Class 8 Chemistry — Important Questions with Answers

What it is

Metals are shiny, bendable conductors of heat and electricity; non-metals are dull, brittle insulators.

Answer

Metals are generally lustrous (shiny), malleable and ductile, good conductors of heat and electricity, and sonorous. Non-metals are usually dull, brittle (break easily) and poor conductors of heat and electricity. For example, iron is a shiny conductor while sulphur is a dull, brittle insulator.

What it is

Metals are usually hard, shiny, malleable, ductile and good conductors; non-metals are usually the opposite.

Answer

Metals are generally hard, lustrous (shiny), malleable (can be beaten into sheets), ductile (can be drawn into wires), sonorous and good conductors of heat and electricity, for example iron and copper. Non-metals are generally soft (or brittle if solid), dull, non-malleable, non-ductile and poor conductors of heat and electricity, for example sulphur and carbon; exceptions include graphite, which conducts electricity.

What it is

Metals react with oxygen to form basic oxides, and reactive metals react with water and acids to release hydrogen.

Answer

When a metal reacts with oxygen it forms a metal oxide, which is basic in nature; for example, 2Mg + O2 -> 2MgO. When a reactive metal reacts with a dilute acid it forms a salt and releases hydrogen gas; for example, Zn + H2SO4 -> ZnSO4 + H2. The hydrogen gas burns with a pop sound.

What it is

A more reactive metal displaces a less reactive metal from its salt solution.

Answer

A displacement reaction is one in which a more reactive metal displaces a less reactive metal from its salt solution. For example, when an iron nail is dipped in blue copper sulphate solution, iron being more reactive displaces copper: Fe + CuSO4 -> FeSO4 + Cu. The blue colour fades and a reddish-brown copper coating forms on the nail.

What it is

Metals and non-metals are chosen for uses that match their properties.

Answer

Metals: copper and aluminium are used to make electrical wires because they conduct electricity well, and iron and steel are used to make machines, vehicles and utensils because they are strong. Non-metals: oxygen is used for respiration and combustion, and chlorine is used to disinfect drinking water; carbon (graphite) is used in pencils.

What it is

Burning needs three partners together: fuel, oxygen and enough heat.

Answer

For combustion, three things must be present together: a combustible substance (fuel), a supporter of combustion (usually oxygen from air) and heat to raise the fuel to its ignition temperature. If any one is removed, burning stops - which is why covering a fire with a blanket (cutting off oxygen) or water (cooling below the ignition temperature) puts it out.

What it is

A good fuel gives lots of heat, is cheap, easy to store and burns cleanly.

Answer

A good fuel should have a high calorific value (release a large amount of heat per kilogram), a moderate ignition temperature, be readily available and cheap, be easy to store and transport, and burn without leaving much ash or releasing harmful smoke. For example, LPG is a good domestic fuel because it has a high calorific value and burns cleanly.

What it is

Combustion needs a combustible substance (fuel), a supporter of combustion (oxygen/air) and heat to reach the ignition temperature.

Answer

For combustion to occur there must be a combustible substance (a fuel), a supporter of combustion (oxygen, usually from air), and the temperature must reach the ignition temperature of the fuel - the lowest temperature at which it catches fire. If any one of these is removed, combustion stops.

What it is

Combustion can be rapid, spontaneous or explosive depending on how fast it happens.

Answer

Rapid combustion is combustion that takes place quickly with the production of heat and light when a substance is brought near a flame, for example a burning matchstick or LPG on a stove. Spontaneous combustion is combustion in which a material suddenly bursts into flames without any external heat, for example white phosphorus in air. Explosive combustion is a very fast reaction that produces heat, light, sound and a large amount of gas suddenly, for example the bursting of firecrackers.

What it is

A candle flame has three zones - a dark inner zone, a luminous middle zone and a hot, non-luminous outer zone.

Answer

A candle flame has three zones. The innermost dark zone contains unburnt wax vapour and is the least hot. The middle luminous yellow zone shows partial combustion and gives the flame its light. The outermost non-luminous blue zone shows complete combustion and is the hottest part of the flame, which is why goldsmiths use it with a metal blowpipe.

