Water test -- two anhydrous reagents
Both tests detect the PRESENCE of water only. The reagent must START anhydrous (containing no water of crystallisation) so the colour change can only be caused by the water added. Anhydrous cobalt(II) chloride: blue to pink. Anhydrous copper(II) sulfate: white to blue. The added water becomes chemically bonded into the crystal structure as water of crystallisation, giving the hydrated form -- white CuSO4 becomes blue CuSO4.5H2O.
Purity -- fixed melting and boiling points
A pure substance melts and boils at a single fixed, sharp temperature. Pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure. A dissolved solute lowers the melting point below 0 °C, raises the boiling point above 100 °C, and makes the sample melt or boil over a range instead of sharply. Distilled water is used in practical chemistry because distillation has removed the dissolved chemical impurities that tap water carries.
Domestic water treatment -- three stages
Water from a river or reservoir is treated in three stages, each removing ONE specific type of impurity. 1. Sedimentation and filtration -- the water stands so heavier suspended particles settle out, then passes through a filter bed of sand, removing suspended solids and debris. 2. Carbon -- activated carbon removes substances causing unpleasant tastes and odours. 3. Chlorination -- a small, controlled amount of chlorine kills harmful microbes/pathogens.
Drawn from real examiner reports.
Presence test ≠ purity test
The colour-change test answers "is water present?" -- it works on pure and impure water alike and says nothing about purity. The melting/boiling point test answers "is this water pure?" -- it assumes the liquid is already known to be water. Litmus paper and boiling point are NOT credited as a test for the presence of water.
Flagged Jun 2023 P11 Q23; P32 Q5v
Vague water-treatment answers
"Describe how the domestic water supply is treated" is marked stage by stage, so "the water is filtered and cleaned" scores nothing. Name sedimentation/filtration, carbon and chlorination as three DISTINCT stages and give each its own specific purpose. Saying chlorination "purifies" or "cleans" the water misses the marking point that it kills harmful microbes/pathogens.
Flagged Jun 2022 P53; Nov 2022 P43 Q8a
Swapping the treatment stages' jobs
Each stage removes a different impurity and the purposes are not interchangeable. Filtration does not kill microbes -- that is chlorination. Chlorination does not remove solid particles -- that is sedimentation and filtration. Carbon removes tastes and odours only. Match every named stage to its own impurity rather than assuming any stage "makes the water clean".
Only one colour is half an answer
A test-for-water answer needs the reagent named as anhydrous plus BOTH colours, before and after. "It turns pink" omits the starting blue; "cobalt(II) chloride turns pink" omits anhydrous, so the colour change could no longer be attributed to the water added. Reversing the direction -- pink to blue, or blue to white -- also loses the mark.
Tap water is impure, not unsafe
Tap water is safe to drink; the objection to using it in practical chemistry is chemical, not hygienic. It contains dissolved mineral ions that could react with reagents or interfere with the reaction or measurement being made. "Tap water is dirty" or "tap water is not safe" does not score -- the required idea is dissolved chemical impurities contaminating the results.
Multiply the whole water group
In a hydrated formula the hydration number multiplies the ENTIRE water group, not one atom. For CuSO4.5H2O the water contributes 5 × 18 = 90, giving 160 + 90 = 250; counting a single water gives 178. For CoCl2.6H2O the water contributes 6 × 18 = 108, giving 130 + 108 = 238. Use Ar(Cu) = 64 and Ar(Cl) = 35.5, never 35.
Sharp point vs melting range
Purity shows in the SHAPE of the result, not only its value. A pure substance melts at one fixed sharp temperature; a mixture melts gradually over a range. Quoting a single number and ignoring whether melting was sharp or spread out misses half the reasoning -- a sample that melts from -3 °C to 0 °C is impure even though it finishes at 0 °C.
Flagged Jun 2023 P42 Q2c
Spot which water skill is being asked
Four distinct skills live in this topic: the presence test, the purity test, distilled-vs-tap reasoning, and the treatment stages. Decide which one the question wants before you write -- a scientifically true answer about a different one earns nothing.
Match the command word and the tariff
State wants the term or value only; describe wants the observable steps; explain wants the reason. Then count the marks and give that many separate points -- a 3-mark treatment question expects three stages, not one long sentence.
One stage, one purpose, one mark
Write each treatment stage on its own line with its own specific purpose beside it. The marker can then see three separate marking points, and the purposes cannot drift between stages the way they do in a single flowing paragraph.
Quote the fixed values, then compare
For a purity question, first state that pure water melts at 0 °C and boils at 100 °C, then compare the sample against those values -- reporting both the shift away from them and whether melting or boiling happened sharply or over a range.
Cambridge 0654 spec reference: Section C10 "Chemistry of the environment", sub-topic C10.1 "Water" (Core only, no Supplement statement). This leaf covers: chemical tests for the presence of water; testing water purity via melting/boiling point; why distilled water (not tap water) is used in practical chemistry; and the treatment of the domestic water supply.
| Term | Mark-scheme-precise meaning |
|---|---|
| Anhydrous | Containing no water of crystallisation -- "without water" |
| Hydrated | Containing water chemically combined within the solid (water of crystallisation), e.g. |
| Pure substance | A single substance with nothing else mixed in; melts and boils at one fixed, sharp temperature |
| Distillation | A liquid is boiled and its vapour condensed and collected separately, leaving dissolved impurities behind, to give a purer liquid |
Define anhydrous and hydrated.
When water is added to white anhydrous copper(II) sulfate, blue hydrated crystals form with the formula CuSO4.5H2O.
Calculate the relative formula mass, Mr, of CuSO4.5H2O. (Ar: Cu = 64, S = 32, O = 16, H = 1) (2 marks)
