Question

An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:

• A. Calcium
• B. Carbon
• C. Silicon
• D. Iron

Answer 1

The correct option is A

Calcium

The explanation for the correct option:

(A) Calcium reacts with oxygen to give calcium oxide.

• $\underset{\left(\mathrm{Calcium}\right)}{\mathrm{Ca}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{Calcium}\mathrm{oxide})}{\mathrm{CaO}\left(\mathrm{s}\right)}$
• The calcium ion $\left({\mathrm{Ca}}^{+2}\right)$ and the oxide ion $\left({\mathrm{O}}^{-2}\right)$ are held together by strong ionic forces of attraction which makes Calcium oxide an ionic compound.
• It has a high melting point due to the strong electrostatic force of attraction between ions.
• Calcium oxide is an odorless, white, or grey-white solid.
• Calcium oxide is soluble in water and forms calcium hydroxide as shown below.
• $\underset{(\mathrm{Calcium}\mathrm{oxide})}{\mathrm{CaO}\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}$

The explanation for the incorrect options:

(B) Carbon reacts with oxygen to form a gas that cannot possess a high melting point.

$\underset{\left(\mathrm{Carbon}\right)}{\mathrm{C}\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

(C) Silicon reacts with oxygen to form monoxide and dioxide which have a high melting point but are insoluble in water.

$\underset{\left(\mathrm{Silicon}\right)}{\mathrm{Si}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\left(\mathrm{Silica}\right)}{{\mathrm{SiO}}_2\left(\mathrm{s}\right)}$

(D) Iron reacts with oxygen to form Iron (II) oxide which is insoluble in water.

$\underset{\left(\mathrm{Iron}\right)}{2\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Iron}\left(\mathrm{II}\right)\mathrm{oxide}}{\mathrm{FeO}\left(\mathrm{s}\right)}$

Hence, option (A) is correct.