3.1 Physical and Chemical Properties of Metals and Non-metals

Look around your kitchen. The tawa, the pressure cooker, the spoons and the electric wires behind the switchboard are all made of metals; the handles of the pans, the sockets of the plugs and the sulphur in a matchhead are made of materials that are not. Even the air you breathe is mostly a non-metal. Everything around you is built from about a hundred elements, and nearly all of them fall into one of these two families. Knowing which family an element belongs to lets you predict how it looks, how it is shaped, and — most important for a chemist — how it reacts.

In this section you will first compare the physical properties of metals and non-metals, noticing the exceptions that make science interesting. Then you will see how they behave chemically: with oxygen, with water, with dilute acids and with the salt solutions of other metals. Metals turn out to differ enormously in how eagerly they react, and the order of that eagerness — the reactivity series — is the key that unlocks the whole chapter: it explains why sodium is stored under kerosene, why gold never rusts, and how metals are extracted from their ores in Section 3.3.

Physical Properties of Metals and Non-metals

Take a piece of freshly cut iron, copper or aluminium and compare it with a lump of sulphur or a stick of charcoal. The metals are shiny; the non-metals are dull. Strike each with a hammer: the metals flatten, the non-metals shatter. Connect each in a simple circuit with a bulb and a cell: the metals light the bulb, the non-metals do not. These differences are the physical properties by which we sort elements.

Silver and copper are the best conductors of electricity, which is why household wires are copper; lead and mercury are poor conductors among metals. Gold and silver are so malleable that a gram of gold can be beaten into a sheet about $1\ \mathrm{m^{2}}$ in area, and silver foil (vark) decorates Indian sweets. Metals conduct heat so well that a steel spoon left in hot tea soon becomes too hot to hold, which is why cooking vessels have metal bases and plastic or wooden handles.

Metals and Non-metals with Oxygen

Hold a piece of magnesium ribbon with tongs in a flame: it burns with a dazzling white flame, leaving a white ash of magnesium oxide. Heat a copper powder in a china dish and the reddish-brown powder slowly turns black, coated with copper(II) oxide. Almost every metal combines with oxygen to form a metal oxide: $$\mathrm{2Mg(s) + O_{2}(g) \rightarrow 2MgO(s)}$$ $$\mathrm{2Cu(s) + O_{2}(g) \rightarrow 2CuO(s)}$$ $$\mathrm{4Al(s) + 3O_{2}(g) \rightarrow 2Al_{2}O_{3}(s)}$$ Metals differ greatly in how fast they do this. Sodium and potassium react so vigorously that they catch fire in air even at room temperature, so they are stored under kerosene. Magnesium burns only once lit. Iron burns when its filings are sprinkled into a flame (iron wire or nails do not). Copper reacts only on strong heating. Silver, and especially gold and platinum, do not react with oxygen at all, even when heated; that is why gold stays bright for centuries. Aluminium is a surprise: it is very reactive, but the thin layer of oxide that forms on its surface at once protects the metal underneath from further attack. Anodising deliberately makes this oxide layer thicker, to make aluminium resist corrosion and to colour it.

Most metal oxides are basic, but two common ones are special. Aluminium oxide and zinc oxide react with an acid and with a base, each time giving a salt and water: $$\mathrm{Al_{2}O_{3}(s) + 6HCl(aq) \rightarrow 2AlCl_{3}(aq) + 3H_{2}O(l)}$$ $$\mathrm{Al_{2}O_{3}(s) + 2NaOH(aq) \rightarrow 2NaAlO_{2}(aq) + H_{2}O(l)}$$ The salt formed with the base is sodium aluminate. In the same way zinc oxide gives zinc chloride with hydrochloric acid and sodium zincate, $\mathrm{Na_{2}ZnO_{2}}$, with sodium hydroxide.

