1. It is dangerous to sleep in an unventilated room with fire burning inside because the fire produces carbon monoxide and carbon dioxide gases. Carbon monoxide is poisonous and can cause death.
2. The filament of an electric bulb is made of tungsten because it has a high melting point and can be heated to a high temperature to emit light.
3. Water kept in an earthen pitcher becomes cold because the pitcher has minute pores on its surface which absorb water. Water in the pores evaporates when it comes in contact with air, and produces a cooling effect.
4. A sliced apple, when exposed to air, turns brown after some time as it contains iron which gets oxidised and gives the fruit a brownish colour.
5. The freezing compartment inside a refrigerator is at the top because the air in contact with it becomes cold and heavy and sinks downwards, cooling the rest of the compartment.
6. A copper vessel left in the air for a long time turns green. This is due to the formation of copper carbonate when copper reacts with carbon dioxide and moisture present in the air.
7. A wick in a stove keeps burning continuously as kerosene rises in the wick due to capillary action.
8. The Earth rotates on its axis from west to east. This rotation makes the sun and the stars appear to be moving across the sky from east to west.
9. The sky appears blue because the light of the sun is spread or scattered by the dust particles in the air. In space the sky would appear black as there are no dust or air particles to scatter the light.
10. Food is cooked quickly in a pressure cooker because the boiling point of water increases at high pressure. Food cooks faster at high temperature.
11. In mountainous regions, the atmospheric pressure is less than it is at seal level water, therefore, boils at a lower temperature(less than 100° C) and food takes more time to cook.
12. When ice floating in a glass of water melts, the level of water remains unchanged because as a solid, ice displace an equal volume of water.
13. A man weigh more at the poles than at the equator because the polar radius of the Earth is less than the equatorial radius .Hence the gravitational pull is more at the poles than at the equator.
14. Standing in double-decker buses, particularly on the upper floor, is not allowed because on tilting, the centre of gravity of the bus gets changed and it is likely to overturn.
15. The boiling point of seawater will be more than the boiling point of pure water because the former contains salt and other impurities.
16. An ordinary clock loses time in summer because the length of its pendulum increases, and therefore, its time period also increases. The pendulum takes more time to complete each oscillation and thus loses time.
17. Whenever there is water loss from the body, secretion of saliva is reduced resulting in dryness of the mouth and stimulating the sensation of thirst. Intake of fluid then helps in restoring the loss of the water.
18. A swimmer just out of the river feels cold particularly if it is windy, because of the evaporation of water from his body surface. The evaporation is more on a windy day.
19. Alcohol is sometimes rubbed on the body of a person suffering from fever. As soon as it is applied on the body, it evaporates taking away some heat from the body. Since evaporation has a cooling effect, the body temperature can be reduced by rubbing alcohol.
20. Soft iron is used as an electromagnet because it remains a magnet only while the current passes through the coil around it and loses its magnetism when the current is switched off.
21. The person jumping out of a moving train is carried forward in the direction of the train because the person himself is in motion sharing the velocity of the train and will continue in its state of uniform motion unless it exercises some force to prevent it (Newton’s first law of motion).
22. A lightning conductor is fixed to tall buildings to protect them from the destructive effects of the lightning.
23. An electric bulb makes a bang when it is broken because there is a vacuum inside the electric bulb; when the bulb is broken air rushes in at great speed from all sides to fill the vacuum. The rushing of air produces a noise generally referred to as the “bang”.
24. A small space is left between each set of two rails of railway line to allow for their expansion in summer.
25. A bad egg floats in water because the up-thrust produced on account of displaced water by the immersed portion of the bad egg is greater than the weight of the egg.
26. Moisture gather on the outer surface of a glass tumbler containing cold water because the water vapors present in the air get cooled and appear as droplets of water on coming in contact with the cold surface of the glass tumbler.
27. The launching of Earth satellites should be from a place near the equator to take the fullest advantage of the earth’s movements. The regions of the Earth closer to the equator.
28. We experience difficulty in breathing on mountains because the pressure of the air out side is less as compared to the pressure of the pressure of air in side the lungs.
29. If a highly corked glass bottle full of water is left out of doors on a frosty night and convert into ice. There is no room available for the increased volume and this may result in bursting of the bottle.
30. Water extinguishes fire because as it evaporates, the temperature of the burning body is lowered, thus retarding or stopping the burning action also the resulting vapour surround the burning substance cutting off the oxygen supply thus inhibiting the burning process. In fact hot water will extinguish fire more quickly than cold water as hot water will vaporize faster.