) and one is dilute sodium hydroxide (an alkali).
Describe how the student could use phenolphthalein to tell which solution is the acid and which is the alkali. State the colour seen in each.
