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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

ScienceClass 8CBSE

Manu was heating oil to fry potato chips. The cooking oil, all of a sudden, caught fire; he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done?

No, the step taken by Manu to extinguish fire from oil is not suitable. Because along with water oil also spread which will increase the flame. Steps that should have been taken by Manu Manu should have put the flame of the burner off and put a lid on a frying pan. This cuts the contact between flame and oxygen and the flame will be gradually extinguished

ScienceClass 8CBSE

Fuel efficiency is determined by its calorific value, which is the amount of heat energy produced on the complete combustion of 1 kg of fuel. The calorific value of a fuel is expressed in kJ/kg.

x Watchglass containing petrol will catch fire first because petrol is a highly combustible liquid, and its calorific value is 45.8 MJ/kg. This is very less when compared to milk or mustard oil.

ScienceClass 8CBSE

What do you understand by fuel efficiency?

Fuel efficiency is determined by its calorific value, which is the amount of heat energy produced on the complete combustion of 1 kg of fuel. The calorific value of a fuel is expressed in kJ/kg.

ScienceClass 8CBSE

Cracker on the ignition produces sound. Why?

Upon ignition, the cracker releases gas is released out and the gas is heated by the amount of heat generated by the cracker. This led to an explosion resulting in the production of sound.

ScienceClass 8CBSE

True/False against the following statements and also correct the false statement. 1. A physical process in which a substance reacts with oxygen to give off heat is called combustion. 2. Water is the best extinguisher for fires involving electrical equipment. 3. Alcohol, CNG and LPG are inflammable substances. 4. Increased concentration of nitrogen in the air is believed to cause global warming. 5. Greater the calorific value, the better the fuel. 6. The middle zone is the hottest zone of a flame. 7. The substances, which vaporise during burning, give flame.

False- Carbon-di-oxide is the best extinguisher for fires involving electrical False- Increased concentration of carbon-di-oxide in the air is believed to cause global False- Outer zone is the hottest zone of a

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.

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