What it is

Calorific value is the amount of heat produced by completely burning 1 kg of a fuel.

Answer

The calorific value of a fuel is the amount of heat energy produced when 1 kilogram of the fuel is completely burnt. Its unit is the kilojoule per kilogram (kJ/kg). A good fuel is one that has a high calorific value, a moderate ignition temperature, low cost, easy availability, and produces little smoke and residue.

What it is

A fire is put out by removing fuel, cutting off oxygen or lowering the temperature below the ignition point.

Answer

Water extinguishes a fire by cooling the burning substance below its ignition temperature and by cutting off the supply of air (oxygen). Water is not used on oil fires because oil is lighter than water, so it floats and the fire spreads; it is not used on electrical fires because water conducts electricity and can give a shock. For such fires, a carbon dioxide extinguisher is used, which cuts off oxygen and cools the fuel.

What it is

Coal and petroleum are fuels formed from ancient buried life over millions of years.

Answer

Coal and petroleum are called fossil fuels because they were formed from the dead remains of prehistoric plants and animals buried under the earth for millions of years under high heat and pressure. They are exhaustible natural resources - they take millions of years to form but are being used up fast - so they must be conserved and used carefully.

What it is

Inexhaustible resources are unlimited (sunlight, air); exhaustible resources are limited and can run out (coal, petroleum).

Answer

Inexhaustible natural resources are present in unlimited quantity in nature and are not likely to be used up by human activities, for example sunlight and air. Exhaustible natural resources are present in limited quantity and can be used up by human activities, for example coal, petroleum and natural gas, which are fossil fuels formed over millions of years.

What it is

Processing (destructive distillation) of coal gives useful products - coke, coal tar and coal gas.

Answer

When coal is processed in industry (destructive distillation), it gives coke, coal tar and coal gas. Coke is an almost pure form of carbon used in the extraction of metals and to make steel. Coal tar is a black liquid used to make dyes, paints, explosives and medicines. Coal gas is used as a fuel in industries.

What it is

Petroleum is a mixture separated by refining into useful fractions such as petrol, diesel, kerosene and LPG.

Answer

Refining is the process of separating the various useful constituents (fractions) of petroleum in a petroleum refinery, based on their different boiling points. Some fractions obtained are petrol (fuel for cars and motorcycles), diesel (fuel for heavy vehicles and generators) and kerosene (fuel for stoves and lamps); others include LPG, lubricating oil and bitumen.

What it is

Natural gas is a clean fossil fuel; compressed to CNG it is used as a vehicle fuel and piped as PNG.

Answer

CNG stands for Compressed Natural Gas, which is natural gas stored under high pressure. It is considered a cleaner fuel because it burns almost completely and produces very little smoke and fewer harmful gases than petrol or diesel, so it causes less air pollution. It can also be supplied through pipes as Piped Natural Gas (PNG) to homes and factories.

What it is

Most plastics do not rot (they are non-biodegradable), so they pile up for centuries.

Answer

Most plastics are non-biodegradable, which means micro-organisms cannot break them down, so they stay in the environment for hundreds of years. Burning plastic releases harmful gases, and discarded plastic chokes drains, harms animals that swallow it and pollutes soil and water. The best approach is to reduce, reuse and recycle plastic.

What it is

Synthetic fibres such as rayon, nylon, polyester and acrylic are man-made from chemicals (mostly petroleum products).

Answer

Four common synthetic fibres are rayon, used to make clothes and mixed with cotton; nylon, used to make ropes, parachutes, socks and toothbrush bristles because it is very strong; polyester, used to make wrinkle-resistant clothes such as terylene and PET bottles; and acrylic, used to make sweaters and blankets as an inexpensive substitute for wool.

What it is

Thermoplastics soften on heating and can be remoulded; thermosetting plastics do not soften again once set.

Answer

Thermoplastics are plastics that soften on heating and can be bent and remoulded many times, for example polythene and PVC, used for toys, bottles and pipes. Thermosetting plastics are plastics that once moulded and set do not soften on heating and cannot be remoulded, for example bakelite and melamine, used for electrical switches, handles and crockery.

What it is

Plastics are non-biodegradable, so they pollute the environment; the 4 R's help manage them.

