(a) Polluted water can contain harmful substances such as plastics and phosphates.
State two other types of harmful substance in polluted water.
(b) Table 3.1 shows the masses of ions, in mg, present in a 1000 cm3 sample of polluted water.
Answer these questions using the information from Table 3.1.
(i) Name the negative ion that has the lowest concentration.
(ii) State the name of the NO3– ion.
(iii) Calculate the mass of sodium ions in 250 cm3 of polluted water.
mass = ______ mg
(c) Fig. 3.1 shows some of the stages in the treatment of the domestic water supply.
(i) Complete Fig. 3.1 by adding the final stage.
(ii) State why carbon is added to drinking water.
(d) Describe how to test the purity of water using melting point.
(e) Complete the symbol equation for the reaction of phosphorus(V) chloride, PCl5, with water.
2PCl5 + 8H2O → ….H3PO4 + ….HCl
▶️ Answer/Explanation
(a) Any two of: metal compounds, sewage, nitrates
Polluted water can contain various harmful substances beyond plastics and phosphates. Metal compounds from industrial waste, sewage containing organic matter and pathogens, and nitrates from agricultural runoff are common contaminants that make water unsafe for consumption or harmful to aquatic life.
(b)(i) bromide
Looking at Table 3.1, the bromide ion (Br–) has the lowest concentration at 0.3 mg per 1000 cm3 among all the negative ions listed.
(b)(ii) nitrate
The NO3– ion is called nitrate, which is a common polyatomic ion containing nitrogen and oxygen.
(b)(iii) 2.3 mg
Calculation: The table shows 9.2 mg of Na+ in 1000 cm3. For 250 cm3, the mass would be (9.2 mg × 250)/1000 = 2.3 mg.
(c)(i) chlorination
The final stage in water treatment is typically chlorination, where chlorine is added to kill any remaining bacteria or pathogens in the water.
(c)(ii) to remove tastes/odors
Activated carbon is added to water treatment processes primarily to adsorb and remove unpleasant tastes and odors caused by organic compounds or chlorine byproducts.
(d) Heat the water sample to its melting point (or cool to freezing point) and measure the temperature. Pure water will melt/freeze at exactly 0°C, while impure water will melt/freeze at a lower temperature or over a range of temperatures.
This works because impurities lower the melting/freezing point of water (freezing point depression). The more impurities present, the greater the deviation from 0°C.
(e) 2PCl5 + 8H2O → 2H3PO4 + 10HCl
Balancing the equation:
Phosphorus atoms: 2 on left (in 2PCl5) → 2 on right (2H3PO4)
Chlorine atoms: 10 on left (2×5) → 10 on right (10HCl)
Hydrogen atoms: 16 on left (8×2) → 6 (from H3PO4) + 10 (from HCl) = 16 on right
Oxygen atoms: 8 on left (8×1) → 8 on right (2×4 from H3PO4)
(a) The list shows some substances present in water from natural sources.
dissolved oxygen
calcium compounds
plastics
harmful microbes
State which one of these substances provides essential minerals for aquatic life.
(b) Explain why phosphates present in polluted water are harmful to aquatic life.
(c) Table 3.1 shows the masses of ions, in mg, present in a 1000 cm³ sample of polluted water.
(i) Name the negative ion present in the highest concentration.
(ii) State the name of the NO₃⁻ ion.
(iii) Calculate the mass of phosphate ions present in 200 cm³ of polluted water.
(d) Fig. 3.1 shows some of the stages in the purification of drinking water.
(i) State the purpose of sedimentation.
(ii) State why chlorine is added to drinking water.
(e) Describe how to test for the purity of water using boiling point.
(f) Complete the symbol equation for the reaction of disulfur dichloride, \( S_2Cl_2 \), with water.
\[ S_2Cl_2 + ….H_2O \rightarrow ….HCl + H_2SO_3 + H_2S \]
▶️ Answer/Explanation
(a) calcium compounds
Calcium compounds provide essential minerals like calcium that are necessary for the growth and development of aquatic organisms, particularly for shell-forming creatures.
(b) (lead to) deoxygenation (of water)
Phosphates act as nutrients for algae, causing excessive algal growth (algal blooms). When these algae die, their decomposition by bacteria consumes large amounts of oxygen, leading to oxygen depletion in the water which can suffocate aquatic life.
(c)(i) hydrogencarbonate
With 10.0 mg in 1000 cm³, HCO₃⁻ has the highest concentration among negative ions in the table.
(c)(ii) nitrate
The NO₃⁻ ion is called nitrate, a common polyatomic ion in chemistry.
(c)(iii) 0.4 mg
Calculation: If 1000 cm³ contains 2.0 mg of phosphate ions, then 200 cm³ would contain (2.0 mg × 200)/1000 = 0.4 mg.
(d)(i) to remove solids
Sedimentation allows heavier solid particles to settle at the bottom of the water by gravity, making them easier to remove in subsequent filtration steps.
(d)(ii) to kill (harmful) microbes
Chlorine is added as a disinfectant to kill harmful bacteria, viruses and other pathogens that could cause waterborne diseases.
(e)
Method 1: Heat the water sample to its boiling point and measure the temperature. Pure water boils at exactly 100°C at standard pressure. If the boiling point is higher or occurs over a range of temperatures, the water is impure.
Method 2: Heat the sample to 100°C and observe if it boils. Pure water will boil exactly at 100°C, while impure water will either boil at a higher temperature or over a range of temperatures.
(f) \( 3H_2O \) and \( 2HCl \) (coefficients)
Balanced equation: \( S_2Cl_2 + 3H_2O \rightarrow 2HCl + H_2SO_3 + H_2S \)
Explanation: To balance the equation, we need 3 water molecules to provide enough oxygen and hydrogen atoms. This produces 2 HCl molecules while maintaining the balance of sulfur atoms (2 on each side).