Environmental chemistry

Ten questions of mixed difficulty, covering Environmental chemistry. Print it, or work through it on screen — the answer key starts on its own page.

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Environmental chemistry

Science 9 · maddyhelps.com

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A lake next to farm fields turns bright green with algae in July. By August, many fish have died. Which explanation best fits?
1. a) Fertilizer runoff fed the algae; when they died, decomposers used up the oxygen
2. b) The algae put so much oxygen into the water that the fish could not survive
3. c) Fertilizer runoff poisoned the fish directly, and the algae grew on the remains
4. d) The extra nutrients made the lake water too basic for the fish to breathe
A pesticide is measured along a lake food chain: water, 0.00005 ppm; plankton, 0.04 ppm; small fish, 0.5 ppm; large fish, 2 ppm; fish-eating birds, 25 ppm. What do these data best show?
1. a) Dilution: the level falls as the pesticide spreads out
2. b) Bioaccumulation: the level rises in one animal as it ages
3. c) Biodegradation: the pesticide breaks down up the chain
4. d) Biomagnification: the level rises at each step up the chain
Four pesticides have these LD50 values: P, 5 mg/kg; Q, 50 mg/kg; R, 500 mg/kg; S, 5000 mg/kg. Which is the most toxic?
1. a) Pesticide Q
2. b) Pesticide R
3. c) Pesticide S
4. d) Pesticide P
The three numbers on a bag of fertilizer, such as 10-20-10, stand for the amounts of
1. a) calcium, phosphorus and sodium, in that order
2. b) nitrogen, potassium and phosphorus, in that order
3. c) carbon, hydrogen and oxygen, in that order
4. d) nitrogen, phosphorus and potassium, in that order
Bacteria are used to clean up a fuel spill in soil. They will break down the fuel fastest where the soil is
1. a) waterlogged, with very little oxygen in it
2. b) dry, so the fuel stays in a single place
3. c) warm, moist and well supplied with oxygen
4. d) frozen, so the fuel cannot spread further
A lab has 50 mL of a solution with a concentration of 400 ppm. Water is added until there is 200 mL of solution. The new concentration is
1. a) 350 ppm
2. b) 100 ppm
3. c) 1600 ppm
4. d) 400 ppm
A neutral solution, such as pure water at room temperature, has a pH of
1. a) 0
2. b) 7
3. c) 14
4. d) 1
Which of these is an organic compound?
1. a) Carbon dioxide, CO₂
2. b) Table salt, NaCl
3. c) Water, H₂O
4. d) Glucose, C₆H₁₂O₆
A student blows through a straw into water containing bromothymol blue. The blue solution turns green and then yellow. This shows that the breath
1. a) made the water neutral, because oxygen from the breath dissolved
2. b) made the water acidic, because carbon dioxide forms an acid in water
3. c) made the water basic, because carbon dioxide forms a base when it dissolves
4. d) had no chemical effect; the warm air changed the indicator's colour
Students sampling a stream find many mayfly and stonefly nymphs. This most likely means the water
1. a) is clean and has plenty of dissolved oxygen
2. b) is warm, still and low in dissolved oxygen
3. c) is polluted with sewage or fertilizer runoff
4. d) is too acidic for most other insects to live

Answer key · Environmental chemistry

Science 9 · maddyhelps.com

a) Fertilizer runoff fed the algae; when they died, decomposers used up the oxygen — Nitrogen and phosphorus from fertilizer let algae grow far faster than usual. When the algae die, bacteria decompose them and use up the dissolved oxygen, so the fish suffocate; the fertilizer never had to be toxic to kill them.
d) Biomagnification: the level rises at each step up the chain — Each level eats many organisms from the level below and keeps their pesticide, so the concentration multiplies, here 500 000 times from water to birds. Bioaccumulation is real too, but it means build-up in one organism over its life, and these data compare different levels instead.
d) Pesticide P — LD50 is the dose, per kilogram of body mass, that kills half of a test group. The smaller the LD50, the less it takes to kill, so the smallest number is the most toxic; picking the biggest number is the classic mistake.
d) nitrogen, phosphorus and potassium, in that order — N-P-K: nitrogen, phosphorus and potassium are the three macronutrients plants need in the largest amounts. The order never changes, so a bag reading 10-20-10 is highest in phosphorus.
c) warm, moist and well supplied with oxygen — Biodegradation is done by living microorganisms, and they work fastest where conditions suit living things: warmth, moisture and oxygen. Freezing may stop the fuel spreading, but it nearly stops the bacteria too.
b) 100 ppm — The same amount of substance is now spread through four times the volume, so the concentration is a quarter: 400 ÷ 4 = 100 ppm. Multiplying by 4 gives 1600 ppm, but adding water can only lower a concentration.
b) 7 — The pH scale runs from 0 to 14, with 7 in the middle as neutral. Below 7 is acidic and above 7 is basic, so a pH of 0 is not neutral at all; it is extremely acidic.
d) Glucose, C₆H₁₂O₆ — Organic compounds contain carbon, usually bonded to hydrogen, and they are what living things are built from. Carbon dioxide contains carbon but is classed as inorganic, along with carbon monoxide and carbonates, which is the usual trap.
b) made the water acidic, because carbon dioxide forms an acid in water — Bromothymol blue is blue in a base, green near neutral and yellow in an acid. Carbon dioxide from breath dissolves to form a weak acid, carbonic acid, and that is what turns the indicator yellow.
a) is clean and has plenty of dissolved oxygen — Mayfly and stonefly nymphs need clean water with plenty of oxygen, so finding many of them is a sign of good water quality. Living things used this way are biological indicators, and they show the conditions over weeks, not just on the day of sampling.