Answer

Plastics are harmful because they are non-biodegradable, meaning micro-organisms cannot break them down, so they remain in the environment for a very long time, choke drains and harm animals that swallow them; burning plastic releases poisonous gases. We can manage plastic waste by following the 4 R's - reduce, reuse, recycle and recover - and by avoiding single-use plastic and using cloth or jute bags instead.

What it is

Greenhouse gases act like a blanket that traps the Sun's heat around the Earth.

Answer

The greenhouse effect is the trapping of the Sun's heat near the Earth's surface by gases like carbon dioxide, methane and water vapour, which keeps the planet warm enough for life. It becomes a concern when human activities release extra greenhouse gases, trapping too much heat and causing global warming and climate change.

What it is

Harmful substances in air (pollutants) such as sulphur dioxide and nitrogen oxides cause acid rain and health problems.

Answer

Acid rain is rain that has become acidic due to the presence of pollutant gases in the air. Gases such as sulphur dioxide and nitrogen dioxide, released mainly by burning fossil fuels, dissolve in rain water to form sulphuric acid and nitric acid, which fall as acid rain. Acid rain damages buildings and monuments such as the Taj Mahal, corrodes metals, harms plants and makes lakes acidic, killing aquatic life.

What it is

Water pollution is the contamination of water by harmful substances; potable water is water safe for drinking.

Answer

Potable water is water that is safe (fit) for drinking. Water can be made potable by physical and chemical methods such as boiling, which kills germs, filtration, which removes suspended impurities, and chlorination, that is adding a small amount of chlorine or bleaching powder to disinfect it and kill harmful micro-organisms.

What it is

In a physical change no new substance forms and it is usually reversible; in a chemical change a new substance forms and it is usually not reversible.

Answer

A physical change is one in which no new substance is formed and only properties such as shape, size or state change; it is usually reversible, for example the melting of ice into water. A chemical change is one in which one or more new substances with new properties are formed; it is usually irreversible and often involves energy change, for example the burning of paper into ash and gases.

What it is

Chemical changes show signs such as change in colour, evolution of gas, formation of a precipitate, and change in temperature.

Answer

A chemical change may be recognised by a change in colour (iron rusting to reddish-brown), the evolution of a gas (bubbles when an acid meets a metal), the formation of a precipitate (an insoluble solid), and a change in temperature or the giving out of heat and light (burning). A change in smell may also occur.

What it is

An element is made of one kind of atom, a compound is two or more elements chemically combined in a fixed ratio, and a mixture is substances physically mixed in any ratio.

Answer

An element is a pure substance made up of only one kind of atom and cannot be broken down by chemical means, for example oxygen or gold. A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion, giving new properties, for example water (H2O). A mixture contains two or more substances mixed physically in any proportion, keeping their own properties, for example air or a salt solution.

What it is

A homogeneous mixture has a uniform composition throughout; a heterogeneous mixture does not.

Answer

A homogeneous mixture has the same composition and properties throughout, and its components cannot be seen separately, for example salt dissolved in water or air. A heterogeneous mixture has a non-uniform composition, and its different components can often be seen and separated easily, for example a mixture of sand and iron filings or oil and water.

What it is

An atom is made of protons and neutrons in a central nucleus, with electrons moving around it.

Answer

The three subatomic particles are the proton, the neutron and the electron. The proton carries a positive charge and the neutron carries no charge; both are present in the central nucleus. The electron carries a negative charge and revolves around the nucleus in shells. In a neutral atom the number of protons equals the number of electrons.

What it is

Atomic number is the number of protons; mass number is the total of protons and neutrons.

Answer

The atomic number (Z) of an element is the number of protons present in the nucleus of its atom. The mass number (A) is the total number of protons and neutrons in the nucleus. The number of neutrons is found by subtracting the atomic number from the mass number, that is neutrons = A - Z.

What it is

A symbol is a short form for an element; valency is its combining capacity - the number of electrons an atom loses, gains or shares.

Answer

Valency is the combining capacity of an element, that is, the number of electrons an atom loses, gains or shares to complete its outermost shell and become stable. Hydrogen has a valency of 1 and oxygen has a valency of 2, which is why they combine as H2O.