Acid, alkali or neutral? — answer key

1. MQ1 · Practice

Model answer. Limewater is an alkali, so each indicator shows its alkali colour: litmus blue, methyl orange yellow and thymolphthalein blue. Row D.

A full answer includes

• I chose row D.
• I took litmus to be blue in an alkali.
• I took methyl orange to be yellow in an alkali.
• I took thymolphthalein to be blue in an alkali.

Also accept

• Row D

Watch for

• “Litmus turns red in an alkali.” — It is the other way round: an acid turns litmus red, and an alkali turns it blue.
• “Methyl orange is red in an alkali.” — Methyl orange is red in an acid. In an alkali, and in a neutral solution, it is yellow.
• “Row C.” — Thymolphthalein turns blue in an alkali; colourless is its colour in an acid and in a neutral solution.

2. MQ2 · Practice

Options as numbered on the pupil sheet: 1 — colourless; 2 — red; 3 — blue; 4 — yellow.

Model answer. In an acid, litmus is red, methyl orange is red and thymolphthalein is colourless. In an alkali, methyl orange is yellow.

A full answer includes

• 1. Litmus in dilute sulfuric acid: red.
• 2. Methyl orange in dilute sulfuric acid: red.
• 3. Thymolphthalein in dilute sulfuric acid: colourless.
• 4. Methyl orange in potassium hydroxide solution: yellow.

Also accept

• red; red; colourless; yellow

Watch for

• “Methyl orange is red in an alkali.” — Methyl orange is red in an acid. In an alkali, and in a neutral solution, it is yellow.
• “Thymolphthalein turns blue in an acid.” — Thymolphthalein is blue only in an alkali. In an acid, and in a neutral solution, it is colourless.

3. MQ3 · Practice

Model answer. Sodium hydroxide solution, pH 13, is strongly alkaline: it would turn thymolphthalein blue. Thymolphthalein is colourless in an acid and in a neutral solution, so the bottle holds the hydrochloric acid or the sodium chloride solution. Litmus would tell these two apart: red in the acid, purple in the neutral solution. (With these three candidates the deduction is safe; a weakly alkaline solution could leave thymolphthalein colourless too.)

A full answer includes

• It is the hydrochloric acid or the sodium chloride solution, not the sodium hydroxide solution.

Also accept

• The acid or the sodium chloride solution, not the sodium hydroxide

Watch for

• “It must be the hydrochloric acid.” — Thymolphthalein is colourless in a neutral solution too, so it cannot prove an acid.
• “It must be the sodium chloride solution.” — Thymolphthalein is colourless in an acid too, so it cannot prove a neutral solution.
• “It must be the sodium hydroxide solution.” — Sodium hydroxide solution, an alkali at pH 13, would turn thymolphthalein blue.
• “Thymolphthalein turns blue in an acid.” — Thymolphthalein is blue only in an alkali. In an acid, and in a neutral solution, it is colourless.

4. MQ4 · Practice

Model answer. Green on universal indicator means pH 7: the solution is neutral. It is full of potassium ions, K⁺, and nitrate ions, NO₃⁻, but it has no spare H⁺ or OH⁻ ions, so litmus stays purple.

A full answer includes

• I chose row C.
• I took the pH to be 7.
• I took the solution to contain K⁺ and NO₃⁻ ions, with no spare H⁺ or OH⁻ ions.
• I took litmus to stay purple.
• I did not choose a row with no ions at all.

Also accept

• Row C

Watch for

• “Row A.” — A neutral salt solution is full of ions. Neutral means it has no spare H⁺ or OH⁻ ions.
• “Row B.” — Green is pH 7, neutral, and spare H⁺ ions would make the solution acidic.
• “Row D.” — Litmus is red only in an acid. In a neutral solution it is purple.

5. MQ5 · Practice

Options as numbered on the pupil sheet: 1 — seawater; 2 — rainwater; 3 — stomach juice; 4 — oven cleaner; 5 — black coffee.

Model answer. The higher the pH above 7, the more alkaline; the lower the pH, the more acidic. From pH 13 down to pH 1: oven cleaner, seawater, rainwater, black coffee, stomach juice. Only oven cleaner and seawater are alkaline; the other three are acidic.

A full answer includes

• Oven cleaner (13), seawater (8), rainwater (6), black coffee (5), stomach juice (1).

Also accept

• oven cleaner, seawater, rainwater, black coffee, stomach juice

Watch for

• “Stomach juice is the most alkaline, because 1 is the first number.” — pH 1 is at the acid end of the scale: it is the most acidic of the five.
• “Rainwater is neutral, because it is water.” — Rainwater at pH 6 is slightly acidic: carbon dioxide from the air dissolves in it.

6. MQ6 · Practice

Model answer. Both solutions are acidic. The lower the pH, the higher the concentration of hydrogen ions, so X, at pH 3, has more H⁺ ions in each cm³ than Y, at pH 6: X is the more acidic.

A full answer includes

• X has the higher concentration of hydrogen ions.

Also accept

• X has the higher concentration of H⁺ ions

Watch for

• “Two acids with different pH values are equally acidic: both are just acid.” — Both are acidic, but the lower the pH, the more acidic the solution: the acid with the lower pH has a much higher concentration of hydrogen ions.
• “Y has more H⁺ ions, because 6 is bigger than 3.” — A bigger pH means fewer H⁺ ions: the pH scale runs the other way.
• “Y is alkaline, because its pH is higher.” — Only a pH above 7 is alkaline. pH 6 is still acidic.

7. MQ7 · Practice

Model answer. (a) 6.4, 6.5 and 6.3 lie within 0.2 of each other; 7.9 is far from them, so it is the anomaly, perhaps from a probe that was not rinsed. (b) Mean = (6.4 + 6.5 + 6.3) ÷ 3 = 19.2 ÷ 3 = 6.4. The pond is very slightly acidic.

Checked automatically

• (a) Anomalous reading: 7.9 (exact value)
• (b) Mean pH: 6.4 (within ±0.05)
  • Common wrong answer 6.8 : That includes the anomaly. Leave 7.9 out and average the other three.

A full answer includes

• (a) 7.9: it is far from the other three.
• (b) Method: (6.4 + 6.5 + 6.3) ÷ 3.
• (b) Answer: 6.4.
• I did not include the anomaly in the mean.

Also accept

• 7.9; 6.4

Watch for

• “Average all four readings.” — An anomaly is left out of the mean: with it, the mean comes out at 6.8, which none of the good readings supports.

8. MT1 · Apply

Model answer. (a) Turmeric changes colour only between neutral and alkaline: vinegar (an acid) and salt water (neutral) both leave it yellow. (b) The indicator needs a different colour in an acid and in a neutral solution. Litmus is red in vinegar and purple in salt water; universal indicator is orange or red in vinegar and green in salt water; methyl orange is red in vinegar and yellow in salt water. Thymolphthalein would fail for the same reason as turmeric: it is colourless in both.

A full answer includes

• (a) Turmeric is yellow in both an acid and a neutral solution, so both liquids look the same.
• (b) I named litmus, methyl orange or universal indicator.
• (b) Its colour in vinegar: red (litmus or methyl orange), or orange or red (universal indicator).
• (b) Its colour in salt water: purple (litmus), yellow (methyl orange) or green (universal indicator).
• I did not name thymolphthalein, which is colourless in both.

Also accept

• Both yellow, because turmeric changes only in an alkali; litmus, red and purple

Watch for

• “Thymolphthalein can tell them apart.” — Thymolphthalein is colourless in an acid and in a neutral solution, so it has the same problem as turmeric.