Test: Chemical Reactions - Year 8 MCQ

Ice melting is an endothermic process where thermal energy is absorbed from the surroundings to convert solid ice into liquid water. This absorption of heat causes the surrounding temperature to decrease, making the area feel cooler.

• Iron can be protected from rusting by galvanizing (coating with zinc) and by painting (covering to prevent contact with air and moisture).
• Keeping it in water actually increases rusting, so option c is wrong.

An exothermic reaction is characterized by the release of thermal energy to the surroundings, which increases the temperature of the environment. This type of reaction occurs in processes like combustion, where chemical energy is converted into heat and light.

When magnesium is burned in air, it reacts with oxygen to form magnesium oxide, releasing significant heat and light in the process. This reaction is a clear example of an exothermic reaction, showcasing the energy release associated with combustion.

The reaction between magnesium and dilute hydrochloric acid produces magnesium chloride and hydrogen gas. This reaction is exothermic, causing the test tube to become warm due to the thermal energy released.

Sodium is known for its vigorous reaction with water, producing hydrogen gas and sodium hydroxide. Due to its high reactivity, sodium is typically stored under oil to prevent accidental reactions with moisture in the air.

Gold is an inert metal that does not react with either water or steam. Its lack of reactivity makes it suitable for use in jewelry and electronics, as it does not corrode or tarnish over time.

Rust is primarily composed of iron oxide, which forms when iron reacts with oxygen in the presence of moisture. This process, known as oxidation, weakens iron structures over time, leading to potential structural failures.

When potassium reacts with water, the reaction is:

2K + 2H₂O → 2KOH + H₂

So, the products are potassium hydroxide (KOH) and hydrogen gas (H₂).

Correct answer: a) Both A and C

Endothermic reactions are defined by their absorption of thermal energy from the surroundings, resulting in a decrease in the temperature of the environment. This is evident in reactions like the one between sodium hydrogen carbonate and citric acid, which absorb heat and feel cool to the touch.

A combustion reaction, commonly known as burning, requires three essential components: fuel, oxygen, and heat. The fuel can be any combustible material, such as wood or gasoline, while oxygen is usually supplied from the air. The heat initiates the reaction, allowing the fuel to combine with oxygen, resulting in the release of energy in the form of thermal and light energy. This principle explains why fire extinguishers often work by removing heat or oxygen from the equation.

The reaction between potassium and water is classified as an exothermic reaction. During this process, potassium reacts with water to produce potassium hydroxide and hydrogen gas, releasing significant thermal energy in the form of heat. Exothermic reactions are characterized by the release of energy, which causes the surroundings to warm up. This is why such reactions can be quite dramatic and are handled with caution in laboratory settings.

Magnesium reacts with water less vigorously than sodium or potassium. In the presence of water, magnesium produces hydrogen gas and magnesium hydroxide. Its reaction is slower, especially with liquid water, but faster with steam.

Exothermic processes are characterized by the release of thermal energy into the surroundings, which raises the temperature of the environment. In contrast, endothermic processes absorb thermal energy from their surroundings, resulting in a decrease in temperature around them. This fundamental difference is crucial in understanding various chemical reactions and physical changes, including how they can be harnessed in applications like heating or cooling systems.

Zinc reacts with dilute hydrochloric acid to produce zinc chloride (a salt) and hydrogen gas. This is a typical metal-acid reaction, where the metal displaces hydrogen from the acid, releasing thermal energy in the process.