Tests for water — CuSO₄ and CoCl₂
Two anhydrous reagents test for water. Anhydrous copper(II) sulfate is a white powder that turns blue with water; anhydrous cobalt(II) chloride is blue and turns pink. Both changes reverse on heating, which drives off the water of crystallisation and restores the anhydrous colour. A positive change proves only that water is PRESENT, not pure. To show purity, the liquid must boil at exactly 100 °C and freeze at 0 °C at standard pressure.
Purifying the water supply — three stages
Public water is treated in three stages. (1) Sedimentation: suspended particles (soil, silt) settle under gravity; alum may be added to make fine particles clump and settle faster. (2) Filtration: water passes through sand and gravel that trap smaller particles and some microorganisms. (3) Chlorination: a little chlorine kills disease-causing bacteria, making the water safe to drink. Give the stages in this order, each with a purpose.
Anhydrous vs hydrated vs aqueous
Three terms are easily confused. Anhydrous means no water of crystallisation — anhydrous copper(II) sulfate is a white powder. Hydrated means water molecules are chemically combined in the crystal lattice in a fixed ratio (water of crystallisation); hydrated copper(II) sulfate, CuSO₄·5H₂O, is a blue solid. An aqueous solution is different — the salt is dissolved and the water is only the solvent. Relative formula masses: CuSO₄ = 159.5, CuSO₄·5H₂O = 249.5 (Cu = 63.5).
Drawn from real examiner reports.
Colour changes written in reverse
The anhydrous form is always the STARTING colour and adding water produces the change: copper(II) sulfate white → blue, cobalt(II) chloride blue → pink. Writing "blue → white" for CuSO₄ or "pink → blue" for CoCl₂ describes heating (removing water), not a test for water, and scores zero. Mnemonics: CuSO₄ White-Before-Blue; CoCl₂ Blue-Before-Pink.
W22 Paper 11 Q15; S23 Paper 32 Q3(c) — colour changes of CuSO4 and CoCl2 consistently poorly recalled across sittings.
Mixing up the two reagents
The two tests are often swapped. Copper(II) sulfate turns blue with water; cobalt(II) chloride turns pink. In multiple-choice and short-answer questions, giving the pink change for copper or the blue change for cobalt is a common slip. Tie each reagent to its own colour pair: copper → blue, cobalt → pink, and recall which starts white (copper) and which starts blue (cobalt).
Colour change ≠ proof of purity
A positive colour change (CuSO₄ white → blue, or CoCl₂ blue → pink) confirms only that water is PRESENT. It does NOT show the water is pure. Stating "the test proves the liquid is pure water" scores zero. Purity must be checked separately: pure water boils at exactly 100 °C and freezes at exactly 0 °C at standard pressure, and a dissolved impurity would shift these values.
Hydrated ≠ aqueous (dissolved)
In a hydrated salt the water is chemically combined in the crystal lattice in a fixed ratio (water of crystallisation). In an aqueous solution the salt is dissolved and the water is just the solvent. Writing "hydrated = dissolved" or "anhydrous = concentrated" both fail the mark scheme. Hydrated and aqueous copper(II) sulfate are both blue but chemically distinct.
S23 Paper 31 Q3(d)(i) — confusing a hydrated salt with an aqueous solution is flagged explicitly.
Describing only two purification stages
Candidates frequently name sedimentation and filtration but omit chlorination, the most commonly dropped stage. All three are required, in order: sedimentation (settling), filtration (sand and gravel remove fine particles), then chlorination (chlorine kills bacteria). Putting chlorination before filtration also loses marks.
Filtration does not kill bacteria
Filtration and chlorination do different jobs. Filtration is a PHYSICAL process — sand and gravel trap suspended particles and some microorganisms, but bacteria can pass through. Chlorination is a CHEMICAL process that kills the remaining disease-causing bacteria. Assuming filtered water is already safe to drink is wrong; it must still be chlorinated before supply.
Answering "test for water" questions
Give three things: the reagent, the starting anhydrous colour, and the colour after water is added — CuSO₄ white → blue, or CoCl₂ blue → pink. One mark is the observation alone; two marks is reagent plus observation. A positive result shows water is present, not pure.
Describing water purification
Name all three stages in order with a purpose each: sedimentation (particles settle under gravity), filtration (sand and gravel remove fine particles), chlorination (chlorine kills bacteria). Each stage scores for the name plus what it achieves. Never omit chlorination.
"Describe" vs "State" commands
"Describe" asks for what you do AND what you observe — "add anhydrous copper(II) sulfate; it turns white to blue." "State" asks only for the fact — "turns blue." Match the command word: too brief loses the description mark; over-writing a "state" question wastes time.
Anhydrous: A substance that contains no water of crystallisation. Appears as a powder or dry solid.
Hydrated (hydrate): A crystalline substance in which water molecules are chemically combined within the crystal lattice in a fixed molar ratio. These incorporated water molecules are called water of crystallisation.
Aqueous solution: A substance dissolved in water, where water acts as the solvent (not part of the chemical structure).
Definition-precision trap: "Hydrated" does NOT mean dissolved in water. The water in a hydrated salt is chemically combined, not acting as a solvent.
Define the term "anhydrous".
Anhydrous copper(II) sulfate has the formula .
Calculate the relative formula mass () of anhydrous copper(II) sulfate.
(: Cu = 63.5, S = 32, O = 16)
