Indicators and pH testing

An indicator is a substance that changes colour depending on whether the solution it is placed in is acidic, neutral, or basic. The three indicators you must know by name and colour:

Each of these is a single indicator — it only tells you whether a solution is acidic, basic, or neutral, never how strongly. A universal indicator solves that limit: it is a carefully blended mixture of several dyes that shifts through a graded rainbow of colours as acidity or basicity increases, and it is read against a printed colour chart to estimate an approximate pH value. The pH scale runs from 0 to 14 and expresses how acidic or basic a solution is, based on the hydrogen-ion concentration:

where: $[\text{H}^+]$ is the molar concentration of hydrogen ions in the solution, and the negative logarithm turns a very small, awkward concentration number into a convenient scale running roughly 0 to 14. At this level the formula is used only conceptually, to see that a higher $\text{H}^+$ concentration gives a lower pH number — it is never evaluated numerically from a given concentration in this syllabus.

pH matters well beyond the laboratory. Soil pH affects which crops grow well — most prefer close to neutral soil, and strongly acidic or alkaline soils need chemical treatment before sowing. Digestion relies on the strongly acidic stomach (gastric juice, roughly pH 1.5–3.5) to activate digestive enzymes and kill pathogens in food. Tooth decay is linked to pH — bacteria ferment sugar left on teeth into acids that lower the pH on the enamel surface, and below about pH 5.5 the enamel begins to demineralise. Blood pH is tightly held near 7.4, slightly alkaline; even a small deviation from this narrow band is dangerous. All of these are examined qualitatively — you are expected to relate the situation to the pH scale, not to calculate a value.

The reasoning behind indicators and the pH scale rests on three linked ideas that are easy to blur together.

Step 1. An indicator's colour change is a chemical response, not a label. Litmus, phenolphthalein, and methyl orange are themselves weak acids or bases whose own molecular structure — and hence the colour of light they absorb — changes shape when they gain or lose $\text{H}^+$ in an acidic or basic environment. This is why the same indicator gives a reliable, repeatable colour in acid versus base: it is reacting, not guessing.

Step 2. A single indicator carries only one bit of information — acidic, basic, or neutral — because it has essentially one colour-change threshold. A universal indicator is deliberately built from several dyes with different thresholds layered together, so the blended colour shifts smoothly across the whole range and can be matched to a chart calibrated in pH units. This is exactly why litmus alone can never tell you "how strongly" acidic something is, while universal indicator can.

Step 3. The pH number itself is built to run opposite to hydrogen-ion concentration because of the minus sign and the logarithm in $\text{pH} = -\log_{10}[\text{H}^+]$: as $[\text{H}^+]$ rises, its logarithm rises too, but the leading minus sign flips the direction, so pH falls. This single design choice is the source of the rule "lower pH means more acidic" — it is not an arbitrary convention but a direct consequence of the formula's structure, even though you are not asked to compute with it here.

Question: A student is given three unlabelled colourless solutions — dilute HCl, dilute NaOH, and pure water — plus four solutions with known universal-indicator readings: P = pH 2, Q = pH 7, R = pH 10, S = pH 5. (a) Using only litmus paper, identify the first three solutions. (b) Arrange P, Q, R, S in order of increasing acidity and name the neutral one and the strongest acid.

Step 1. Dip a strip of blue litmus and a strip of red litmus into each of the first three solutions separately and note any colour change. Solution A turns blue litmus red while red litmus stays red, so A is acidic — this must be the dilute HCl. Solution B turns red litmus blue while blue litmus stays blue, so B is basic — this must be the dilute NaOH. Solution C produces no colour change in either strip, so C is neutral — this must be the pure water.

Step 2. For part (b), recall the rule fixed by the formula: a lower pH number means a higher $\text{H}^+$ concentration and stronger acidity; pH = 7 is neutral; a higher pH means stronger basicity. Classify each: P (pH 2) is strongly acidic, S (pH 5) is mildly acidic, Q (pH 7) is neutral, R (pH 10) is basic.

Step 3. Increasing acidity means moving from the highest pH toward the lowest, so the order is $R \rightarrow Q \rightarrow S \rightarrow P$.

Answer: litmus identifies A as dilute HCl (acidic), B as dilute NaOH (basic), C as pure water (neutral); among P, Q, R, S the neutral solution is Q (pH 7) and the strongest acid is P (pH 2), since the lowest pH corresponds to the highest hydrogen-ion concentration.

• Thinking a higher pH number always means "more acidic" — it is the reverse: pH falls as acidity rises, because of the minus sign in $\text{pH} = -\log_{10}[\text{H}^+]$.
• Using litmus alone to claim "this is a strong acid" — litmus only sorts acidic versus basic versus neutral; only a universal indicator with its colour chart can grade strength on the pH scale.
• Trying to calculate a numeric pH value from a given molar $\text{H}^+$ concentration — this lies beyond the ICSE syllabus; the formula is used conceptually only here.
• Assuming pH 7 is always the "ideal" or "healthy" value everywhere — the stomach is deliberately maintained at a very low pH, and blood is deliberately maintained slightly above 7, both for good biological reasons.

Ask the tutor: Want to quiz yourself on indicator colour changes, or talk through why stomach acid sits so low on the pH scale without harming the body?