Use a copy of the Periodic Table where you need one.
Some statements about acids are given below.
I pH is a measure of the H+ concentration in a solution.
II The strength of an acid depends on its concentration.
III Universal Indicator turns orange when placed in a solution of any acid.
IV When acids react with metals, hydrogen ions gain electrons.
Which statements about acids are correct?
Source: 2024 SMSS Sec 3 End-of-Year P1 Q11
Which statements about strong and weak acids are always correct?
1 Strong acids ionise to a greater extent in water than weak acids.
2 Strong acids are more concentrated than weak acids.
3 Strong acids have a lower pH than weak acids.
Source: 2024 CHS Sec 3 End-of-Year Section A Q21
The reactions of four different oxides W, X, Y and Z with hydrochloric acid and sodium hydroxide solution are shown (✓ means there is a reaction and ✗ means there is no reaction). Which row shows the correct types of oxide?
| oxide | hydrochloric acid | sodium hydroxide |
|---|---|---|
| W | ✓ | ✗ |
| X | ✓ | ✓ |
| Y | ✗ | ✗ |
| Z | ✗ | ✓ |
| W | X | Y | Z | |
|---|---|---|---|---|
| A | acidic | amphoteric | neutral | basic |
| B | amphoteric | acidic | basic | neutral |
| C | basic | amphoteric | neutral | acidic |
| D | neutral | basic | amphoteric | acidic |
Source: SJI 2020 Year 3 End-of-Year P1 Q25
A farmer adds ammonium nitrate to the soil to increase its nitrogen content. He then adds calcium hydroxide to neutralise the excess acid in the soil. Why is this not a good idea?
Source: SJI 2020 Year 3 End-of-Year P1 Q22
Rat poison needs to be insoluble in rainwater, but soluble in the low pH of stomach contents. Which is a suitable barium compound to use as rat poison?
Source: 2023 HYSS Sec 3 End-of-Year P1 Q16
Acids in our daily lives. The stomach secretes gastric juice, which contains hydrochloric acid. Barium compounds that react with gastric juice produce aqueous barium ions, which are highly poisonous.
pKa is a measure of the extent to which an acid ionises. The larger the pKa value, the less the molecules ionise in solution, and so the weaker the acid. Table 8.1 shows the pKa values of some acids, and the volume of carbon dioxide produced when 1 g of calcium carbonate reacts with each acid for 1 minute.
| acid | pKa | volume of CO2 produced / cm3 |
|---|---|---|
| ethanoic acid | 4.7 | 2.32 |
| citric acid | 3.1 | 4.02 |
| lactic acid | 3.9 | 3.62 |
| carbonic acid | 6.3 | 0.95 |
| pH | pKa | |
|---|---|---|
| strong acid | high / low | high / low |
| weak acid | high / low | high / low |
Source: 2023 XMS Sec 3 WA3 Q8, adapted
Oxalic acid, H2C2O4, and sulfuric acid, H2SO4, each have two hydrogen atoms per molecule that can form H+ ions.
Source: 2026 NAS Prelim P2 Q10, adapted
One industrial use of ethanoic acid is to remove limescale (calcium carbonate) from steel pipes.
Source: 2024 BLSS Prelim P2 Q10(b) and (c)
BI and IV only[1]
II is wrong: strength is how much of the acid ionises, not how much is dissolved. III is wrong: a strong acid turns Universal Indicator red. In IV, H+ ions gain electrons to form hydrogen gas.
A1 only[1]
Strength is about the extent of ionisation. A dilute strong acid can be less concentrated than a weak acid, and can then have a higher pH, so 2 and 3 are not always true.
Cbasic, amphoteric, neutral, acidic[1]
W reacts with acid only (basic), X with both (amphoteric), Y with neither (neutral) and Z with alkali only (acidic).
BCalcium hydroxide will react with ammonium nitrate to release ammonia gas.[1]
A base reacts with an ammonium salt to give off ammonia, so the nitrogen the farmer added is lost to the air.
Abarium carbonate[1]
Barium carbonate is insoluble in water, but it reacts with the acid in the stomach to form soluble barium chloride. Barium chloride and barium nitrate dissolve in rainwater, and barium sulfate does not react with acid.
Barium sulfate is insoluble, so it does not produce barium ions. [1]
Although barium carbonate is insoluble, it reacts with the acid in gastric juice (hydrochloric acid) in the stomach [1]
to form soluble barium chloride, which produces barium ions and causes poisoning. [1]
Bubble the gas through limewater [1]
and a white precipitate forms (or the limewater turns milky). [1]
In a strong acid such as HCl, the molecules ionise completely in water, releasing a higher concentration of H+ ions. [1]
In a weak acid, the molecules ionise partially, giving a lower concentration of H+ ions. [1]
Strong acid: pH low, pKa low. Weak acid: pH high, pKa high. [2]
One mark for every two correct.
Carbonic acid is the weakest. [1]
It has the highest pKa, 6.3, so it ionises to produce the fewest H+ ions. [1]
So there are fewer H+ ions to react with the carbonate, and the least carbon dioxide, 0.95 cm3, is produced. [1]
Carbon monoxide is a neutral oxide. [1]
Moles = 0.452(12) + 2(1) + 4(16) = 0.4590 = 0.00500 mol [1]
Sulfuric acid is a strong acid: it ionises completely in water to form a high concentration of H+ ions, so its pH is lower. [1]
Oxalic acid is a weak acid: it ionises partially, giving a lower concentration of H+ ions. [1]
Hydrochloric acid and ethanoic acid each have one hydrogen per molecule that forms an H+ ion, so they contain the same amount of hydrogen ions and need the same volume of NaOH to neutralise. [1]
Sulfuric acid has two, so the same volume and concentration contains twice the amount of hydrogen ions [1]
and needs twice the volume of NaOH to neutralise. [1]
25.01000 × 0.40 = 0.0100 mol [1]
16.70 cm3: 0.00500 mol of oxalic acid contains 0.0100 mol of H+, the same as the hydrochloric acid in (d). [1]
2CH3COOH + CaCO3 → (CH3COO)2Ca + CO2 + H2O [2]
Add a few drops of Universal Indicator to separate samples of hydrochloric acid and ethanoic acid. [1]
Hydrochloric acid turns it red, so it is the strong acid. Ethanoic acid turns it orange, so it is the weak acid. [1]
Both colours are needed for this mark.
Strong acids ionise completely in water to form H+ ions [1]
while weak acids ionise partially in water. [1]
Hydrochloric acid is a strong acid that will also react with the iron in the steel and damage the pipes. [1]