Mixed review · Chemical change and the environment

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

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Mixed review · Chemical change and the environment

Science 9 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

A family has half a can of leftover oil-based paint. The safest way to get rid of it is to
1. a) take it to a household hazardous waste drop-off
2. b) pour it on gravel so it soaks into the ground
3. c) seal the can and put it out with the garbage
4. d) pour it down a storm drain on the street
A farmer tests her field and finds the soil has a pH of 5. To raise the pH closer to 7, she should add
1. a) a substance with a pH of 0 to balance it
2. b) lime, a base, to neutralize some of the acid
3. c) vinegar, since adding an acid will cancel the acid
4. d) table salt, because salts are neutral
A student drops identical antacid tablets into equal volumes of water at 10 °C, 25 °C and 40 °C and times how long each one fizzes. The manipulated variable is
1. a) the temperature of the water
2. b) the volume of water used
3. c) the time the tablet fizzes
4. d) the type of antacid tablet
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
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, phosphorus and potassium, in that order
3. c) carbon, hydrogen and oxygen, in that order
4. d) nitrogen, potassium and phosphorus, in that order
Which of these is an organic compound?
1. a) Water, H₂O
2. b) Carbon dioxide, CO₂
3. c) Table salt, NaCl
4. d) Glucose, C₆H₁₂O₆
Students sampling a stream find many mayfly and stonefly nymphs. This most likely means the water
1. a) is too acidic for most other insects to live
2. b) is warm, still and low in dissolved oxygen
3. c) is clean and has plenty of dissolved oxygen
4. d) is polluted with sewage or fertilizer runoff
Which of these is a chemical change?
1. a) An ice cube melting in a glass
2. b) A glass bottle shattering
3. c) Sugar dissolving in hot tea
4. d) A silver spoon tarnishing black
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 poisoned the fish directly, and the algae grew on the remains
2. b) The extra nutrients made the lake water too basic for the fish to breathe
3. c) Fertilizer runoff fed the algae; when they died, decomposers used up the oxygen
4. d) The algae put so much oxygen into the water that the fish could not survive
A log burns completely in a fire pit, leaving a small pile of ash that is much lighter than the log was. The best explanation is that
1. a) burning turned most of the log's mass into heat energy
2. b) the mass was destroyed, and it was given off as the fire's heat and light
3. c) the ash is lighter because the fire shrank the atoms in it
4. d) most of it left as gases, such as carbon dioxide and water vapour

Answer key · Mixed review · Chemical change and the environment

Science 9 · maddyhelps.com

a) take it to a household hazardous waste drop-off — A hazardous waste site can recycle or destroy the paint safely. In most towns storm drains flow straight into a river without treatment, and anything poured on the ground can seep through the soil into groundwater.
b) lime, a base, to neutralize some of the acid — A base neutralizes an acid and moves the pH up toward 7, and lime is what farmers spread on acidic soil. Adding salt does nothing to the acid: salt is a product of neutralization, not a cause of it.
a) the temperature of the water — The manipulated variable is the one the student deliberately changes, and here that is temperature. The fizzing time is the responding variable, which is measured; volume and tablet type are kept the same, so they are controlled variables.
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) 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.
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.
c) 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.
d) A silver spoon tarnishing black — Tarnish is a new substance formed when silver reacts with sulfur compounds in the air, and a colour change like that is evidence of a chemical change. Melting, dissolving and breaking change only the form or state; the substance is the same afterwards.
c) 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) most of it left as gases, such as carbon dioxide and water vapour — Carbon and hydrogen in the wood combined with oxygen and escaped as invisible gases. Collect those gases along with the ash and they weigh more than the log did, because oxygen from the air joined in: mass is conserved even when it looks lost.