8E Combustion

What is a hydrocarbon made of?

• Carbon and hydrogen

• Oxygen and sulfur

• Hydrogen and nitrogen

• Carbon and oxygen

Answer explanation

A hydrocarbon is a compound consisting exclusively of carbon and hydrogen atoms. Therefore, the correct answer is 'Carbon and hydrogen', as the other options include elements not found in hydrocarbons.

What type of reaction is combustion?

• Isothermal

• Endothermic

• Reversible

• Exothermic

Answer explanation

Combustion is an exothermic reaction because it releases energy in the form of heat and light when a substance reacts with oxygen. This distinguishes it from endothermic reactions, which absorb energy.

What does carbon dioxide do to limewater?

• Turns it blue

• Turns it milky

• Turns it green

• Turns it red

Answer explanation

When carbon dioxide is bubbled through limewater (calcium hydroxide solution), it reacts to form calcium carbonate, which is insoluble and causes the solution to turn milky. Thus, the correct answer is 'Turns it milky'.

What happens to hydrated copper sulfate when it undergoes thermal decomposition?

• It turns pink

• It loses water

• It gains water

• It turns blue

Answer explanation

When hydrated copper sulfate is heated, it undergoes thermal decomposition and loses water, resulting in anhydrous copper sulfate, which is a white powder. Therefore, the correct answer is that it loses water.

What is the law of conservation of mass?

• Mass is created in reactions

• Mass is lost in reactions

• Mass of reactants equals mass of products

• Mass can be destroyed

Answer explanation

The law of conservation of mass states that in a chemical reaction, the mass of the reactants must equal the mass of the products. Therefore, the correct choice is that the mass of reactants equals mass of products.

What is phlogiston?

• A type of gas

• A hydrocarbon

• A substance once thought to be released during combustion

• A metal oxide

Answer explanation

Phlogiston was a hypothetical substance believed to be released during combustion. It was part of an outdated theory that explained fire and oxidation before the discovery of oxygen.

What are the three factors needed for a fire to burn?

• Oxygen, carbon, heat

• Heat, fuel, oxygen

• Heat, water, fuel

• Fuel, nitrogen, heat

Answer explanation

A fire requires three essential elements: heat to ignite, fuel to burn, and oxygen to sustain combustion. The correct answer is 'Heat, fuel, oxygen' as these are the fundamental components of the fire triangle.

How do water extinguishers extinguish a fire?

• Remove oxygen

• Add carbon dioxide

• Remove heat

• Add fuel

Answer explanation

Water extinguishers work by removing heat from the fire. When water is applied, it cools the burning material below its ignition temperature, effectively stopping the combustion process.

What is produced during complete combustion of a hydrocarbon?

• Carbon monoxide and water

• Soot and carbon dioxide

• Carbon dioxide and water

• Nitrogen oxides and water

Answer explanation

During complete combustion of a hydrocarbon, the fuel reacts with oxygen to produce carbon dioxide and water. This is the ideal reaction, unlike incomplete combustion which can produce carbon monoxide or soot.

What is acid rain caused by?

• Dissolved metals

• Dissolved oxygen

• Dissolved sulfur dioxide and nitrogen oxides

• Dissolved carbon dioxide

Answer explanation

Acid rain is primarily caused by the release of sulfur dioxide and nitrogen oxides into the atmosphere. These compounds react with water vapor, forming sulfuric and nitric acids, which then fall as acid rain.