Chemguide: Support for CIE A level Chemistry

These statements want you to know about some simple uses of magnesium and calcium compounds.

Before you go on, you should find and read the statements in your copy of the syllabus.

Magnesium oxide

Magnesium oxide is used to make linings for some furnaces. It is known as a refractory material - which just means that it is resistant to heat. You may have come across this on the CIE page about ceramics.

Magnesium oxide has a high melting point (2852°C) and so resists the high temperatures in a furnace. Its melting point is high because of the strong ionic bonds between the small Mg²⁺ and O^2- ions.

Magnesium oxide is a basic oxide, and so can't be used in furnaces where acids may be present. However, it doesn't react in any way with other basic materials such as calcium oxide.

For that reason, it can be used in the furnaces involved in iron and steel making. In the Blast Furnace, limestone is added as one of the raw materials. That decomposes to form basic calcium oxide in the heat of the furnace. This would have no effect on a refractory furnace lining like magnesium oxide which is itself basic.

In the basic oxygen steel making process, quicklime (calcium oxide) is added to the furnace to remove impurities in the iron. Again, this won't affect the magnesium oxide lining.

The use of lime in agriculture

This is quite confusing, because the word "lime" can mean three entirely different things!

• Powdered limestone (calcium carbonate) is often called lime.

• On heating, calcium carbonate decomposes to make calcium oxide, which is known as "quicklime", but is often just called lime.

• If you add water to calcium oxide, it reacts to form calcium hydroxide, which is known as "slaked lime" or "hydrated lime", but is also often just called lime.

It isn't totally clear from the CIE syllabus or the past papers I had available at the time of writing which of these CIE is looking for. I suspect that it is calcium oxide or hydroxide, but I can't be sure.

To some extent that doesn't actually matter, because all three types of lime are used for exactly the same purpose - to raise the pH of the soil.

Many crop plants grow best at a pH of around 6 - 6.5. However, over time, soils tend to become more acidic than this. All of the various kinds of lime react with acids in the soil, and therefore raise the pH. Calcium carbonate and oxide and hydroxide all react with acids.

The use of lime and fertilisers together

A multiple choice question that I looked at asked about what would happen if a farmer treated a field with lime together with a nitrogen fertiliser containing an ammonium compound. If you knew about ammonium salts (which you will come across later if you haven't already done so), you would know that they react with bases to give off ammonia gas.

So you could work out what happens - but it is easier if you have met it before!

If you added a strong base like calcium oxide or hydroxide at the same time as the ammonium compound, then they would quickly react to release ammonia into the atmosphere - wasting the fertiliser.

If you just stir together a solid mixture of calcium oxide or hydroxide and an ammonium compound in the lab, you can smell (cautiously!) the ammonia being released.

Go to the Section 9.2 Menu . . .

To return to the list of learning outcomes in Section 9.2

Go to the CIE Main Menu . . .

To return to the list of all the CIE sections

Go to Chemguide Main Menu . . .

This will take you to the main part of Chemguide.