A boat-repair company is selecting a metal for a valve body in a seawater cooling pump. Fig. 1 compares a regular row of copper atoms with atoms in brass. B...
Question 1 Report
A boat-repair company is selecting a metal for a valve body in a seawater cooling pump. Fig. 1 compares a regular row of copper atoms with atoms in brass. Brass is an alloy containing mostly copper with zinc added. The lower drawing is not to scale, but it shows that zinc atoms are a different size from copper atoms. The company wants a valve that resists wear when sand passes through the pump.
Table 1 gives data from samples made by the company.
| sample | mass of copper / g | mass of zinc / g | mean scratch width / mm |
|---|---|---|---|
| A | 90 | 10 | 0.42 |
| B | 70 | 30 | 0.31 |
| pure copper | 100 | 0 | 0.58 |
(a) Describe two differences between the particle arrangements in pure copper and brass shown in Fig. 1. [3]
(b) Use Fig. 1 to explain why brass is harder than pure copper. [3]
(c) Use Table 1 to calculate the percentage by mass of zinc in sample B. [3]
(d) What conclusion about wear resistance can be made from the scratch-width results? [2]
(e) How could the company improve this scratch test so that the comparison is more reliable? [3]
(f) Give two properties of metals that allow brass to be made into a shaped valve body. [2]
Answer Details
(a) Pure copper contains one type of atom [1]. Brass contains both copper and zinc atoms [1], and the atoms are different sizes, making the arrangement less regular [1].
(b) Different-sized atoms distort the metal layers [1]. The layers cannot slide past one another as easily [1], so brass is harder and more resistant to scratching [1].
(c) \[\text{total mass}=70+30=100\text{ g}\] [1]
\[\text{percentage zinc}=\frac{30}{100}\times100=30\%\] [2]
(d) Sample B has the narrowest scratch, so it is the most wear resistant [1]. Pure copper has the widest scratch, so it is the least wear resistant [1].
(e) Repeat each scratch test and calculate a mean [1]. Use the same load on the scratching point [1] and the same type and size of point [1]. Keeping scratch distance constant and using the same calibrated measuring instrument are also valid controls.
(f) Brass is malleable, so it can be shaped [1], and it has a high melting temperature [1]. Strength or conductivity are also acceptable metal properties.
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