Alloys and amalgams

What an alloy actually is. An alloy is formed by melting two or more elements — usually metals, sometimes a metal with a non-metal — together and letting the melt cool and solidify as one piece. The result is a homogeneous mixture: under a microscope you cannot pick out separate crystals of the starting metals the way you could pick raisins out of dough. But it is still a mixture, not a compound. There is no fixed atom-to-atom ratio the way there is in a compound such as $\text{Fe}_2\text{O}_3$; instead one kind of atom sits dissolved inside the crystal lattice of the other, and the exact proportion can be varied over a range depending on what property the metallurgist is chasing. This is why an alloy is best pictured as a solid solution — the same idea as sugar dissolved in water, except both the "solvent" and the "solute" are metals, and the whole thing has frozen solid.

Why bother alloying at all. Pure metals, fresh out of extraction, are often unsuited to the job they are needed for. Alloying is a controlled fix for four recurring weaknesses:

• Softness. Pure metals like gold, iron, and aluminium are frequently too soft or too malleable for structural or wearable use; alloying introduces atoms of a different size into the lattice, which jams the sliding of atomic layers past each other and raises hardness.
• Corrosion. Pure iron reacts steadily with moist air to form rust; the right alloying element can form a protective barrier that pure iron cannot form on its own.
• Melting point. An alloy's melting point is not simply the average of its components — it can fall well below the lower of the two, which is exploited whenever a low-temperature joint is needed.
• Appearance and other properties. Colour, lustre, electrical behaviour, and machinability can all be tuned by choosing the second ingredient, which is why 22-carat gold jewellery is actually gold alloyed with copper or silver rather than pure gold.

The four alloys the syllabus expects you to know cold:

where: Cu is copper, Zn is zinc, Sn is tin, Pb is lead, Fe is iron, Cr is chromium and Ni is nickel — every symbol here is a pure element, and the "+" signs mean "melted together as a solid solution," not "chemically combined in a fixed ratio." Brass is harder than copper and resists corrosion, used for utensils and fittings; bronze is harder still and prized for statues and medals; solder has a low melting point and is used to join electrical wires; stainless steel resists rusting and is used for cutlery and surgical instruments. Steel itself (plain carbon steel, iron with a small percentage of carbon) is the standard example of an alloy whose second ingredient is a non-metal, which is a favourite exam trap in itself.

Amalgams: the mercury special case. An amalgam is simply an alloy in which one of the components is mercury, the only metal that is liquid at room temperature. Dental amalgam (silver, tin, and mercury) has historically been used to fill cavities, and sodium amalgam (Na–Hg) is used in the laboratory as a reducing agent. Because mercury contributes a liquid phase, the amount of mercury present decides whether the finished amalgam behaves as a liquid, a soft paste, or a solid — there is no rule that an amalgam must be liquid.

Alloying is not magic — it is a direct, physical consequence of mixing atoms of different sizes and chemistries into one shared crystal structure, and each syllabus alloy is a clean illustration of one mechanism.

Hardening by lattice mismatch. In a pure metal, all the atoms are the same size, so layers of atoms can slide past each other under stress fairly easily — this is why pure metals are often soft and malleable. When a second element with a different atomic size is dissolved into the lattice (zinc into copper for brass, tin into copper for bronze), the mismatched atoms distort the regular rows and physically obstruct that sliding. More force is needed to deform the material, so the alloy is measurably harder than either pure component.

Corrosion resistance by a protective barrier. Chromium added to iron does not simply dilute the iron's tendency to rust — it reacts with atmospheric oxygen to form a very thin, tough, transparent, self-repairing layer of chromium oxide on the surface. This layer physically seals the iron underneath away from moisture and oxygen, which is why stainless steel keeps its shine while plain iron pits and flakes with rust. Nickel adds further toughness and resistance to the alloy.

Melting-point depression. When you mix two metals that do not react chemically with each other, the mixture's melting point is often lower than either pure component's melting point — an effect called melting-point depression. This is why solder (lead + tin) melts around $183\,^{\circ}\text{C}$, well below pure lead's $327\,^{\circ}\text{C}$: the disordered mixed lattice is easier to break apart into a liquid than either pure metal's own regular lattice, which is exactly what a low-temperature joining material needs.

Appearance and workability. Copper added to gold changes both the colour (giving rose or red-gold shades) and the hardness (pure gold is far too soft to hold its shape as jewellery), which is why fine jewellery is almost always a gold alloy rather than pure 24-carat gold.

Question: A student is given a block of pure lead (melting point about $327\,^{\circ}\text{C}$) and a sample of solder, an alloy of lead and tin with a melting point of about $183\,^{\circ}\text{C}$, and is asked to explain, using the idea of alloying, why solder rather than pure lead is used to join electrical wires.

Step 1. Identify the practical constraint. Joining electrical wires means melting a metal onto the wire junction without damaging the plastic insulation or heat-sensitive components nearby, so the joining metal must melt at as low a temperature as is practically achievable.

Step 2. Compare the two melting points. Pure lead melts only at $327\,^{\circ}\text{C}$, a temperature high enough to scorch insulation and damage nearby electronics if used directly for soldering.

Step 3. Apply melting-point depression. Solder is an alloy of lead and tin. Alloying the two metals produces a mixture whose melting point, $183\,^{\circ}\text{C}$, is lower than that of either pure lead or pure tin on its own — this is the general effect that mixing two non-reacting metals usually depresses the melting point below both components.

Step 4. Connect the property to the use. Because solder melts $144\,^{\circ}\text{C}$ lower than pure lead, a joint can be made quickly at a temperature safe for the wire's insulation, and the melt still solidifies into a strong, electrically conductive bond once it cools. Solder is chosen over pure lead specifically because alloying with tin has usefully lowered the melting point to about $183\,^{\circ}\text{C}$, a full $144\,^{\circ}\text{C}$ below pure lead's $327\,^{\circ}\text{C}$.

Competitive stretch

If asked instead why stainless steel is chosen for cutlery over plain iron, the reasoning shifts to corrosion resistance: chromium forms a self-repairing oxide layer sealing the iron from oxygen and moisture — a check on whether you match the right benefit to the right example, not one memorised reason for every alloy.

• Calling an alloy a compound. An alloy is a homogeneous mixture (solid solution) with a composition that can vary within limits, not a compound with a fixed formula and fixed atomic ratio.
• Forgetting alloys can include non-metals. Steel (iron plus a small percentage of carbon) is the standard syllabus example where the second component is a non-metal, not another metal.
• Swapping brass and bronze. Brass is copper plus zinc; bronze is copper plus tin. These two are the most frequently confused pair in exams.
• Assuming an amalgam must be liquid. An amalgam is any alloy containing mercury; depending on how much mercury is present it can be liquid, a soft paste, or fully solid.

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