Chemistry Questions and Answers

Answer: D

Phosphorus is kept under water to stop it catching fire in air.

White phosphorus ignites spontaneously at about 30 °C, close to room temperature in a warm climate. It is insoluble in water and denser than it, so a covering layer keeps oxygen away without dissolving or reacting with the element.

Answer: C

Air is a mixture.

Its components are not chemically combined, keep their own properties, and are present in proportions that vary with place and altitude. They can be separated by the purely physical process of fractional distillation of liquid air.

Answer: D

Nitrogen forms a maximum of four covalent bonds.

It has five valence electrons and no d-orbitals, so it can form three ordinary covalent bonds and one further coordinate bond by donating its lone pair - as in the ammonium ion \(\mathrm{NH_4^+}\). The absence of d-orbitals is why nitrogen cannot expand its octet as phosphorus does in \(\mathrm{PCl_5}\).

Answer: A

Argon is the most abundant noble gas in the atmosphere, at 0.93 per cent by volume.

Nearly all of it is argon-40, produced by the radioactive decay of potassium-40 in the crust. It is the third commonest gas in air, after nitrogen and oxygen.

Answer: B

Monazite sand is found on the Travancore coast of Kerala.

The beach sands of Chavara and Manavalakurichi are among the richest monazite deposits in the world. Monazite is a phosphate of cerium and other rare earths containing about 8–10 per cent thorium, and India's thorium reserves are the basis of the third stage of its nuclear power programme.

Answer: A

The electron makes 4 waves.

A magnetic quantum number of \(m_l=+3\) requires \(l\ge 3\), and since \(l\) can be at most \(n-1\), the smallest possible principal quantum number is \(n=4\).

De Broglie's condition for a stable orbit is \[2\pi r = n\lambda\] so the number of complete waves in the orbit equals \(n\), giving 4.

Answer: C

Shaving creams use soft soaps.

Soft soaps are the potassium salts of fatty acids, made with potassium hydroxide instead of sodium. They are more soluble than the sodium (hard) soaps, lather freely and are gentler on skin. Calcium and lead soaps are insoluble and are used as greases and driers.

Answer: A

Ozone is concentrated in the stratosphere.

The ozone layer lies between about 15 and 35 km, where ultraviolet light splits \(\mathrm{O_2}\) and the free atoms combine with more \(\mathrm{O_2}\). The heat released is why the stratosphere, uniquely, grows warmer with height.

Answer: C

The ozone layer is depleted by man-made compounds containing both fluorine and chlorine.

Chlorofluorocarbons drift up to the stratosphere, where ultraviolet light frees a chlorine atom that then destroys ozone catalytically: \[\mathrm{Cl + O_3 \rightarrow ClO + O_2},\qquad \mathrm{ClO + O \rightarrow Cl + O_2}\] One chlorine atom can destroy tens of thousands of ozone molecules before it is removed.

Answer: D

Natural rubber is a polymer of isoprene.

It is cis-1,4-polyisoprene, and the all-cis geometry is what gives it elasticity, since the chains cannot pack neatly. Gutta-percha is the trans isomer of the same polymer and is hard and inelastic.