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ScienceClass 8CBSE

Which non-metal is essential for our life and all living beings inhale it during breathing?

Oxygen

ScienceClass 8CBSE

Name two soft metals which can be cut with a knife.

sodium (ii) potassium

ScienceClass 8CBSE

Which of the following property is not responsible for copper to be used as electrical conduction wires? (a) ductility (b) colour (c) good conductor of electricity it is solid

Answer is (b) colour

ScienceClass 8CBSE

Generally metallic oxides are basic and non-metallic oxides are acidic in nature. Solution of which of the following oxides in water will change the colour of blue litmus to red? (a) sulphur dioxide (b) magnesium oxide (c) iron oxide copper oxide

Soln: Answer is (a) sulphur dioxide Explanation: Sulphur forms Sulphur-di-oxide which is acidic in nature and it will convert blue litmus to red.

ScienceClass 8CBSE

Which of the following non-metals reacts and catches fire on exposure to air? (a) phosphorus (b) nitrogen (c) sulphur (d) hydrogen

Answer is (a) phosphorus

ScienceClass 8CBSE

The metal which produces hydrogen gas on reaction with dilute hydrochloric acid as well as sodium hydroxide solution is (a) copper (b) iron (c) aluminium sodium

Soln: Answer is (c) aluminium Explanation: Aluminium reacts with dilute HCl as well as NaOH to produce Hydrogen Gas.

ScienceClass 8CBSE

Which of the following reacts with cold water vigorously? (a) carbon (b) sodium (c) magnesium sulphur

Answer is (b) sodium Explanation: Sodium reacts vigorously with water, hence it is stored in kerosene to avoid reaction of sodium with moisture in the atmosphere and oxygen.

ScienceClass 8CBSE

Metals generally react with dilute acids to produce hydrogen gas. Which one of the following metals does not react with dilute hydrochloric acid? (a) magnesium (b) aluminium (c) iron (d) copper

Answer is (d) copper Explanation: Less reactive metals like Copper, gold and silver does not react with dilute HCl to produce Hydrogen gas.

ScienceClass 8CBSE

Metals are generally solid. Which of the following metals is in the liquid state at room temperature? (a) mercury (b) silver (c) aluminium sodium

Answer is (a) mercury

ScienceClass 8CBSE

Metals are generally hard. Which of the following metals is an exception and can be cut with a knife? (a) iron (b) sodium (c) gold magnesium

Soln: Answer is (b) sodium Explanation: Sodium is highly reactive metals which looks soft and can be cut into pieces by using knife.

ScienceClass 8CBSE

Materials which can be drawn into wires are called ductile. Which of the following is not a ductile material? (a) silver (b) copper (c) sulphur aluminium

Soln: Answer is (c) sulphur Explanation: Ductility is the property of metals. Here Sulphur is non-metal which is not have ductile property.

ScienceClass 8CBSE

Boojho has learnt that non-metals on beating with a hammer are generally broken into pieces. Which of the following is a nonmetal? (a) iron nail (b) aluminium wire (c) copper plate piece of coal

Soln: Answer is (d) piece of coal Explanation: Iron, Copper and aluminum are metals whereas coal is a non-metal hence answer is (d) piece of coal.

ScienceClass 8CBSE

The substance that will be flattened on beating with a hammer is (a) crystal of iodine (b) lump of sulphur (c) piece of coal zinc granule

Soln: Answer is (d) zinc granule Explanation: Metals can be beaten into thin sheets whereas non-metals break into pieces on hitting hence answer is (d) zinc granule.

ScienceClass 8CBSE

Which of the following is not a metal? (a) copper (b) sulphur (c) aluminium (d) iron

Answer is (b) sulphur

ScienceClass 8CBSE

Forest fire produces a lot of air pollution. Write in brief about the reasons for forest fires.

Reasons for the forest fires are given below Lightning causes a forest Campfire may also be a Human negligence is one of the main reasons for a forest At high temperature, sometimes dry grass catches fire which spreads throughout the

ScienceClass 8CBSE

Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

This is because wood comes from the cutting of trees and forests, which affects the environment adversely. Cutting of trees results in an increase of CO2 Content in the air, as a result, global warming takes place. Also, burning of wood releases a lot of harmful gases, including carbon-di-oxide, carbon-monoxide and sulphur-di-oxides.

ScienceClass 8CBSE

The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have a vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

I prefer CNG over petrol because CNG is a clean fuel and it produces very less harmful gases. This will reduce the air pollution that happens due to burning of fuel. The calorific value of CNG is higher than that of petrol hence it can be economical.

ScienceClass 8CBSE

Give two examples each for a solid, liquid and gaseous fuel along with some important uses.

Coal- Is used to produce electricity through steam Coke- Coke is used as reducing agent in the extraction of Liquid Fuels Petrol: Used to run small automobiles like bike and Kerosene- Used for domestic heating purpose and jet engines as Gaseous fuel CNG – CNG is used to run Natural Gas – It is used for industrial

ScienceClass 8CBSE

The fire extinguishers are useful for controlling the fire in the following ways:

Fire extinguishers extinguish the fire by cutting the supply of air or by bringing down the temperature of fuel or both. The most common fire extinguisher is water. But waterworks only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Water is also not suitable for fires involving oil and Do you recall that water is heavier than oil? So, it sinks below the oil, and oil keeps burning on the top. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases it does not harm the electrical equipment.

ScienceClass 8CBSE

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling fire.

Three essential requirements to produce gas are: Combustible substance or Heta to acquire ignition

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