Everyday Science

Chromium is added to steel to make stainless steel because it forms a thin, stable oxide layer on the surface that prevents rusting and corrosion, giving stainless steel its characteristic resistance to staining.

Silver melts at approximately 961.8 °C, which is the correct melting point among the options provided. The other temperatures correspond to melting points of different substances or are too low for silver.

Ammonia is highly soluble in water at room temperature due to its ability to form hydrogen bonds with water molecules, making it readily soluble compared to the other gases listed.

People with short-sightedness (myopia) have difficulty seeing distant objects clearly because their eyes focus images in front of the retina. Concave lenses diverge light rays before they enter the eye, helping to focus images correctly on the retina, thus correcting short-sightedness.

Fats provide approximately 9 calories (or kilocalories) per gram. Therefore, 1 gram of fat gives about 9.3 K Calories of energy, making option B correct.

Human hearing is most sensitive to frequencies in the range of approximately 3000 to 4000 hertz, where the threshold of audibility is lowest. Among the given options, 5000 hertz (option D) is the closest to this peak sensitivity range. Frequencies like 20 hertz and 20,000 hertz are at the edges of the audible spectrum and are less easily perceived, while 400 hertz is well below the peak sensitivity range. Therefore, the correct answer is D (5000 hertz).

Bauxite is the primary ore of aluminium, containing aluminium oxide minerals from which aluminium metal is extracted. It does not contain copper, zinc, or iron as the main metal.

The normal human hearing range is typically from 20 Hz to 20 kHz, which covers the frequencies most people can perceive. Options A, C, and D do not represent the standard range accurately.

An aneroid barometer does not use any liquid; it measures atmospheric pressure using a small, flexible metal box called an aneroid cell. Therefore, the correct answer is 'None'.

Radon is a naturally occurring radioactive gas that is colorless, odorless, and tasteless. It is produced by the decay of uranium in soil and rocks and can accumulate in buildings, posing health risks.