Question

1. what is the main difference between a physical and chemical change?

• a. physical changes alter the appearance but not the composition, while chemical changes create new substances
• b. physical changes always involve heat, while chemical changes do not
• c. chemical changes are always reversible, while physical changes are not
• d. physical changes occur quickly, while chemical changes take days to complete

1. which of the following is an example of a chemical reaction?

• a. melting an ice cube
• b. cutting a piece of paper
• c. baking a cake
• d. dissolving salt in water

1. what evidence indicates that a chemical reaction has occurred?

• a. a change in temperature only
• b. a change in color only
• c. a change in shape only
• d. formation of a gas, color change, temperature change, or precipitate formation

1. which of the following is a physical change?

• a. rusting of iron
• b. burning of wood
• c. freezing of water
• d. digestion of food

Explanation:

• Question 1: Physical changes alter appearance (e.g., melting ice - change from solid to liquid) but not composition (still water). Chemical changes create new substances (e.g., burning wood - new substances like ash, carbon dioxide).
• Question 2: Baking a cake involves chemical reactions (e.g., proteins denaturing, starches gelatinizing, and new flavor compounds forming). Melting ice (physical - state change), cutting paper (physical - shape change), dissolving salt (physical - mixture formation).
• Question 3: Evidence of a chemical reaction includes formation of a gas (e.g., baking soda and vinegar reaction), color change (e.g., rusting - iron oxide formation), temperature change (exothermic/endothermic reactions), or precipitate formation (e.g., mixing silver nitrate and sodium chloride). A single change (like only temperature or only color) might not be conclusive, but multiple or these specific signs are.
• Question 4: Freezing water is a physical change (state change from liquid to solid, but still water). Rusting of iron (chemical - iron oxide formed), burning wood (chemical - new substances), digestion of food (chemical - complex molecules broken down into simpler ones).

Answer:

1. A. Physical changes alter the appearance but not the composition, while chemical changes create new substances
2. C. Baking a cake
3. D. Formation of a gas, color change, temperature change, or precipitate formation
4. C. Freezing of water