Video Transcript
Which of the following is a major advantage of copper over steel when used as a roofing material? A) Higher thermal conductivity. B) Lower density. C) Higher corrosion resistance. D) Higher melting point. Or E) Higher strength.
What the question is looking for is an advantage that’s pertinent to the application of the metal to roofing. One of the most important features of a roof is that it protects the building from weather, particularly the rain. It should be durable and long-lasting.
Option A, having a higher thermal conductivity, does not make a great deal of sense for a roof. Heat is lost from the inside of a building to the outside world, some of it through the roof. So, having a material with a higher thermal conductivity will actually make that worse.
Steel is mostly iron, so we can compare the thermal conductivities of copper and iron. Copper displays a thermal conductivity about five times higher than that of iron. However, as we’ve mentioned even though the thermal conductivity of copper is higher, this is not an advantage when it comes to building a roof. Therefore, this is not a correct answer.
What about a lower density? All else being equal, using a lower density material will make the roof lighter, making it easier to install and putting less stress onto the rest of the building. Copper has a density of 8.96 grams per cubic centimetre while steel, depending on the type, has a density of about eight grams per cubic centimetre. This means copper is actually more dense than steel. So, it does not have a lower density, and therefore, it cannot be an advantage. Therefore, this too is an incorrect answer.
For this question, you wouldn’t necessarily have to look up the densities for copper and steel. If you looked at your periodic table, you’d see that copper and iron are in the same period of the periodic table. Since they’re both in the transition metal block, their densities would be similar. So, the difference between their densities would likely be quite small and would confer a small advantage at best.
Next, we have higher corrosion resistance. This is a very good feature for a roof material. A roof with higher corrosion resistance will last longer and resist the elements. Copper is a very stable element. It does not react with acid, for instance, in acid rain, and is very slow to oxidise. Meanwhile, steel rusts quite quickly. Remember that the question refers to steel and not to stainless steel. Stainless steel would resist corrosion but it’s much more expensive than regular steel.
Most forms of steel are alloys of iron and carbon while stainless steel also contains a high proportion of chromium. So, having a higher corrosion resistance would confer a major advantage on copper over the use of steel when used as a roofing material. Therefore, this is a correct answer. However, to be safe, I’m going to look at the last two options as well.
What would be the benefit of having a roof with a higher melting point? As long as the melting point of the roof is above ambient temperature, it’s probably not important what the melting point is. The hottest outside temperature ever recorded was 56.7 degrees C, recorded in Death Valley, California in 1913. Copper has a melting point well above that at 1085 degrees Celsius. However, steal depending on the type of steel, can have a melting point much higher between 1425 degrees Celsius and 1540 degrees Celsius. So, saying that copper has a higher melting point than steel is not even true. Therefore, this is an incorrect answer.
Finally, we come to the last option, having a higher strength. For most roofs, having a high strength isn’t really an issue. Roofs don’t support very much mass themselves, apart from their own weight. So, as long as the material is strong enough, having a higher strength is not going to make a great deal of difference.
If we look at the hardness of copper, depending on how it’s treated, it can have a strength of anywhere between 200 and 350 megapascals. Depending on the type of steel, steel can have a strength of anywhere between 370 and 1760 megapascals. This shows that it is not true that copper has a higher strength than steel. Therefore, this is an incorrect answer.
Again, you need not have looked up the values of the strengths of copper and steel because steel is well-known to be a strong building material while copper is not used for such purposes.
So, of the five options given the one that is a major advantage of copper over steel when used as roofing material is copper’s higher corrosion resistance. But wait, don’t copper roofs turn green? Yes, it’s true, copper roofs do form a green patina over time. The copper in copper roofs slowly reacts with oxygen in the air forming pink-red copper(I) oxide and then black copper(II) oxide. It can also react with sulphur to form copper sulfide which is also black.
The copper oxides can go on to react with carbon dioxide and water from the air, forming various copper compounds, some of which are green, some of which are blue. Alternatively, copper(II) oxide can react with sulphurous pollutants, like sulphur trioxide, and water to form copper compounds containing the sulfate anion. This particular example is also green. The green patina that forms on copper roofs will be a mixture of these compounds. And the colour of the patina will vary depending on the concentration of pollutants in the atmosphere.
This patina has no substantial effect on the ability of the copper roof to do its job. It would take many decades, or many hundreds of years, for a copper roof to completely corrode away. While in the same time, a steel roof would have had to be replaced repeatedly. So, the major advantage of copper by far over steel is its higher corrosion resistance.
