Grade 7 Science Practice Test: Chemical Reactions and Ecosystems
- Chemical Reactions and Ecosystems
- 12 questions
- 15 minutes
This test covers atoms and chemical reactions, photosynthesis and respiration, and interactions in ecosystems, following the NGSS integrated sequence for Grade 7 (some states order middle-school science topics differently). Tip: atoms are rearranged in a reaction, never created or destroyed, so count them on both sides.
Tap an option to mark your answer. You can change your mind, or tap the same option again to leave the question blank. The correct answers and explanations appear when you finish the test.Tap an option to answer; tap it again to leave the question blank.
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Question 1
The model shows one molecule. C stands for a carbon atom, and H stands for a hydrogen atom.
What is the chemical formula of this molecule?
Correct answer: D
The model has one carbon atom bonded to four hydrogen atoms, so the formula is CH4 (methane). The small number after a symbol tells how many atoms of that element are in one molecule, so C4H would mean four carbon atoms and only one hydrogen atom.
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Question 2
A student did three experiments and recorded what she observed.
Experiment Before After 1 white solid wax clear liquid wax that turns back into white solid wax when it cools 2 two clear, colorless liquids a bright yellow solid that does not dissolve in water, in a clear liquid 3 salt and water a clear liquid; when the water evaporates, salt crystals are left In which experiment did a chemical reaction take place?
Correct answer: B
In Experiment 2 a yellow solid appeared with properties that neither starting liquid had, so a new substance formed. Melting wax only changes its state, and dissolved salt is still salt, as the crystals left after evaporation show.
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Question 3
Methane burns in oxygen according to this equation:
CH4 + 2O2 → CO2 + 2H2OHow many oxygen atoms are on each side of the equation?
Correct answer: C
On the left, 2O2 means two oxygen molecules with two atoms each, or 4 oxygen atoms. On the right, CO2 has 2 and the two water molecules have 1 each, so there are 4 again. The answer 2 comes from ignoring the coefficient 2 in front of O2.
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Question 4
A student heats a pad of steel wool, which is mostly iron, in an open dish until it glows and turns dark. She weighs it before heating and again after it cools.
Measurement Mass Before heating 6.0 g After heating and cooling 6.8 g Which statement correctly explains the change in mass?
Correct answer: A
The iron reacted with oxygen from the air to form iron oxide. The oxygen atoms that joined the iron add their mass, so the product weighs more than the steel wool did. No atoms are created in a reaction, and the thermal energy that goes in does not add any mass you could measure.
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Question 5
Which of these is a synthetic material, made from natural resources through chemical reactions?
Correct answer: D
Nylon does not exist in nature: it is made by reacting chemicals that come from crude oil (petroleum), a natural resource. Cotton, wool and granite are natural materials that are only picked, sheared or cut before people use them.
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Question 6
Students want to design an instant cold pack. They dissolve 10 grams of four different solids, each in 50 mL of water that starts at 20°C, and record the water's temperature afterward.
Solid Water temperature after dissolving P 29°C Q 19°C R 8°C S 24°C Which solid should they choose, and why?
Correct answer: C
A cold pack has to take in thermal energy from its surroundings. Dissolving Solid R lowered the water from 20°C to 8°C, the largest drop, so it absorbed the most thermal energy. Solid P released energy and would suit a hot pack instead.
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Question 7
Using energy from sunlight, a plant makes sugar during photosynthesis. Which two substances does it use as raw materials?
Correct answer: A
In photosynthesis, chloroplasts use light energy to rearrange the atoms of carbon dioxide and water into sugar, and oxygen is given off. Oxygen is a product of photosynthesis, not one of its raw materials.
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Question 8
Bromothymol blue (BTB) is a blue liquid that turns yellow when the amount of carbon dioxide in it increases. A student fills four tubes with BTB solution, adds different things, seals them and checks them after one day.
Tube Added Kept in Color after 1 day 1 water plant bright light blue 2 water plant darkness yellow 3 snail bright light yellow 4 nothing darkness blue Which statement best explains why Tube 2 turned yellow?
Correct answer: D
Plants carry out cellular respiration all the time, giving off carbon dioxide. In light, photosynthesis used up that carbon dioxide, so Tube 1 stayed blue; in darkness only respiration went on, so carbon dioxide built up and Tube 2 turned yellow. Tube 4 shows that darkness alone does not change the color.
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Question 9
Deer in a forest feed heavily on acorns in fall and winter. Scientists rated the acorn crop each fall and counted the deer the next spring.
Year Acorn crop in fall Deer counted the next spring 1 good 120 2 good 134 3 very poor 88 4 good 101 Which conclusion is best supported by the data?
Correct answer: A
After the poor acorn crop in Year 3, the deer count dropped from 134 to 88, and it started to recover after the next good crop. A smaller food supply means fewer deer survive and reproduce. The data cannot show that deer caused the poor crop, since the crop was good the year before, when there were even more deer.
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Question 10
Bees collect nectar from flowers for food. While they do this, they carry pollen from one flower to another, which helps the plants make seeds. What kind of relationship is this?
Correct answer: C
In mutualism both species benefit: the bee gets food, and the plant gets its pollen carried to other flowers. In parasitism one organism benefits while harming the other, and in competition both try to use the same limited resource.
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Question 11
In a food chain, about 10% of the energy at each level is passed on to the next level. In the food chain grass → grasshoppers → mice → snakes, the grass captures 20,000 units of energy. About how many units of that energy reach the snakes?
Correct answer: B
Each step keeps about 10%: 20,000 → 2,000 (grasshoppers) → 200 (mice) → 20 (snakes). The rest of the energy is used for life processes and lost as heat at each level. The answer 200 is the energy that reaches the mice, one step too early.
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Question 12
A fish species from another continent is released into a lake. It eats the same insect larvae as the native fish, and nothing in the lake eats it. What is the most likely effect on the native fish?
Correct answer: B
The two species now compete for the same limited food. With no predators to keep its numbers down, the new species can multiply quickly and leave less food for the native fish, so their population is likely to shrink. The native fish do not eat the newcomer, since nothing in the lake does.
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