Experiment: Effect of alloying on the hardness of metals

Comparing a pure metal with its alloy is the standard Form 4 practical for the Manufactured Substances in Industry chapter, and it links the everyday idea that alloys are stronger to the arrangement of atoms you learn in structure and bonding. In this experiment we set out to show, fairly and by measurement, that an alloy is harder than the pure metal it is made from. This guide gives the method for comparing a piece of a pure metal with a piece of its alloy, the observations to expect, and the science process skills the practical papers reward.

Aim

To investigate the effect of alloying on the hardness of a metal, that is, to compare the hardness of a pure metal with the hardness of its alloy, for example pure copper with bronze, or pure iron with steel.

Apparatus and materials

• A block of a pure metal (for example copper)
• A block of the corresponding alloy of the same shape and size (for example bronze)
• A steel ball bearing
• A weight or heavy load
• A metre rule or a fixed measuring scale
• A retort stand with clamp, or a glass tube to guide the load
• A hand lens or a ruler graduated in millimetres to view the dent
• A plasticine or plastic support to hold each metal block steady

Procedure

1. Place the block of pure metal on the bench and rest a steel ball bearing on its flat surface.
2. Clamp a metre rule vertically beside the block, or set up a vertical glass tube directly above the ball bearing to guide the falling load.
3. Raise a fixed weight to a fixed height above the ball bearing and release it so that it falls freely and strikes the ball bearing.
4. Remove the ball bearing and examine the dent made on the surface of the pure metal. Measure the diameter (or depth) of the dent with a hand lens or millimetre ruler and record it.
5. Repeat steps 1 to 4 using the block of the alloy, keeping the same ball bearing, the same weight and the same height of fall.
6. Compare the size of the dent on the pure metal with the size of the dent on the alloy.
7. Repeat the whole comparison two or three more times on fresh, undamaged parts of each block and take the average dent size to make the result reliable.

Expected observations

The falling load makes a clearly visible dent on the surface of the pure metal. When the same load is dropped from the same height onto the alloy, the dent is noticeably smaller and shallower. In other words, the pure metal is dented more easily than the alloy under identical conditions. The alloy resists the deforming force better and keeps its surface closer to its original shape. Describe what you see, a larger dent on the pure metal and a smaller dent on the alloy, rather than quoting exact millimetre readings you have not measured, because the size of the dent depends on the particular metals and load you use.

Inference and conclusion

A smaller dent means the surface resists indentation more strongly, and resistance to indentation is what hardness means. Because the alloy is dented less than the pure metal under the same load and height, the alloy is harder than the pure metal. The reason lies in the arrangement of atoms. In a pure metal the atoms are all the same size and pack into orderly layers; when a force is applied these layers can slide over one another, so the pure metal is relatively soft and easily dented. In an alloy, atoms of a second element with a different size are mixed among the atoms of the first metal. These foreign atoms disturb the orderly layers and prevent them from sliding smoothly, so the alloy is stronger and harder. The conclusion is that alloying increases the hardness of a metal, which is why alloys such as bronze, brass, steel and duralumin are used in place of the pure metals for tools, structures and machine parts.

Science process skills (Paper 3 style)

Stating a hypothesis. A suitable hypothesis is: an alloy is harder than the pure metal it is made from, so the alloy is dented less than the pure metal when the same load falls from the same height.

Identifying variables. The manipulated variable is the type of metal used (pure metal or its alloy). The responding variable is the size of the dent produced (its diameter or depth). The controlled variables are the mass of the load, the height of fall, the size of the ball bearing and the size and shape of the metal block.

Operational definition of hardness. Hardness is defined operationally here as resistance to denting: the metal that shows the smaller dent under the same load dropped from the same height is the harder metal.

Tabulating data. Draw a table with columns for the type of metal, the dent size for each trial, and the average dent size, so the pure metal and the alloy can be compared directly.

Making an inference. From a smaller dent on the alloy, infer that the alloy is harder, and link this to foreign atoms disrupting the sliding of atomic layers.

Communicating. Write a conclusion that states which metal is harder and gives the reason in terms of the arrangement of atoms, not just the observation.

Safety precautions

• Keep fingers and hands clear of the area directly below the falling load, so that the load or ball bearing does not crush them.
• Use a guide tube or clamp so the load falls straight and cannot bounce sideways towards anyone, preventing injury from a stray load.
• Place the apparatus on a firm, level bench away from the edge, so the heavy blocks and load do not fall off and hurt feet.
• Wear safety goggles, because a hard load striking metal can send small chips or the ball bearing flying towards the eyes.

Common errors

• Changing the height or the load between metals. If the height of fall or the mass of the load is different for the two metals, the comparison is not fair; keep both exactly the same.
• Testing on a damaged spot. Dropping the load twice on the same place gives a bigger, misleading dent; always test on a fresh part of the surface.
• Using blocks of different thickness. A thin block may dent differently from a thick one for reasons unrelated to hardness; use blocks of the same size and shape.
• Reading the dent carelessly. Estimating the dent by eye leads to large errors; measure the diameter or depth with a millimetre scale or hand lens, and repeat to take an average.
• Confusing hardness with strength or mass. The experiment tests resistance to denting, so describe the result as hardness, not as the metal being heavier or generally stronger.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can connect the observation to the theory, the smaller dent on the alloy, and the disrupted layers of atoms that explain it, because Paper 2 and Paper 3 both reward the reason, not just the result. The idea that alloying improves hardness runs right through the Manufactured Substances in Industry chapter of SPM Chemistry, and it is exactly the kind of fair-test reasoning assessed in the Paper 3 practical (Paper 3 is a practical test assessing science process skills). Securing the method here, one manipulated variable, everything else controlled, also builds the habit you will reuse in every other practical, from rate of reaction to electrochemistry.