Question

Students of a school had gone for an overnight camp. They gathered some wood and wanted to light a fire. They tried to light the fire with a matchstick but the wood would not catch fire. Someone suggested pouring kerosene over the wood, but no kerosene was available. Someone else suggested setting dry leaves on fire first with the matchstick and using it to light the wood. But they could not find any dry leaves.
Then Tanya brought a bottle of Vaseline(petroleum jelly) that she had brought with her to apply on her skin. She smeared it over a piece of cloth and set it on fire. She used it to light the wood.
Answer these questions.
1. Why could the wood not be set on fire by the matchstick?
2.How would kerosene have helped?
3.Why were dry leaves suggested and not green leaves?
4.Which two properties of petroleum jelly were used to light the fire?

Answer 1

1. The ignition temperature of the wood is very high. Matchstick is not able to produce enough heat and raise the temperature to the ignition temperature of the wood. So, wood does not set on fire by the matchstick.

2. Kerosene has a much lower ignition temperature than wood. When kerosene starts burning, due to the heat produced the temperature increases and equals to the ignition temperature of the wood, and the wood starts burning too.

3. Green leaves contain water molecules, so they can't be burn easily but dry leaves have no water content so they can be burnt easily.

4. Petroleum jelly is a mixture of hydrocarbons.
Two properties of petroleum jelly used to light the fire are:
(i) Petroleum jelly contains carbon, and carbon can be easily burnt.
(ii) Petroleum jellies have a high calorific value.