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Zinc oxide and exam questions

Well after last time, I thought about exam questions. I know that exams are unpopular with students but they can serve an important purpose. I was interested to read how the RSC had done some research on the topic, they tried to test the hypothesis that “over the last 50 years O-level and GCSE have become more easy”

Their results did show that todays children found the older exams harder than modern exams but that is not perfect evidence to suggest that the new exams are more easy. Now I am sure some of you are going to be very disappointed that I do not join in the debate on “grade inflation”, I know it is a hot topic but I want to concentrate on something else. This something else is even more dangerous than “grade inflation” it is learning by rote without obtaining an understanding !

One of the older exam questions was

“Which of the following substances do not form a gas on moderate heating ?”

1. Potassium nitrate

2. Lead dioxide

3. Iron nitrate

4. Zinc oxide

5. Potassium permanganate

Note I have changed several of the substances, partly for copyright reasons and partly because I can not recall the list of five and also because I do not want to give out the answers to someone elses exam.

Before we get going I want to ask the question of “what is moderate heating”, is it a gentle warming with boiling water, is it holding the test tube in your hand or is it a quick squirt with a chef’s propane blowtorch (but not the oxyacetylene welders torch) ? This to me is a good question in itself but let us assume that the question means heating with a bunsen burner.

Having learnt a vast number of reactions by heart I can tell you

The potassium nitrate will form oxygen and potassium nitrite

The lead dioxide will form oxygen and lead monoxide

Metal nitrates form a mixture of nitrogen dioxide, oxygen, iron nitrite and iron oxide on heating. In general the metals which have a greater charge to size ratio than the group I metals tend to form more of the metal oxides and nitrogen dioxide on heating.

Zinc oxide does not make noticeable amounts of gas when heated

Potassium permanganate decomposes to form oxygen on heating.

So zinc oxide is the odd one out, but when you know your chemistry in greater detail you will discover that metal oxide formation is reversible and as the temperature is increased metal oxides tend to partly revert to oxygen and the metal. In the following film I will show you what happens when zinc oxide is heated. The white zinc oxide loses a little oxygen to become oxygen deficient and yellow when hot. When it cools it reverts to white.

Knowing that zinc oxide (like lead monoxide) is thermochromic and that the hot oxide is yellow while the cold zinc oxide is white is a small bit of trivial knowledge. An understanding of what happens when you heat zinc oxide with a blowtorch is far more important at university. The heating of the zinc oxide causes it to lose some oxygen, the zinc rich (or oxygen deficient) oxide is then a doped semiconductor which has the same colour as cadmium sulphide.


One Response

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