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

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

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

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

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.