Chemistry · Solutions and electrolytes · Grade 7

Acids, bases and indicators

Why does red cabbage juice turn red in lemon and blue in soda?

The situation

In a kitchen in Tashkent, red cabbage has been chopped for a salad — purple juice is left on the board. When lemon juice is dripped on it the stain turns red, and where baking soda is sprinkled it turns blue. Which of the lemon juice, vinegar, water, baking soda, soap and limewater in the kitchen are acids and which are bases? How can we tell without tasting them, and how can an acid be neutralised exactly with a base?

Topic

Acids, bases and indicators *(sample)*. Kitchen laboratory: lemon, baking soda, soap and red cabbage juice

Key ideas

pH scale
Indicators
Neutralisation

Formulas

pH = - \lg [H^{+}]

pH is the negative decimal logarithm of the hydrogen-ion concentration (mol/L). pH 7 is neutral, < 7 is acidic, > 7 is alkaline.

[H^{+}] \cdot [O H^{-}] = 10^{- 14}

The ionic product of water (25 °C): in pure water $[H^{+}] = [O H^{-}] = 10^{- 7}$ mol/L.

HCl + NaOH \overset{}{\to} NaCl + H_{2} O

Neutralisation: acid + base → salt + water; in ionic form: $H^{+} + OH^{-} \overset{}{\to} H_{2} O$ .

CH_{3} COOH + NaHCO_{3} \overset{}{\to} CH_{3} COONa + H_{2} O + CO_{2} ↑

Vinegar and baking soda: neutralisation releases carbon dioxide gas (fizzing).

c_{1} V_{1} = c_{2} V_{2}

Titration: at the equivalence point the amounts (moles) of acid and alkali are equal (for a monobasic acid).

Theory

Acids, bases and pH

In water, acids give hydrogen ions H⁺ (more precisely H₃O⁺), and alkalis give hydroxide ions OH⁻. Pure water itself contains very little of either: at 25 °C $[H^{+}] \cdot [O H^{-}] = 10^{- 14}$ . So it is convenient to express the medium by a single number, pH = $- \lg [H^{+}]$ : pH < 7 is acidic, 7 is neutral, > 7 is alkaline. The scale is logarithmic: if pH falls by 1, H⁺ increases 10 times. Lemon juice (pH ≈ 2.2) has about 60 000 times more H⁺ than pure water.

Indicators

An indicator is a weak organic acid or base whose colour depends on pH: its protonated and unprotonated forms are different colours. Each indicator changes colour within its own range: phenolphthalein 8.2–10 (colourless → raspberry), methyl orange 3.1–4.4 (red → yellow), litmus 4.5–8.3 (red → blue). A universal indicator is a mixture of several indicators; it changes colour across the whole scale and shows pH to within ±1. Phenolphthalein is colourless in an acid and in neutral water alike — it only “recognises” an alkali.

Red cabbage — a natural indicator

Plants such as red cabbage, black grapes, blackcurrants and black mulberries contain anthocyanins — they change colour with pH: red in a strong acid, purple in a neutral medium, blue and green in a weak alkali, and yellow in a strong alkali. That is why red cabbage juice is a “universal” indicator you can make at home. The pH of the samples in the scene: lemon juice 2.0–2.6, vinegar about 2.4, baking soda solution ≈ 8.3, soap solution 9–10, limewater ≈ 12.4.

Neutralisation: vinegar and baking soda

An acid and a base react with each other to form a salt and water — this is neutralisation. When baking soda (NaHCO₃) reacts with ethanoic (acetic) acid, CO₂ gas is also released — dough rising is based on this. 50 mL of 9 % vinegar contains ≈ 0.076 mol of acid; the same number of moles of soda is needed to neutralise it. While there is still excess acid, the solution is a buffer: pH = p $K_{a}$ + lg(acetate/acid), and it changes slowly; when the acid is used up, excess soda makes the medium weakly alkaline (pH ≈ 8.3).

Laboratory: titration

Titration is determining the amount of an unknown acid with an alkali solution of exactly known concentration. NaOH is added drop by drop from a burette, with the solution kept stirred. Near the equivalence point a pink patch appears where each drop falls and vanishes on mixing; the first pale pink colour that does not disappear marks the end of the titration. In a strong acid a single drop makes the pH “jump” from 4 to 10. When vinegar (a weak acid) is titrated, acetate ions remain in the solution at the equivalence point and pH ≈ 8.5–8.7 (depending on dilution), so phenolphthalein is suitable, while methyl orange would change colour far too early. Calculation: $n (NaOH) = c V$ , $m = n M$ , $ω = m / (ρ V_{sample})$ .

Safety and the model

Do not taste acids or alkalis; if an alkali (limewater, NaOH) gets in the eyes, rinse with plenty of water — wear safety goggles. Phenolphthalein solution is alcoholic, so keep it away from flames. The model: the solutions are ideal (activities equal concentrations), 25 °C; the pH of the samples are typical measured values; in the vinegar + soda reaction the CO₂ is assumed to escape completely; in titration the pH is calculated exactly from the charge-balance equation. Distilled water left in the air absorbs CO₂ and reaches pH ≈ 5.6 — the “pure water” in the scene is free of CO₂.

Did you know?

In the 17th century Robert Boyle described how the colour of plant juices changes in acid and alkali; litmus is an ancient indicator obtained from lichens — paper soaked in it is still used today.
The concept of pH was introduced in 1909 by the Danish chemist Søren Sørensen at the Carlsberg Laboratory in Copenhagen while he was studying the action of enzymes.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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