Metals and Non-metals: questions with answers

Metals are generally lustrous, malleable, sonorous and good conductors of electricity, so these four properties are tested together. A single property can mislead, because graphite is a non-metal that conducts and mercury is a metal that is liquid. Here the sample is dull, brittle and non-conducting, so the combined evidence makes it a non-metal.

Most non-metals are poor conductors of electricity. Graphite is the exception, so graphite conducts and is used as an electrode. Sulphur and diamond do not conduct electricity. Copper conducts well, but copper is a metal, not a non-metal.

An amphoteric oxide reacts with both acids and bases. Zinc oxide and aluminium oxide are amphoteric. Sodium oxide and magnesium oxide are basic.

Non-metal oxides are generally acidic or neutral. Carbon dioxide dissolves in water to give an acidic solution and reacts with a base to form a salt and water, so it is an acidic oxide.

How a metal is stored or used depends on its reactivity. Highly reactive metals such as sodium react with air, while metals of low reactivity such as gold do not.

A metal displaces another metal from its salt solution only when the free metal is more reactive than the metal in the salt. B displaces C, so B is more reactive than C. C displaces A, so C is more reactive than A. The order is $B > C > A$.

Nitric acid usually oxidises the hydrogen formed to water. Magnesium and manganese are the exceptions: with very dilute nitric acid they give hydrogen gas.

Only metals above hydrogen in the reactivity series displace hydrogen from dilute acids. Zinc is above hydrogen; copper, silver and gold are below it.

The total charge in an ionic formula must be zero. $Mg^{2+}$ carries +2 and each $Cl^-$ carries -1, so two chloride ions are needed. The formula is $MgCl_2$.

Magnesium loses two electrons and oxygen gains them, forming $Mg^{2+}$ and $O^{2-}$. The ions are held by electrostatic attraction; no electron pair is shared.

In solid sodium chloride the ions are held in fixed positions, so it does not conduct. When molten or dissolved in water, the ions are free to move and it conducts.

Highly reactive metals are obtained by electrolysis of their molten chlorides. Magnesium ions gain electrons at the cathode, so magnesium is deposited there, and chlorine is set free at the anode.

An ore dug out of the earth carries sand, soil and rocky matter along with it. This unwanted earthy matter is the gangue. It is removed during enrichment of the ore, before the metal is obtained from the ore.

In electrolytic refining, the impure metal is made the anode and a thin sheet of the pure metal is made the cathode. So impure silver is the anode and pure silver is the cathode. The electrolyte is a solution of a salt of silver. On passing the current, silver from the anode dissolves and pure silver deposits on the cathode. The insoluble impurities settle below the anode as anode mud.

Galvanisation is coating iron or steel with a thin layer of zinc. The zinc protects the iron even if the coating is broken.