MediumChemistryHigh School (9–10)GeneralMixtures and Solutions › Elements, Compounds and ChangesConceptualBoard Exam
When a candle burns, the wax near the flame melts, and the flame burns…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
- When a candle burns, the wax near the flame melts, and the flame burns. Which part is a physical change, and which is a chemical change?
- A burning candle gets lighter as it burns. Where does the mass go?
Need a reminder? The key ideaShow
A physical change makes no new substance (melting, dissolving, breaking). A chemical change makes new substances (burning, rusting, cooking). Gas, colour, smell, heat, light and a new solid are clues; the test is always: is a new substance made?
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Physical. The wax melts near the flame. Melted wax is still wax; it sets again when it cools.
Step 2. (a) Chemical. In the flame, wax vapour burns with oxygen to make carbon dioxide and water vapour, new substances, with heat and light.
Step 3. (b) The carbon dioxide and water vapour are gases, and they rise into the room. That is where the lost mass goes.
Answer: (a) Melting is physical; burning is chemical. (b) Into the air, as the gases made by burning.
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