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The kinetic theory of matter is one of the models that scientists use to help them to make sense of the world around them. The word kinetic always suggests something is moving, and this theory is about the movement of particles in solids, liquids, and gases. Particles transfer heat energy into kinetic energy.

• 1.

### Which states of matter contract when you cool them?

• A.

Only gases

• B.

Only solids

• C.

Liquids and gases

• D.

All of them

D. All of them
Explanation
All states of matter contract when you cool them because cooling causes the particles within the substance to slow down and come closer together. In solids, the particles vibrate less and become more tightly packed. In liquids, the particles move slower and form stronger bonds. In gases, the particles lose energy and move closer together, resulting in a decrease in volume. Therefore, all three states of matter experience contraction when cooled.

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• 2.

### Which of the following would increase the speed at which a liquid evaporates?

• A.

Covering the liquid with a beaker

• B.

Making sure that there were no draughts

• C.

Spreading it out over a larger area

• D.

All of the above

C. Spreading it out over a larger area
Explanation
Spreading the liquid out over a larger area would increase the speed at which it evaporates because it exposes more surface area to the surrounding air. This allows more molecules of the liquid to escape into the air, increasing the rate of evaporation.

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• 3.

### Some types of thermometer are filled with mercury, a liquid metal. Why do they work?

• A.

Mercury is a metal so it conducts heat into the thermometer

• B.

Mercury behaves in an unusual way

• C.

Particles of liquids expand when they are heated and contract when they are cooled so the mercury fills more or less of the thermometer depending on the temperature

• D.

The particles move faster or slower depending on the temperature so the mercury expands and contracts

D. The particles move faster or slower depending on the temperature so the mercury expands and contracts
Explanation
Mercury thermometers work because the particles of mercury move faster or slower depending on the temperature. When the temperature increases, the particles move faster, causing the mercury to expand and rise in the thermometer. Conversely, when the temperature decreases, the particles slow down, causing the mercury to contract and lower in the thermometer. This expansion and contraction of the mercury allow it to accurately measure and indicate changes in temperature.

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• 4.

### What happens to the particles of water vapour as it changes from a gas into a liquid to form a film of water on the inside surface of a cold window?

• A.

When the particles of the water vapour hit the window, they stick to it

• B.

The particles lose energy to the cold surface, slow down and become closer together

• C.

The particles clump together because of the light coming through the window, forming water

• D.

The particles speed up, forming water droplets

B. The particles lose energy to the cold surface, slow down and become closer together
Explanation
As water vapor changes from a gas into a liquid on the inside surface of a cold window, the particles of water vapor lose energy to the cold surface. This loss of energy causes the particles to slow down and come closer together, eventually forming a film of water on the window.

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• 5.

### What happens to a gas in a sealed container when it is heated?

• A.

The gas explodes

• B.

The pressure of the gas increases

• C.

The gas implodes

• D.

The gas condenses

B. The pressure of the gas increases
Explanation
When a gas in a sealed container is heated, the kinetic energy of the gas molecules increases. This increase in energy causes the molecules to move faster and collide with the walls of the container more frequently and with greater force. As a result, the pressure inside the container increases. Therefore, the correct answer is that the pressure of the gas increases.

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• 6.

### Why does hot air rise?

• A.

Heat naturally goes upwards

• B.

It is pushed there by the cold air

• C.

It is less dense than colder air

• D.

It doesn't, this is a myth

C. It is less dense than colder air
Explanation
Hot air rises because it is less dense than colder air. When air is heated, its molecules move faster and spread out, causing the air to expand and become less dense. As a result, the lighter hot air rises above the denser cold air, creating upward movement. This phenomenon is known as convection and is the reason why hot air balloons and chimneys work.

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• 7.

### What will a substance do when it is heated?

• A.

It will expand

• B.

It will contract

• C.

It will burn

• D.

It will explode

A. It will expand
Explanation
When a substance is heated, its molecules gain energy and move faster. This increased molecular motion causes the substance to expand, as the molecules spread out and take up more space. This expansion can be observed in various materials such as solids, liquids, and gases. Therefore, the correct answer is that the substance will expand when heated.

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• 8.

### When a liquid is evaporating it cools down. Why?

• A.

The faster particles escape leaving the slower particles behind

• B.

There are fewer particles

• C.

Evaporation needs a breeze and breezes cool things down

• D.

It has a smaller volume than before

A. The faster particles escape leaving the slower particles behind
Explanation
When a liquid is evaporating, the faster particles have enough energy to break free from the liquid's surface and escape into the air. As a result, the average kinetic energy of the remaining particles decreases, causing the liquid to cool down. This is because the faster particles, which have higher kinetic energy, are leaving the liquid, leaving behind the slower particles with lower kinetic energy. Therefore, the statement "The faster particles escape leaving the slower particles behind" explains why a liquid cools down when it evaporates.

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• 9.

### Why are warm fluids less dense than their cold equivalent?

• A.

Some of the fluid escapes from the particles so they are lighter

• B.

Their particles are closer together

• C.

Hot particles lose their quarks and become lighter

• D.

Their particles are further apart

D. Their particles are further apart
Explanation
When a fluid is warmed, its particles gain energy and move faster, causing them to spread out and occupy more space. As a result, the average distance between the particles increases, leading to a decrease in density. Therefore, warm fluids are less dense than their cold equivalent because their particles are further apart.

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• 10.

### According to the kinetic theory, what are the particles in a heated solid doing?

• A.

Vibrating more slowly than when the solid was cool

• B.

Vibrating more rapidly than when the solid was cool

• C.

Moving around more slowly than when the solid was cool

• D.

Moving around more rapidly than when the solid was cool