Combustion and flame Sample paper

Q1. The diagram represents the zones of flame, which zone will produce maximum soot?

zones of flame

Solution

Zone 2 represents luminous zone which has unburnt carbon particles.

Q2. A non-combustible substance is

Solution

Stone is a non-combustible substance.

Q3. What is ignition temperature?

Solution

The lowest temperature at which a substance catches fire is called its ignition temperature.

Q4. Which flame zone is hottest?

Solution

The blue zone is the hottest part of a flame.

Q5. Out of cow dung, coal and LPG as fuel. Which fuel would you prefer to boil a given quantity of water? Give reason for your answer.

Solution

LPG will be preferred out of all to boil a given quantity of water. This is because it has the highest calorific value in all and does not cause pollution on burning.

Q6. _______ is a chemical process in which a substance reacts rapidly with oxygen in the air to give off heat and light.

Solution

Combustion is a chemical process in which a substance reacts rapidly with oxygen in the air to give off heat and light.

Q7. The combustion reaction which occurs on its own is called ________ combustion.

Solution

The combustion reaction which occurs on its own is called spontaneous combustion.

Q8. Element showing spontaneous combustion is

Solution

Element showing spontaneous combustion is Potassium.

Q9. Ignition temperature of kerosene oil is higher than that of wood.

  • 1) True
  • 2) False

Solution

False

Q10. Zones of flame interchange based on the substance burning.

  • 1) True
  • 2) False

Solution

False

Q11. Kerosene has a very _______ ignition temperature.

Solution

Kerosene has a very low ignition temperature.

Q12. The flame gets progressively cooler as we move in from the outside edge towards the wick.

  • 1) False
  • 2) True

Solution

True

Q13. You are asked to choose a good fuel. Write four characteristics you will keep in view?

Solution

Characteristics of a good fuel are: Readily available Cheap High calorific value It does not leave behind any harmful substances

Q14. The substances that do not burn are non-combustible.

  • 1) True
  • 2) False

Solution

True

Q15. A student places two lighted candles on a bench. What will happen if he covers one burning candle with a small jar and the other with a larger jar? Why?

Solution

The candle in the larger jar burns longer time. This is because the larger jar contains more amount of oxygen.

Q16. (i) How can burning of fuel cause acid rain? (ii) How can burning of petrol and diesel be responsible for reducing the visibility of air?

Solution

(i) Diesel produces sulphur dioxide gas. Petrol produces oxides of nitrogen. Oxides of nitrogen and sulphur dissolve in rain water and form acids. Such rain is acid rain. (ii) Small suspended particles emitted by automobiles due to burning of petrol and diesel reduce visibility of air.

Q17. Rapid combustion is a form of combustion in which large amounts of heat and light energy are released.

  • 1) True
  • 2) False

Solution

True

Q18. What do you mean by inflammable substances? Give some examples of inflammable substances

Solution

The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG), etc.

Q19. Why is it dangerous to burn coal in closed room?

Solution

A closed room has limited supply of oxygen. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas.

Q20. Which fuel has highest calorific value?

Solution

Hydrogen gas

Q21. Inflammable substances are petrol, alcohol, Liquefied Petroleum Gas and glass.

  • 1) False
  • 2) True

Solution

False

Q22. (a) Why soda-acid fire extinguisher is not successful in controlling fire caused due to burning oils? (b) How does a soda-acid fire extinguisher work?

Solution

(a) Soda-acid fire extinguisher is not successful in controlling fire due to oils because it uses water to cool the combustible substance. But water is heavier than oil and settles down below the oil particles and oil keeps on burning on the top. Hence, fire cannot be controlled.

(b) Soda-fire extinguisher is based on cooling the temperature of the burning object. This type of fire extinguisher contains sulphuric acid and sodium bicarbonate. When the extinguisher becomes functional, the sulphuric acid reacts with sodium bicarbonate to produce carbon dioxide. Carbon dioxide when released cuts off the supply of oxygen and water is released to bring down the ignition temperature of the burning object. In this way, fire is controlled.

Q23. Define combustion. What are the different types of combustion? Describe them with one example each.

Solution

A chemical process in which a substance reacts with the oxygen (of air) to give heat and light is called combustion. The different types of combustion are: Complete combustion – When substance gets sufficient amount of oxygen, the substance burns with a blue flame, it is called complete combustion. Example: Combustion of L.P.G. Incomplete combustion – When a substance does not get sufficient amount of oxygen, it burns with a yellow flame, it is called incomplete combustion.

Example: Combustion of wood. Rapid combustion – The reaction in which a large amount of heat and light are produced in a short time is called rapid combustion. Example: Burning of LPG in a gas stove to give heat and light is an example of rapid combustion. Slow combustion – The type of combustion reaction in which fuel burns at a very slow rate and a part of fuel remains unburnt is called slow combustion. Example: Food oxidized in our body to release energy.