Y8. Revising Chemical Reactions and Properties

Which of the following best describes flammability?

• The ability of a substance to dissolve in water

• The ability of a substance to catch fire and burn

• The ability of a substance to conduct electricity

• The ability of a substance to rust

Answer explanation

Flammability refers to the ability of a substance to catch fire and burn. This distinguishes it from other properties like solubility, conductivity, or rusting, which do not involve combustion.

What is the main product when a hydrocarbon undergoes complete combustion in oxygen?

• Carbon monoxide and water

• Carbon dioxide and water

• Hydrogen and oxygen

• Iron oxide

Answer explanation

During complete combustion of a hydrocarbon in oxygen, the main products formed are carbon dioxide and water. This occurs as the hydrocarbon reacts fully with oxygen, producing these two compounds as the primary outputs.

(a)   is a chemical property.

• Melting point
• Flammability
• Density
• Colour

Answer explanation

Flammability is a chemical property because it describes a substance's ability to undergo a chemical change when exposed to fire. In contrast, melting point, density, and color are physical properties.

What is the word equation for the rusting of iron?

• Iron + water → iron oxide

• Iron + oxygen + water → iron oxide

• Iron + carbon dioxide → iron carbonate

• Iron + hydrogen → iron hydride

Answer explanation

The correct word equation for rusting of iron is 'Iron + oxygen + water → iron oxide'. This reaction occurs when iron reacts with oxygen in the presence of water, forming iron oxide, commonly known as rust.

Which of the following is a sign that a chemical reaction has occurred?

• Change in shape

• Change in temperature

• Change in mass

• Change in size

Answer explanation

A change in temperature is a clear indicator of a chemical reaction, as it often results from the release or absorption of energy. Other options like change in shape, mass, or size do not necessarily indicate a chemical change.

What is the correct symbol equation for the combustion of methane?

Answer explanation

The correct equation for the combustion of methane (CH₄) is CH₄ + 2O₂ → CO₂ + 2H₂O. This reaction fully oxidizes methane, producing carbon dioxide and water, which is characteristic of complete combustion.

Which gas will relight a glowing splint?

• Carbon dioxide

• Oxygen

• Hydrogen

• Nitrogen

Answer explanation

Oxygen is the gas that supports combustion. When a glowing splint is placed in oxygen, it will relight, indicating the presence of this gas. Other gases like carbon dioxide and nitrogen do not support combustion.

What is the test for carbon dioxide gas?

• It turns blue litmus paper red

• It makes a glowing splint relight

• It turns limewater milky

• It pops with a burning splint

Answer explanation

The correct test for carbon dioxide gas is that it turns limewater milky. This occurs because carbon dioxide reacts with calcium hydroxide in limewater to form calcium carbonate, which is insoluble and appears milky.

What is the main cause of rusting in iron?

• Exposure to sunlight

• Contact with water and oxygen

• High temperature

• Contact with acids

Answer explanation

Rusting in iron primarily occurs due to contact with water and oxygen, which leads to the formation of iron oxide. This chemical reaction is essential for rusting, making this option the correct answer.

Which of the following is a correct definition of a chemical bond?

• A force that holds atoms together in a molecule

• A process of mixing two substances

• A type of physical change

• A method of separating mixtures

Answer explanation

A chemical bond is defined as a force that holds atoms together in a molecule, making option A the correct choice. The other options describe different concepts unrelated to the definition of a chemical bond.