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Painting a house

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5 men are hired to paint a house. If an additional man is hired, the painting can be completed 4 days earlier. Calculate the number of additional men to be hired if the painting is to be completed 18 days earlier.

The action of heat on metals and metal carbonates

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Based on the data given, suggest which 2 were the metals and which 2 were the metal carbonates. Explain your answers.

Metals: A and D
Metal carbonates: B and C

Explanation:

Metals, when heated, will be oxidized by oxygen in the air to form metal oxides. The oxygen gained to form the oxide will cause the mass of the product formed to be higher. Mass increase for solids A and D after heating, so these two are metals.
Metal carbonates, when heated, can decompose to form metal oxides and carbon dioxide. The carbon dioxide formed escape to the surroundings, causing the mass of the solid product formed to be smaller.
The mass of B decreased after heating, so it must be one of the metal carbonates.
The mass and appearance of C remained unchanged, so it must be the other metal carbonate, which is not decomposed by the heat.

Describe the effect of temperature increase on the resistance of a metallic conductor.

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The resistance increases as the temperature of a metallic conductor increases, so the resistance is directly proportional to the temperature.
When temperature increases, the amplitude of vibration of atoms increases. As a result, the number of collisions among the electrons and atoms increases, increasing the resistance to the flow of charges through the conductor.

Describe the pollution problems caused by the disposal of non-biodegradable plastics.

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– Plastics are flammable.
In the event of a fire, plastic materials burn readily, causing the fire to spread quickly.

– Burning of plastics gives off toxic gases.
Many plastics produce poisonous gases upon incineration, causing air pollution.

– Plastics are non-biodegradable.
By “non-biodegradable”, it means that plastics cannot be decomposed by bacteria in the soil. Improper disposal of plastics can cause land and water pollution.

Describe what is meant by “wave motion”.

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A wave motion is a movement or a disturbance through a medium, where the individual particles in the medium only vibrate vertically, horizontally, or in a cyclical pattern.

The wave motion can be illustrated:
– In a rope, when one end of a rope is moved in an up-and-down motion in a cyclical pattern.
– In a spring, when the spring is moved in an up-and-down motion, or a back-and-forth motion, in a cyclical pattern.
– In a ripple tank, when the dipper is moved in an up-and-down motion on the water surface, producing water waves.