Physical and Chemical Changes

What is a physical change?

• A physical change alters the chemical structure of a substance.

• A physical change is a process that cannot be reversed.

• A physical change involves a reaction that produces new substances.

• A physical change is a change that does not affect the chemical composition of a substance.

Answer explanation

A physical change is defined as a change that does not alter the chemical composition of a substance, meaning the substance remains the same at the molecular level, unlike chemical changes that create new substances.

What is a chemical change?

• A chemical change is a process that only occurs at high temperatures.

• A chemical change is a process where substances combine or break apart to form new substances.

• A chemical change is a physical alteration of a substance.

• A chemical change involves only the rearrangement of atoms without forming new substances.

Answer explanation

A chemical change involves the transformation of substances through combination or decomposition, resulting in new substances. This distinguishes it from physical changes, which do not alter the chemical composition.

Which of the following is an example of a physical change?

• Dissolving sugar in water

• Freezing water

• Melting ice

• Burning wood

Answer explanation

Melting ice is a physical change because it involves a change in state from solid to liquid without altering the chemical composition. In contrast, burning wood is a chemical change.

Which of the following is an example of a chemical change?

• Water boiling

• Ice melting

• Rust forming on iron

• Sugar dissolving in water

Answer explanation

Rust forming on iron is a chemical change because it involves a reaction between iron and oxygen, resulting in a new substance (rust). In contrast, boiling water, melting ice, and dissolving sugar are physical changes.

What are some indicators of a chemical reaction?

• Increase in pressure

• Formation of crystals

• Change in mass

• Changes in color, temperature, formation of a precipitate, gas production, and changes in odor.

Answer explanation

Indicators of a chemical reaction include changes in color, temperature, formation of a precipitate, gas production, and changes in odor. These signs confirm that a chemical change has occurred, unlike the other options.

How does the conservation of mass apply to chemical reactions?

• In chemical reactions, the total mass of reactants equals the total mass of products.

• The mass of reactants is always greater than the mass of products.

• The total mass of products is irrelevant in chemical reactions.

• Mass can be created or destroyed during a chemical reaction.

Answer explanation

The conservation of mass states that in chemical reactions, the total mass of reactants equals the total mass of products. This means that matter is neither created nor destroyed, making the first answer choice correct.

What is rusting, and how is it classified?

• Rusting is a process that affects all metals equally.

• Rusting is the oxidation of aluminum in water.

• Rusting occurs only in dry environments.

• Rusting is the corrosion of iron due to reaction with oxygen and moisture, classified into uniform and localized rusting.

Answer explanation

Rusting specifically refers to the corrosion of iron when it reacts with oxygen and moisture. It is classified into uniform rusting, which affects the entire surface evenly, and localized rusting, which occurs in specific areas.

What is crystallisation, and how does it occur?

• Crystallisation is the formation of gas bubbles in a liquid.

• Crystallisation occurs only at high temperatures and pressures.

• Crystallisation is the process of breaking down solid crystals into a solution.

• Crystallisation is the process of forming solid crystals from a solution or melt.

Answer explanation

Crystallisation is the process of forming solid crystals from a solution or melt. This occurs when the concentration of solute exceeds its solubility, leading to the formation of solid crystals as the solution cools or evaporates.

Can a physical change be reversed? Give an example.

• No, a physical change cannot be reversed. Example: Burning wood.

• Yes, a physical change can be reversed. Example: Boiling water cannot be turned back into water.

• No, physical changes are permanent. Example: Crushing a can.

• Yes, a physical change can be reversed. Example: Melting ice can be frozen back into ice.

Answer explanation

Yes, a physical change can be reversed. For example, when ice melts into water, it can be frozen back into ice, demonstrating that physical changes are often reversible.

What happens to the chemical composition of substances during a chemical change?

• Chemical changes do not produce new substances.

• The chemical composition remains unchanged during a chemical change.

• The chemical composition of substances changes, resulting in new substances.

• Only physical properties change, not chemical composition.

Answer explanation

During a chemical change, the chemical composition of substances changes, leading to the formation of new substances. This distinguishes chemical changes from physical changes, where the composition remains the same.