Question

1. which of the following is evidence that a chemical reaction has occurred?

a) milk turning sour
b) water freezing into ice
c) salt dissolving in water
d) paper being cut into pieces

1. when wood burns, what new substances are formed?

a) ash, smoke, and gases
b) sandstone and splinters
c) water and ice
d) heat only

1. which statement correctly describes the law of conservation of mass?

a) mass is destroyed in a reaction.
b) mass is created during reactions.
c) mass disappears after chemical change.
d) mass is neither created nor destroyed, only rearranged.

Explanation:

🆕 New Concept Discovered: Chemical vs. Physical Changes
Identifying if new substances are formed

Step 1: Analyze Question 1 (Evidence of a Chemical Reaction)

To determine if a chemical reaction has occurred, we look for the formation of entirely new substances with different chemical properties.

• A) Milk turning sour: This is a chemical change. Bacteria ferment the lactose in milk into lactic acid, changing its chemical composition, taste, and smell.
• B) Water freezing into ice: This is a physical change. It is only a change of state (liquid to solid); the substance remains \( \text{H}_2\text{O} \).
• C) Salt dissolving in water: This is a physical change. The salt and water mix but do not react to form a new chemical compound; the salt can be recovered by evaporating the water.
• D) Paper being cut into pieces: This is a physical change. Only the size and shape of the paper change, not its chemical identity.

Therefore, the correct option is A.

Step 2: Analyze Question 2 (Substances Formed When Wood Burns)

Burning (combustion) is a chemical reaction where a fuel reacts with oxygen to produce entirely new chemical substances.

• When wood burns, the carbon-based compounds in the wood react with oxygen in the air.
• This chemical reaction produces solid residue (ash), unburnt carbon particles suspended in air (smoke), and gaseous products like carbon dioxide and water vapor (gases).
• Options B, C, and D do not represent the actual chemical products of wood combustion (heat is energy, not a substance).

Therefore, the correct option is A.

Step 3: Analyze Question 3 (Law of Conservation of Mass)

The Law of Conservation of Mass is a fundamental principle in chemistry.

• It states that during any physical or chemical change, the total mass of the system remains constant.
• In other words, atoms are not created out of nothing, nor do they vanish into nothing; they are simply rearranged to form new substances.
• This matches option D: "Mass is neither created nor destroyed, only rearranged."

Therefore, the correct option is D.

Answer:

1. A (Milk turning sour)
2. A (Ash, smoke, and gases)
3. D (Mass is neither created nor destroyed, only rearranged)