Everyday Science

1 meter is equal to 1000 millimeters because 1 meter = 100 centimeters and 1 centimeter = 10 millimeters, so 100 × 10 = 1000 millimeters.

Hydrology is the scientific study of the movement, distribution, and quality of water on Earth, which includes rainfall as a key component of the water cycle.

Milk tastes sour when left open because bacteria convert lactose into lactic acid, which causes the sour taste.

A photostat machine works by creating an electrostatic image on a drum or plate, which is then developed with toner to produce a copy. This process is based on electrostatic principles, making option D correct.

Permanent hardness in water is caused by dissolved salts like calcium sulfate and magnesium sulfate, which do not get removed by boiling. Calcium sulfate is a common cause of permanent hardness.

Microwaves are used for communication by artificial satellites because they can penetrate the atmosphere with minimal interference and support high data transmission rates, making them ideal for satellite communication.

Soap is made by the saponification process, which involves boiling fats or oils with caustic soda (sodium hydroxide). This reaction produces soap and glycerine.

Nitrogen is the main constituent of air by volume, making up about 78% of the Earth's atmosphere, which is more than any other gas including oxygen, hydrogen, or helium.

Combustion requires oxygen to occur because oxygen supports the chemical reaction that releases heat and light. While air contains oxygen, it is the oxygen itself that is essential for combustion, not air as a whole.

The iron needle floats on the water surface because of surface tension, which acts like a thin elastic sheet on the water surface preventing the needle from sinking if placed gently. The needle's density is greater than water, so density difference is not the reason.