Question 1
A student drops a bath bomb into warm water and notices vigorous fizzing and bubbles forming. Which sign of a chemical change is BEST shown by the bubbles?No clue? Show me the answer
Correct answerCorrect!
IncorrectStep-by-step walkthrough
Choose a solution method
Method #1Direct ApproachStep 1: Identify what the bubbles represent
Bubbles forming in a liquid during a reaction indicate that a gas is being produced. In this case, the bath bomb reacts with water to release carbon dioxide gas.
Step 2: Match to the signs of chemical change
The five key signs of a chemical change are: gas production, precipitate formation, colour change, energy change, and irreversibility. Fizzing and bubbling clearly match production of a gas.
Step 3: Choose the correct answer
The bubbles are the visible sign of a gas being produced, so the correct answer is Production of a gas.
Method #2Process of EliminationStep 1: Identify what is being asked
The question asks which sign of a chemical change is best shown by the bubbles observed when a bath bomb is dropped into water.
Step 2: Eliminate 'Formation of a precipitate'
A precipitate is a solid that appears in a liquid. Bubbles are not a solid — they are pockets of gas rising through liquid, so this option is incorrect.
Step 3: Eliminate 'Change in colour'
A colour change involves the mixture turning a different colour. Bubbles forming does not describe a colour change, so this option can be eliminated.
Step 4: Eliminate 'Irreversibility'
Irreversibility describes whether the change can be undone — it is not something you directly observe as bubbles. This option does not describe what the bubbles indicate.
Step 5: Select the correct answer
After eliminating the other options, Production of a gas is the sign best shown by the fizzing and bubbles observed.
Question 2
When wood is burned in a fireplace, ash and smoke are produced. Why is this BEST described as a chemical change?No clue? Show me the answer
Correct answerCorrect!IncorrectStep-by-step walkthrough
Choose a solution method
Method #1Direct ApproachStep 1: Recall the definition of a chemical change
A chemical change is a process that produces one or more new substances with different properties from the original materials, and is usually permanent and difficult to reverse.
Step 2: Apply the definition to burning wood
When wood burns, it produces ash, smoke, carbon dioxide, and water vapour — completely different substances from the original wood. You cannot turn ash back into wood, so the change is also irreversible.
Step 3: Choose the correct answer
The best description of a chemical change is that new substances with different properties are formed and the change is irreversible, which is exactly what happens when wood burns.
Method #2Process of EliminationStep 1: Identify what is being asked
The question asks why burning wood is best described as a chemical change — so we need to find the answer that correctly explains the key feature of a chemical change.
Step 2: Eliminate 'changes shape and size'
Changing shape and size describes a physical change, not a chemical one. Physical changes alter form but do not create new substances, so this option is incorrect.
Step 3: Eliminate 'temperature increases'
A temperature increase alone does not confirm a chemical change. For example, heating water raises its temperature but does not create a new substance. This option is not sufficient.
Step 4: Eliminate 'becomes lighter'
The wood becoming lighter is an observation but it does not by itself explain why it is a chemical change — it does not mention new substances or irreversibility.
Step 5: Select the correct answer
The only option that correctly identifies both key features of a chemical change — new substances formed and irreversibility — is the first option.
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