10.1 Water
- Syllabus
- 0620–2026–2027
- Topic
- 10.1
- Level
- —
Learning objectives
10.1.1Chemical tests for the presence of• Describe chemical tests for the presence of water using anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate10.1.2To test for the purity of water using• Describe how to test for the purity of water using melting point and boiling point10.1.3Distilled water is used in practical• Explain that distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities10.1.4Substances found in natural water• State substances found in natural water: dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, fertiliser nitrates, and phosphates from fertilisers/detergents10.1.5Some of these substances are• State: some of these substances are beneficial, (a) dissolved oxygen for aquatic life (b) some metal compounds provide essential minerals for life10.1.6Harmful substances: toxic metal• State harmful substances: toxic metal compounds, plastics harming aquatic life, sewage microbes causing disease, and nitrates/phosphates causing deoxygenation and aquatic damage. Eutrophication details not required10.1.7Treatment of the domestic water supply• Describe treatment of the domestic water supply in terms of: (a) sedimentation and filtration to remove solids (b) use of carbon to remove tastes and odours (c) chlorination to kill microbes
Test chemically for water
| Anhydrous reagent | Colour before | Positive result with water |
|---|---|---|
| cobalt(II) chloride | blue | turns pink |
| copper(II) sulfate | white | turns blue |
Add the suspected water to a small sample of the anhydrous solid, or expose the reagent to the liquid/vapour being tested. Record both the starting and final colour.
These colour changes show that water is present; they do not by themselves prove that a sample is pure water.
The reagents must be anhydrous. Keep the colour pairs separate: cobalt(II) chloride is blue → pink; copper(II) sulfate is white → blue.
Test whether water is pure
A pure substance has sharp, fixed melting and boiling points under stated pressure. Pure water melts/freezes at 0 °C and boils at 100 °C at standard atmospheric pressure.
| Measurement | Pure-water result | Sign of impurity |
|---|---|---|
| melting/freezing point | sharp value at 0 °C | shifted value or melting/freezing over a range |
| boiling point | sharp value at 100 °C | shifted value or boiling over a range |
Measure temperature while the sample changes state and compare with the expected value under the same pressure. Repeating the measurement improves confidence.
A positive cobalt(II)-chloride or copper(II)-sulfate test proves water is present, not that it is pure. Neutral pH alone also does not prove purity.
Choose distilled water for practical chemistry
Distilled water is used instead of tap water in practical chemistry because it contains fewer chemical impurities.
| Water source | Chemical contents | Experimental consequence |
|---|---|---|
| tap water | contains dissolved ions and other impurities | may form precipitates, alter colour/pH or leave residues |
| distilled water | contains fewer chemical impurities | is less likely to introduce an uncontrolled reactant |
Use distilled water when preparing solutions, rinsing apparatus for analysis or carrying out tests where trace dissolved ions could change the observation.
The required comparison is fewer chemical impurities, not that distilled water is guaranteed absolutely pure or sterile.
Identify substances in natural water
| Category | Substance found in natural water | Typical source or form |
|---|---|---|
| gas | dissolved oxygen | air and aquatic photosynthesis |
| dissolved compounds | metal compounds | rocks, soil and human activity |
| solid pollution | plastics | discarded material and fragments |
| biological/organic pollution | sewage | waste entering water |
| biological hazard | harmful microbes | contaminated water and sewage |
| nutrient pollution | nitrates | fertilisers |
| nutrient pollution | phosphates | fertilisers and detergents |
Natural water is a mixture. Its contents can be dissolved, suspended, biological, beneficial or harmful; 'natural' does not mean chemically pure.
State the named substance and, where specified, its source: nitrates from fertilisers; phosphates from fertilisers and detergents.
Do not treat every listed substance as harmful. Dissolved oxygen and some metal compounds can be beneficial, while the effect of other substances depends on type and amount.
Recognise beneficial substances in water
| Substance | Benefit |
|---|---|
| dissolved oxygen | supports respiration and survival of aquatic life |
| some metal compounds | supply essential mineral ions needed by living organisms |
Benefit depends on identity and concentration. The syllabus claim is that some metal compounds provide essential minerals, not that all metal compounds are safe.
The same natural-water inventory contains both useful and damaging substances, so classify by biological effect rather than simply by whether a substance is present.
Dissolved oxygen is beneficial; loss of dissolved oxygen damages aquatic life. Toxic metal compounds are harmful and must not be confused with essential mineral compounds.
Recognise harmful substances in water
| Harmful substance | Required harm |
|---|---|
| toxic metal compounds | poison living organisms |
| plastics | injure or otherwise harm aquatic life |
| sewage containing harmful microbes | spreads disease |
| nitrates and phosphates | deoxygenate water and damage aquatic life |
Name both pollutant and consequence. For example, fertiliser nitrates and detergent/fertiliser phosphates can reduce dissolved oxygen, leaving too little for aquatic organisms.
The required outcome is deoxygenation and harm to aquatic life. Detailed stages of eutrophication are not required.
Do not claim dissolved oxygen is a pollutant. It is beneficial; nitrates and phosphates are harmful here because they lead to its depletion.
Describe domestic water treatment
| Treatment | What it removes or does |
|---|---|
| sedimentation | allows suspended solids to settle |
| filtration | removes remaining insoluble solids |
| carbon treatment | removes unwanted tastes and odours |
| chlorination | kills harmful microbes |
Sedimentation and filtration handle solid particles first. Carbon improves taste and smell. Chlorine disinfects the water before supply.
Filtration removes insoluble solids, not dissolved salts. Chlorination kills microbes but does not remove all dissolved chemical substances.
Do not assign carbon the job of killing microbes or chlorine the job of removing solids. Each treatment stage has a distinct purpose.