Sound — Science STD 8 — Question

Why do you have to use paper or kerosene oil to start fire in wood or coal?

Which properties of chromium metal make it suitable for electroplating it on car bumpers, bath taps and bicycle handlebars, etc., made of iron?

State two ways in which the body shape of boys changes during puberty.

Define foetus. After how many weeks of development, a human embryo is said to become a foetus?

What is protoplasm? Name the elements which make up major part of protoplasm.

Why does an electric bulb glow when a current passes through it?

A is an insect which breeds in ponds of stagnant water. The egg of this insect produces a worm like form B which is entirely different in appearance from the adult insects. The form B undergoes a change C and gets converted into insects A. The female of insects A is a carrier of protozoan D which spreads a disease in humans.

1. What are A, B, C, and D?
2. Name another insect which also undergoes change C.

Which type of reflection of light, regular reflection or diffuse reflection, leads to the formation of images?

What is air? Write the names of various constituents of air.

Whenever we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.