Uses of Metals Worked ExamplesIGCSE Chemistry 0620

(a) Aircraft must be as light as possible: aluminium has a low density.

(b) Overhead cables hang between pylons, so they must be light and carry current: low density and good electrical conductivity.

(c) Food containers must not react with food or moisture: aluminium is resistant to corrosion.

(d) Copper is a very good electrical conductor and is ductile — it can be drawn out into thin wires.

(a) An alloy is a mixture of a metal with other elements. It is not a compound — the elements are not chemically combined in fixed proportions.

(b) Brass is a mixture of copper and zinc.

(c) Stainless steel contains iron with chromium, nickel and carbon (the non-metal). Any two, one of which must be carbon for the non-metal.

(d) It is hard (does not bend or scratch easily) and resistant to rusting.

(a)(i) A — one type of atom, all the same size, in regular layers. (ii) B — atoms of a second element, of a different size, are mixed in among the metal atoms.

(b) D shows positive and negative ions in an alternating lattice — an ionic compound. An alloy contains metal atoms, not oppositely charged ions, and it is a mixture, not a compound.

(c) In C the two metals are in separate regions — the atoms are not mixed together. An alloy has the atoms of the different elements mixed throughout the structure.

Pure metal: all the atoms are the same size and are arranged in regular layers. When a force is applied, the layers can slide over each other easily, so the metal bends and changes shape (it is malleable).

Alloy: atoms of a different element have a different size. They disrupt the regular layers.

So the layers can no longer slide over each other easily. A greater force is needed to change the shape — the alloy is harder and stronger (and less malleable).

Stainless steel is iron mixed with chromium, nickel and carbon. Answer: D.

A confuses brass (copper and zinc) with bronze (copper and tin). B is wrong because an alloy is a mixture, not a compound.

C is the reverse of the truth: the different-sized atoms stop the layers sliding, so alloys are less malleable (harder) than the pure metals.

(a) Aluminium has a much lower density than copper, so the cable is lighter; it still has good electrical conductivity. (Aluminium is also cheaper.) A lighter cable needs fewer, less massive pylons and sags less.

(b) Steel is strong — it takes the weight (tension) of the cable between pylons, so the cable does not stretch or break.

(c) Volume = area × length. Convert 1.00 km to cm: $1.00 \text{ km} = 1.00 \times 10^5 \text{ cm}$.

Mass = density × volume.

(d) Same volume of copper — more than three times as heavy, which is why aluminium is chosen for overhead cables.

(a) The surface of aluminium is covered by a thin layer of aluminium oxide. This layer is unreactive and sticks firmly to the metal, so it stops air and water reaching the aluminium underneath.

So although aluminium is high in the reactivity series, it behaves as if it were unreactive.

(b) An amphoteric oxide reacts with acids (and with bases). The acid in the food can slowly react with and remove the oxide layer, for example

Once the layer is removed, the aluminium underneath is exposed and can react with the acid, so the tray corrodes and aluminium compounds get into the food.

(a) J: it has a low density ($2.7$ g/cm³) so the cable is light, a good electrical conductivity ($61$), good corrosion resistance and low cost. K conducts better but is over three times as dense.

(b) K: it has the highest electrical conductivity ($100$). House wiring does not hang between pylons, so its higher density does not matter. (K matches copper.)

(c) M has a very high cost (and a very high density), so it is only worth using in tiny amounts where a contact must never corrode.

(d) N has poor resistance to corrosion, so it would react with water or acids in the food.

(a) The drop height $h$, the same steel ball (mass/size), and the thickness of the metal sheet. Any two.

(b) For brass, $2.9$ mm is far from the other two readings — it is anomalous and is left out of the mean.

(c) Brass gives the smaller dent, so brass is harder than copper. Brass is an alloy: the different-sized zinc atoms stop the layers of copper atoms sliding.

(d) The tube guides the ball so it falls vertically from the same height and hits the sheet at the same place each time.

(a) Take $18.0\%$ of the mass.

(b) Moles = mass ÷ $A_r$.

(c) Number of atoms = moles × Avogadro constant (use the unrounded value).

(d) Stainless steel is harder than pure iron (the different-sized atoms stop the layers sliding) and it resists rusting, so the spoon does not bend or corrode.

(a) Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from the acid. Copper is below hydrogen, so it does not react.

(b) The plunger is at $60$ cm³, so $60.0 \text{ cm}^3$ of hydrogen.

(c) Convert cm³ to dm³ (÷ 1000), then find moles of hydrogen.

1 mol Zn gives 1 mol $\text{H}_2$.

(d) The unreacted copper remains as a pink-brown (red-brown) solid; the solution is colourless zinc chloride.

(a) At $25\%$ zinc, read up to the line and across: relative hardness $65$.

(b) As the percentage of zinc increases, the hardness increases (steadily, by about $10$ units for every $10\%$ zinc).

Zinc atoms are a different size from copper atoms. The more zinc atoms there are, the more the regular layers are disrupted, so the layers slide over each other less easily.

(c) The graph only covers $0$–$40\%$ zinc, so extending it to $100\%$ is not justified. Pure zinc is a pure metal, not an alloy — all its atoms are the same size, so its layers can slide again and it will not be the hardest.