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ScienceClass 8CBSE

People usually keep Angethi/burning coal in their closed rooms during the winter season. Why is it advised to keep the door open?

Burning of coal produces carbon monoxide, which is poisonous for the people inhaling it. If the door is closed, oxygen level will be less and carbon-monoxide level will be high. This is harmful for the person sleeping in the room. Hence the door should be closed.

ScienceClass 8CBSE

If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame?

Iron will not produce flame rather, it glows red hot.

ScienceClass 8CBSE

Column A Column B Column C (zone) Column C (colour) (a) hottest part (i) innermost zone (x) the blue of unburnt wax vapours (b) moderately (ii) the middle zone of black partial (y) black hot combustion (c) least hot (iii) the outer zone of complete (z) yellow

combustion Soln: Column A Column B Column C (zone) Column C (colour) (a) hottest part (iii) the outer zone of complete combustion (x) the blue of unburnt wax vapours (b) moderately (ii) middle zone of black partial (z) yellow hot combustion (c) least hot (i) innermost zone (y) black

ScienceClass 8CBSE

Match the items of Column A with the items of Column B Column A Column B (a) Oxides of sulphur and nitrogen (i) fire extinguisher (b) CNG (ii) incomplete combustion of coal (c) Oxygen (iii) very low ignition temperature (d) inflammable substance (iv) acid rain (e) carbon dioxide (f) carbon monoxide (v) necessary for combustion vi) fuel for automobiles.

Column A Column B (a) Oxides of sulphur and nitrogen (iv) acid rain (b) CNG vi) fuel for automobiles. (c) Oxygen (v) necessary for combustion (d) inflammable substance (iii) very low ignition temperature (e) carbon dioxide (i) fire extinguisher (f) carbon monoxide (ii) incomplete combustion of coal

ScienceClass 8CBSE

Boojho wants to separate the following materials as combustible and non-combustible. Can you help him? Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, and wood.

Combustible Non-Combustible Straw Chalk Stone Paper Wood Iron rod Charcoal Copper coin Cardboard Glass

ScienceClass 8CBSE

Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities?

Compared to petrol and diesel, CNG is a clean fuel. It produces very less amount of gases which are poisonous hence in big cities CNG is used in place of petrol and diesel.

ScienceClass 8CBSE

Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

Anu should use the outermost zone of the flame, which is the hottest part of the flame.

ScienceClass 8CBSE

Two glass jars, A and B, are filled with carbon dioxide and oxygen gases, respectively. In each jar, a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?

Candle remains in jar B because it is filled with oxygen which is necessary for the combustion and flame. In Jar A candle will not burn due to lack of oxygen.

ScienceClass 8CBSE

Some words (underlined) in the following sentences are jumbled up. Write them in their correct form. (a) Seldie is a combustible substance. (b) Slags is a non-combustible material. (c) Chittsmack does not burn by itself. (d) Some substances on combustion produce thea and mafel. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its Aricolci uelav.

Soln: heat, calorific

ScienceClass 8CBSE

Fill in the blanks in the following sentences. (a) A process in which a substance reacts with to give off heat is called combustion. (b) When the clothes of a person catch , the person is covered with a to extinguish fire. (c) The temperature at which a substance catches fire is called its temperature. (d) The substances which have very ignition temperature and can easily catch fire with a flame are called substances. The substances which vapourise during , give flame.

A Chemical process in which a substance reacts with oxygen to give off heat is called When the clothes of a person catch fire, the person is covered with a blanket to extinguish the The lowest temperature at which a substance catches fire is called its ignition The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable The substances which vapourise during burning give

ScienceClass 8CBSE

Shyam was cooking potato curry on a chulha. To his surprise, he observed that the copper vessel was getting blackened from the outside. It may be due to: (a) proper combustion of fuel. (b) improper cooking of potato curry. (c) improper combustion of the fuel. burning of the copper vessel.

Soln: Answer is (c) improper combustion of the fuel. The copper vessel was blackened from outside because of the insufficiency of oxygen.

ScienceClass 8CBSE

Choose the incorrect statement from the following. A good fuel is one which: (a) is readily available. (b) produces a large amount of heat. (c) leaves behind many undesirable substances. burns easily in the air at a moderate rate.

Soln: Answer is (c) leaves behind many undesirable substances. Explanation: Good fuel is readily available and produces a large amount of heat. A good fuel burns easily in the air at a moderate rate and will not leave behind any undesirable substances.

ScienceClass 8CBSE

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.

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