Class 8 General Science NCERT Based - 2 - Free MCQ Practice Test with solutions,

• Microorganisms contaminate food items. Preservatives are used to prevent the contamination of food items.

• Sodium Benzoate and Sodium Metabisulfite are common food preservatives. These chemicals are used for the manufacture of jams and squash. Salt, sugar, edible oil and vinegar are used to inhibit the multiplication of microorganisms in food items.

• For example, salt, edible oil and vinegar are used to prevent the contamination of pickles; and sugar is used to prevent the contamination of jams and squash.

• Pasteurised milk can be consumed without boiling, because it is free from microorganisms.

• To pasteurise the milk, it is heated up to 70 degree celsius for 15-20 seconds and then cooled rapidly; and then stored or packaged. This decreases the multiplication of microorganisms. Thus, Louis Pasteur invented this method; hence it is named after him.

• The atmospheric nitrogen cannot be taken directly by plants and animals. Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen.

• A polymer is made up of many small units. A polymer can be either natural or synthetic. Cotton is a natural polymer, called cellulose.

• Cellulose comprises many molecules of glucose monomers. Wool, silk and leather are also examples of natural polymers. Synthetic fibres are obtained from the chemical reaction of petrochemicals.

• Polyethylene Terephthalate is a synthetic polymer, a common example of polyester. It is used to make bottles, utensils, film, wires, and other essential products. Like synthetic fibres, plastic is also a polymer. Polythene (polyethene) is an example of plastic. Plastic takes many years to decompose, hence it is not environment friendly.

• Rayon, nylon, terylene and acrylic are all examples of synthetic fibres. Rayon is also called synthetic silk.

• It is made by the synthetic treatment of wood cellulose. It is cheaper than silk. Nylon is synthesised from coal, air and water. It is the first fully synthetic fibre. Nylon wires are more durable than steel wire.

• Terylene is a popular polyester. Fabric made from this fibre does not get wrinkled easily. Acrylic is used to make wool, sweater, shawl and blankets.

• Objects made of acrylic are cheaper and their maintenance is easier.

• Uniforms of firemen have a coating of melamine plastic to make them flame resistant. Melamine is fire-resistant and is more heat resistant as compared to other plastics. Bakelite is a bad conductor of heat and electricity. It is used to make switches, handles of utensils, etc.

• Polyester is a polymer made up of (poly + ester) monomers. Ester is the chemical responsible for the sweet smell of fruits. Teflon is a special plastic, which does not allow oil or water to stick on it. It is used to make the non-sticky layer on the utensils used to cook food.

• Phosphorus, Iodine and Bromine are all non-metals. They do not generally react with water, but are highly reactive in air. Of the three, phosphorus is the most reactive non-metal. It catches fire on being exposed to air.

• To prevent the contact of phosphorus with atmospheric oxygen, it is stored in water.Bromine is the only non- metal which exists as a liquid in room temperature. Sodium is a metal which is highly reactive in nature. It reacts rapidly with air and water.

• A lot of energy is generated in these reactions. Therefore, it is stored in kerosene oil.

Neoprene (CR), also called polychloroprene or chloroprene rubber, synthetic rubber produced by the polymerization (or linking together of single molecules into giant, multiple-unit molecules) of chloroprene.

• Polyester is a generalised term for any fabric or textile, which is made using polyester yarns or fibres. It is a shortened name for a synthetic, man-made polymer, which, as a specific material, is most commonly referred to as a type called polyethylene terephthalate (PET).

• It is made by mixing ethylene glycol and terephthalic acid. That all sounds extremely scientific, but basically, polyester is a kind of plastic. What are some characteristics of polyester fabric

  1. Polyester is very durable and is resistant to many chemical

  2. This is a popular fabric in the fashion industry as it is resistant to shrinking and stretching. It is also resistant to wrinkling and abrasions

  3. The fibres used to create Polyester are very strong yet lightweight

  4. The fibres are easily dyed

  5. It retains its shape very well

  6. Polyester fabric is easy to look after and it can be washed and dried at home

  7. It is a quick drying fabric, so a popular choice for outdoors clothing

The decreasing order of the reactivity of the common metals is given below: Li, K, Na, Ba, Ca, Mg , Al, Mn, Zn, Fe, Ni, Sn, Pb, [H], Cu,Hg, Ag, Au,Pt

• A mixture of Antimony Trisulphide and Potassium Chlorate is applied on the heads of matchsticks. Earlier, white phosphorus was also used along with these two chemicals, but due to its harmful effects, its use was discontinued in the manufacture of safety matches.

• The rubbing surface has powdered glass and a little red phosphorus.. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus.

• This immediately reacts with Potassium Chlorate in the matchstick head to produce enough heat to ignite Antimony Trisulphide and start the combustion.

• Water cools the combustible and the water vapours formed also surround the combustible material, cutting off the supply of air. Thus, the fire is extinguished. The most common fire extinguisher is water.

• But 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.

• It does not harm the electrical equipment. CO₂ can be stored at high pressure as a liquid in cylinders. When released from the cylinder, CO₂ expands enormously in volume and cools down.

• Another way to get CO₂ is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. On coming near the fire, these chemicals decompose to give off CO₂.

• The substances which vapourise on combustion, produce flame. The flame produced during the combustion of candle has three parts: non luminous zone, luminous zone and non combustible zone.

• Unburnt carbon particles are present in the luminous zone of the flame. The non luminous zone of the candle flame has the highest temperature. It is the outermost region of the flame, which is blue in colour.

• Incomplete combustion of fuels produce carbon monoxide. It is an extremely poisonous gas.Combustion of most fuels gives out carbon dioxide in the environment, the excessive quantity of which results in global warming.Combustion of coal and petrol give out sulfur dioxide.

• It is an extremely suffocating and caustic gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.CNG produces the oxides of sulfur and nitrogen in very small amounts.

• Around 67 % of total commercial energy produced in India and 98% of India's total account is found in Gondwana rocks of the Moran region.

• The main regions of Gondwana rocks are found in West Bengal, Jharkhand and Odisha. There are four types of coal found in India: Anthracite (Best quality of coal found only in Jammu & Kashmir); Bituminous (Second best quality of coal); Lignite (Found in Tamil Nadu, Rajasthan, Gujarat and Jammu & Kashmir).

• Coal accounts for approx. 67 % of total commercial energy produced in India and 98% of India’s total account is found in Gondwana rocks of Moran region.

• Carbonization is a slow pyrolysis process in which biomass is converted into a highly carbonaceous, charcoal-like material.

• Typically, carbonization consists of heating the biomass in an oxygen-free or oxygen-limited environment, and reaction conditions are tailored to maximize the production of char.

• Traditionally, charcoal production is one of the oldest chemical conversion processes known to mankind. Even today, charcoal production continues on a large scale worldwide, and is still making use—in part—of tradition charcoal kilns.

• Furthermore, there is renewed interest in carbonization, both from a scientific and a commercial point of view, for the production and application of biochar and torrefied biomass.