Fig. 9.1 shows structural steel beams of the type used to build bridges and large buildings. Steel is an alloy based on the transition metal iron. [Fig. 9.1...

This question is about alloys of the transition metals and why alloying changes the properties of a metal. Note that parts (f) is a percentage-by-mass calculation.

(a) An alloy is a mixture of a metal with one or more other elements [1], usually other metals or carbon, and it still has metallic properties [1].

(b) The two other transition metals in stainless steel are chromium [1] and nickel [1].

(c) The chromium forms a thin, stable protective oxide layer on the surface [1] that stops oxygen and water reaching the iron underneath, so rusting is prevented [1].

(d) One useful property of brass with a matching use, for example it is hard and corrosion-resistant [1], so it is used for taps, musical instruments or electrical fittings [1]. The property named must fit the use given.

(e) Explain hardness in terms of atom arrangement.

1. In a pure metal the atoms are all the same size and arranged in regular layers that can slide over one another [1].
2. The different-sized atoms of the second metal distort these layers [1].
3. So the layers can no longer slide as easily, which makes the alloy harder than the pure metal [1].

(f) The copper is 70% of the 250 g:

\[ \text{mass of copper} = \frac{70}{100} \times 250 = 175 \ \text{g} \]

One mark for the method, one for the answer [2].

(g) Any two transition-metal properties useful for bridges, for example high strength or hardness and a high melting point (also acceptable: high density, malleable or ductile) [2].

(h) The zinc coating gives sacrificial protection: zinc is more reactive than iron, so even when the coating is scratched the zinc is oxidised in preference to the iron, and the iron does not rust while any zinc remains [1].