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Which among the following is considered as the cleanest fuel? (a) cow dung cake (b) petrol (c) kerosene hydrogen gas

Soln: Answer is (d) hydrogen gas Explanation: Hydrogen gas is called a clean fuel as it does not produce any dangerous gases. It can only produce heat and water as products.

ScienceClass 8CBSE

In villages, people use wood as fuel because: (a) it is considered to be an ideal fuel. (b) of its easy availability and low cost. (c) it is environment-friendly. it catches fire easily.

Soln: Answer is (b) of its easy availability and low cost.

ScienceClass 8CBSE

The calorific value of a fuel is expressed in a unit called (a) kilojoule per litre (b) kilogram per millilitre (c) kilojoule per gram kilojoule per kilogram

Soln: Answer is (d) kilojoule per kilogram

ScienceClass 8CBSE

Choose the incorrect statement from the following. Forest fires are usually due to: (a) carelessness of humans (b) heat of sun (c) cutting of trees lightning strike

Soln: Answer is (c) Cutting of trees. Deforestation or Cutting of trees is not the cause for the forests fire. Forest fires are usually caused due to heat of sun, lightning strike and carelessness of humans

ScienceClass 8CBSE

2. Choose the correct statement about inflammable substances from the following. They have: (a) low ignition temperature and cannot catch fire easily. (b) high ignition temperature and can catch fire easily. (c) low ignition temperature and can catch fire easily. high ignition temperature and cannot catch fire easily.

Answer is (c) low ignition temperature and can catch fire easily. Inflammable substances are substances that easily catch fire. They are substances, which have very low ignition temperatures and when exposed to air, they catch fire easily.

ScienceClass 8CBSE

The substance expected to have the highest ignition temperature out of the following is (a) kerosene (b) petrol (c) coal alcohol

Answer is (c) coal Explanation: Ignition temperature is the lowest temperature at which a substance catches fire. Among the given options coal have highest ignition temperature.

ScienceClass 8CBSE

If a person’s clothes catch fire, the best way to extinguish the fire is to: (a) throw water on the clothes. (b) use the fire extinguisher. (c) cover the person with a woollen blanket. cover the person with a polythene sheet.

Soln: Answer is (c) cover the person with a woollen blanket. Explanation: By covering a person’s clothes with a woollen blanket, we are cutting off the oxygen required for the burning of clothes. Hence fire will be distinguished.

ScienceClass 8CBSE

On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of the non-availability of (a) oxygen (b) water vapours (c) carbon dioxide wax

Soln: Answer is (a) oxygen Explanation: Oxygen is necessary for combustion. When we place an inverted tumbler over a burning candle, o xygen supply will be stopped, this extinguishes the flame.

ScienceClass 8CBSE

The substance that does not burn with flame is (a) LPG (b) camphor (c) dry grass charcoal

Soln: Answer is (d) charcoal Explanation: The charcoal will not vaporize on heating. Hence charcoal will not burn by producing flame. Charcoal glows on combustion.

ScienceClass 8CBSE

Which of the following is not a combustible substance? (a) camphor (b) glass (c) straw alcohol

Soln: Answer is (b) glass Explanation: Camphor, glass, straw can burn in the air, and they are combustible substances. Glass will not burn in the air, and it is a non-combustible substance.

ScienceClass 8CBSE

Magnesium ribbon on burning in air produces: (a) magnesium oxide, water and light. (b) magnesium oxide and heat. (c) magnesium oxide, heat and light. magnesium oxide, water and heat.

oln: Answer is (c) magnesium oxide, heat and light Explanation: Magnesium reacts with atmospheric oxygen to get magnesium oxide by liberating heat and light.

ScienceClass 8CBSE

Which one of the following has the highest calorific value? (a) kerosene (b) biogas (c) LPG petrol

Soln: Answer is (c) LPG Explanation: Calorific represents the amount of heat generated when fuel burns completely. The calorific value of LPG is 55000-kilo joules/ Kg, which is high when compared to the calorific value of Kerosene, Biogas and petrol.

ScienceClass 8CBSE

A substance which reacts with oxygen giving heat, is called a combustible substance. Which one of the following is a combustible substance? (a) iron nail (b) glass (c) stone piece wood

Soln: Answer is (d) wood Explanation: Glass, iron nail and stone piece will not burn like wood hence they are not combustible. Wood burns in the air, and it is called a combustible substance.

ScienceClass 8CBSE

What steps would you suggest for the judicious use of fossil fuels?

Steps to take for the judicious use of fossil fuels Use fossil fuels only when it is Switch off automobile engine while standing in a Use solar energy wherever We can use natural gas in place of fossil fuels Use Alternate sources of energy like, solar, wind and biomass should be used in place of fossil

ScienceClass 8CBSE

Coal reserves are said to be enough to last for another hundred years. Do you think we need to worry in such case? Why or why not?

Coal reserves are said to be enough to last for another hundred years. Because there will be no availability of coal after the exhaust of existing coal resource. To produce coal it will take millions of years. Dead organisms should be buried in the earth and it should decompose in the absence oxygen. For all these process coal formation needs millions of years.

ScienceClass 8CBSE

Write some important uses of the various constituents of petroleum.

Soln: Constituents of Petroleum are petroleum gas, petrol, diesel, kerosene, lubricating oil, paraffin wax and Bitumen. Constituents of petroleum Uses Petroleum Gas (LPG) Fuel for home and industry Petrol Used as fuel for light automobiles. Kerosene Fuel for lamps, stoves and jet aircrafts Diesel Used as fuel for large automobiles and electric generators Lubricating oil Used as lubricant for engines Paraffin wax Used in preparation of Ointments, candles, vaseline etc. Bitumen Used in preparation of Paints, road surfacing

ScienceClass 8CBSE

Why petrol is exhaustible natural resource, whereas sunlight is not? Explain.

Petrol is a fossil fuel which is produced after years of decomposition of buried and plants deep down the earth. For producing petrol we need millions of years. If we use petrol higher than the requirement. Perol will get exhausted. Whereas sun is a continuous and unlimited source of energy. Sun is renewable resource of energy which will not get exhausted.

ScienceClass 8CBSE

Imagine that all the exhaustible natural resources are exhausted by human activities. Do you think survival of living beings would be possible.? If yes, why?, If not, why not?

If all the exhaustible natural resources are exhausted by human activities then survival of living beings would be impossible. Fossil fuels are very important source of energy for us today. On burning fossil fuel and give off heat and light. Heat produced can be used to cook food or to run engines such as automobile engines. It can be used to generate electricity as in powerhouse where the most common fuel used is coal.

ScienceClass 8CBSE

While driving what are the tips we must follow to save petrol/ diesel/natural gas?

Steps to follow while driving to save petrol/ diesel/natural gas Drive at a constant and moderate speed as far as possible, Switch off the engine at traffic lights or at a place where you have to wait, Ensure correct tyre Ensure regular maintenance of the

ScienceClass 8CBSE

We read in newspapers that burning of fuels is a major cause of global warming. Explain why.

Burning of fuels lead to release of carbon-di-oxide, carbon-monoxide and SO2 gases. These are the green houses gases that not only increases the air pollution but also contribute for the global warming by trapping radiations from sun.

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