Grade 7 Science Practice Test: Atoms, Molecules and Thermal Energy
- Atoms, Molecules and Thermal Energy
- 15 questions
- 18 minutes
This test covers atoms, molecules and extended structures, particle motion in solids, liquids and gases, the difference between temperature and thermal energy, and how energy moves as heat. Tip: picture the particles and ask whether they are speeding up, slowing down or changing their arrangement.
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Question 1
Which of these substances is a compound?
Correct answer: D
A compound is made of two or more different elements bonded together; carbon dioxide contains carbon and oxygen. O2 is a molecule, but both of its atoms are oxygen, so it is an element.
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Question 2
The chemical formula for glucose, a sugar, is C6H12O6. How many atoms are in one molecule of glucose?
Correct answer: B
Add the subscripts: 6 carbon + 12 hydrogen + 6 oxygen = 24 atoms. 3 is the number of different elements, not the number of atoms.
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Question 3
In which state of matter are the particles close together but able to slide past one another?
Correct answer: A
In a liquid, particles stay close together but can move past each other, so a liquid flows and takes the shape of its container. In a solid they only vibrate in fixed positions, and in a gas they are far apart.
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Question 4
A grain of table salt (sodium chloride) is made of sodium and chlorine in a 1:1 ratio. Which description of its structure is correct?
Correct answer: C
Sodium chloride does not form separate molecules. Its particles repeat in a pattern that extends through the whole crystal, which is why the formula NaCl gives a ratio rather than a molecule size. It is a compound, not a mixture of sodium and chlorine.
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Question 5
A beaker of crushed ice is heated at a steady rate. The temperature rises to 0°C and then stays at 0°C until all the ice has melted. What happens to the energy added while the temperature stays at 0°C?
Correct answer: B
During melting, the added energy changes the arrangement of the particles, freeing them from fixed positions, instead of raising their average speed, so the temperature stays the same. Energy is not lost, and no new atoms are made.
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Question 6
Data Set A: bath and tea
A bathtub holds 150 liters of water at 40°C. A cup holds 0.25 liter of tea at 80°C.
In which one do the water molecules have the greater average speed?
Correct answer: D
Temperature measures the average kinetic energy of the particles, so the hotter tea has faster-moving molecules on average. The amount of water does not change the average speed.
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Question 7
Data Set A: bath and tea (see above):
Which one has more total thermal energy?
Correct answer: A
Thermal energy is the total kinetic energy of all the particles. The bath has 600 times as much water, so even at a lower temperature it holds much more thermal energy. Twice the temperature does not make up for so many fewer particles.
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Question 8
Ice cubes are dropped into a glass of lemonade at room temperature. The lemonade gets colder. Which statement describes the energy transfer?
Correct answer: C
Heat always flows from the warmer object to the colder one. The lemonade loses energy to the ice, which warms and melts, so the lemonade cools. "Cold" is not something that flows; it is just less thermal energy.
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Question 9
A space heater sits on the floor of a room. Air near the heater warms, rises toward the ceiling and is replaced by cooler air that sinks, so the air circulates around the room. What is this kind of heat transfer called?
Correct answer: B
Convection moves energy through the flow of a liquid or gas: warm, less dense air rises and cool air sinks, forming a current. Conduction needs direct contact between particles, and radiation does not need matter at all.
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Question 10
Data Set B: cocoa cups
Students poured 200 mL of cocoa at 80°C into four cups made of different materials. The cups were the same size and shape and had the same lid. After 20 minutes they measured the temperature.
Cup material Temperature after 20 min (°C) Aluminum 52 Glass 61 Plastic 64 Foam 73 Which material is the best insulator?
Correct answer: A
The cocoa in the foam cup cooled the least (from 80°C to 73°C), so foam let the least energy escape. Aluminum is the worst insulator here; its cocoa lost 28°C because metals conduct heat well.
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Question 11
Data Set B: cocoa cups (see above):
Why did the students use the same amount of cocoa at the same starting temperature in every cup?
Correct answer: D
In a fair test only one variable, here the cup material, is changed. Keeping the volume, starting temperature, lid and time the same means any difference in cooling comes from the material.
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Question 12
The same amount of energy is added to two pots of water that start at the same temperature. Pot 1 holds 1 kg of water, and its temperature rises by 10°C. Pot 2 holds 2 kg of water. By about how much does the temperature of Pot 2 rise?
Correct answer: C
The same energy is shared among twice as many water particles, so each one speeds up half as much and the temperature rises about 10 ÷ 2 = 5°C. 20°C would mean more mass heats up more, which is backward.
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Question 13
On a cold day, Liam rubs his hands together quickly and they feel warmer. Which energy change explains this?
Correct answer: B
Friction between the moving hands transfers kinetic energy into thermal energy, so the particles in the skin move faster and feel warmer. Energy is converted, not created.
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Question 14
The red liquid in a glass thermometer rises when the thermometer is placed in warm water. Why does the liquid rise?
Correct answer: D
Heating makes the particles move faster and spread out, so the liquid expands and rises in the narrow tube. The particles themselves do not get bigger, which is a common misconception.
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Question 15
After a run, sweat evaporates from your skin and you feel cooler. Why does evaporation cool the skin?
Correct answer: A
To evaporate, a water molecule needs enough energy to break away from the liquid. The fastest ones leave and take energy from the skin, so the skin's temperature drops. Water vapor is not colder than ice, and molecules do not change size.
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