Non-metals also burn in oxygen, but the oxides they form are different. Carbon gives carbon dioxide, sulphur gives sulphur dioxide, and phosphorus gives phosphorus pentoxide; each dissolves in water to make an acidic solution, and each reacts with sodium hydroxide: $$\mathrm{C(s) + O_{2}(g) \rightarrow CO_{2}(g)}$$ $$\mathrm{S(s) + O_{2}(g) \rightarrow SO_{2}(g)}$$ $$\mathrm{CO_{2}(g) + 2NaOH(aq) \rightarrow Na_{2}CO_{3}(aq) + H_{2}O(l)}$$ A few non-metal oxides, such as carbon monoxide $\mathrm{CO}$ and water $\mathrm{H_{2}O}$, are neutral: they do not turn litmus either way.

Metals with Water and Acids

Drop a small piece of sodium into water (with the proper precautions: a thick-walled glass trough, a small piece, a distance). It races over the surface, fizzing, melts into a silvery ball, and the hydrogen it releases may burst into flame. Calcium reacts less violently: it fizzes, floats because hydrogen bubbles stick to it, and the water turns milky with calcium hydroxide. Magnesium does not react with cold water, but in hot water it also floats on bubbles of hydrogen. Aluminium, iron and zinc show no reaction with cold or hot water but react with steam, giving the metal oxide and hydrogen. Lead, copper, silver and gold do not react with water at all. $$\mathrm{2Na(s) + 2H_{2}O(l) \rightarrow 2NaOH(aq) + H_{2}(g)}$$ $$\mathrm{Ca(s) + 2H_{2}O(l) \rightarrow Ca(OH)_{2}(aq) + H_{2}(g)}$$ $$\mathrm{Mg(s) + 2H_{2}O(l) \rightarrow Mg(OH)_{2}(s) + H_{2}(g)}$$ $$\mathrm{2Al(s) + 3H_{2}O(g) \rightarrow Al_{2}O_{3}(s) + 3H_{2}(g)}$$ $$\mathrm{3Fe(s) + 4H_{2}O(g) \rightarrow Fe_{3}O_{4}(s) + 4H_{2}(g)}$$

The same ranking appears when metals meet a dilute acid. In test tubes of dilute hydrochloric acid, magnesium bubbles furiously, aluminium and zinc less so, and iron slowly, while copper does nothing. The gas is hydrogen (the pop test). The reaction of each metal with the acid is a displacement of hydrogen, as in Principle 2.1: $$\mathrm{Mg(s) + 2HCl(aq) \rightarrow MgCl_{2}(aq) + H_{2}(g)}$$ $$\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_{2}(aq) + H_{2}(g)}$$ $$\mathrm{Fe(s) + 2HCl(aq) \rightarrow FeCl_{2}(aq) + H_{2}(g)}$$ Why do sodium and potassium never appear in this list? They are far too violent with acids to be tried in a school laboratory. Why does copper show no reaction? It is less reactive than hydrogen, so it cannot push hydrogen out of an acid.

Because it dissolves metals that nothing else can touch, a mixture of one part concentrated nitric acid and three parts concentrated hydrochloric acid, called aqua regia, is a special reagent.

Salt Solutions and the Reactivity Series

Place a clean iron nail in a blue solution of copper(II) sulphate. After half an hour the nail is coated with a reddish-brown layer and the blue colour of the solution has faded to pale green. The iron has pushed copper out of the solution: the more reactive metal has displaced the less reactive one from its salt (a displacement reaction, Section 1.2). $$\mathrm{Fe(s) + CuSO_{4}(aq) \rightarrow FeSO_{4}(aq) + Cu(s)}$$ Put a copper wire in silver nitrate solution and a grey, shining deposit of silver grows on it while the solution turns blue: $$\mathrm{Cu(s) + 2AgNO_{3}(aq) \rightarrow Cu(NO_{3})_{2}(aq) + 2Ag(s)}$$ But a copper wire in iron(II) sulphate solution does nothing, and neither does an iron nail in zinc sulphate. Doing many such experiments — with every pair of metals — lets us rank the metals.

most reactive ← K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au → least reactive

The series also summarises everything else in this section. Metals at the top (K, Na, Ca) react with cold water; Mg needs hot water; Al, Zn, Fe need steam; the metals above hydrogen give hydrogen with dilute acids; Cu and those below give none. Gold and silver, at the bottom, are called noble metals because they hardly react with anything, which is why they are used in jewellery and why coins and ornaments are made of them.

Exercises