Consider the following statements regarding burning of magnesium ribbon in air : 1. White powder of MgO is formed. 2. It is an example of combustion reaction. 3. Heat and light are produced. Which of the statements given above are correct?
- (a)1 and 2 only
- (b)1 and 3 only
- (c)2 and 3 only
- (d)1, 2 and 3
Correct — D, 1, 2 and 3. All three statements describe what happens when a strip of magnesium ribbon is lit in air. The metal combines with atmospheric oxygen to give magnesium oxide, which is left behind as a light white powder, so statement 1 holds. The reaction is a combustion — a substance burning in oxygen — so statement 2 holds. And it is famously vigorous: magnesium burns with a dazzling white light while giving out a great deal of heat, which is statement 3. The chemistry is 2Mg + O2 giving 2MgO, and the reason the ribbon is rubbed with sandpaper first is to scrape off the dull oxide coating that would otherwise stop it catching.
- (a)1 and 2 only — Leaves out the heat and light, which is the most conspicuous thing about this reaction — the glare is bright enough that students are told not to look straight at it. A reaction cannot be called a combustion and then be denied its heat.
- (b)1 and 3 only — Drops statement 2, but burning a metal in oxygen is the textbook definition of a combustion reaction. It is also an oxidation and a combination reaction; the categories overlap rather than exclude one another.
- (c)2 and 3 only — Drops the product. What is left on the tile after the ribbon has burned is a white ash of magnesium oxide, and identifying that residue is half of what the classroom demonstration is for.
Burning magnesium is the standard school illustration of a combination reaction that is also an oxidation and a combustion. Two atoms of magnesium give up electrons to one molecule of oxygen, forming the ionic solid magnesium oxide, and the large amount of energy released appears as heat and as intense white light. The white ash is basic — dissolved in water it gives magnesium hydroxide, which turns red litmus blue, a common follow-on test in the same practical.
An all-of-the-above key is often treated with suspicion, and here the suspicion is misplaced: the three statements are simply three true things about one familiar reaction, described at three different levels — the product, the class of reaction, and the observation. One point of accuracy is worth carrying beyond the exam. Magnesium is reactive enough to burn in nitrogen as well, so a ribbon burned in ordinary air leaves a little magnesium nitride along with the oxide, and a greenish-yellow tinge sometimes seen in the ash comes from that. The paper's statement 1 describes the main product, which is what a school-level item is after.
- Burning magnesium in air gives magnesium oxide: 2Mg + O2 gives 2MgO, a white powder.
- The reaction is simultaneously a combination, an oxidation and a combustion — the three names describe different aspects of the same change.
- Magnesium burns with a dazzling white flame, which is why magnesium powder was once used in photographic flash and is still used in flares and fireworks.
- A magnesium ribbon is cleaned with sandpaper before the experiment to remove the layer of magnesium oxide that would otherwise prevent it lighting.
- Magnesium oxide is basic; shaken with water it gives magnesium hydroxide, which turns red litmus blue.
- Rejecting an all-of-the-above option on principle rather than on the statements.
- Assuming the categories combination, oxidation and combustion are mutually exclusive.
- Forgetting the sandpaper step and treating the oxide coating as irrelevant.
As this statements item on a single classroom reaction, or as an identify-the-type-of-reaction item, or as an odd-one-out on which change is chemical rather than physical.
Which one of the following processes involves chemical reaction ?
- (a) Evaporation of petrol
- (b) Dissolution of salt in water
- (c) Storing of oxygen gas under pressure in a gas cylinder
- (d) Burning of magnesium ribbon in air
Answer(d) Burning of magnesium ribbon in air
The same reaction used as the example of a genuine chemical change. A new substance with new properties, magnesium oxide, is formed, which is why this item can then go on to ask what the change should be called and what it looks like.
- practice — not a real PYQ
A magnesium ribbon is cleaned with sandpaper before being burnt in air mainly in order to
- (a)make it thinner so that it burns faster
- (b)remove the layer of magnesium oxide on its surface
- (c)make it a better conductor of heat
- (d)remove moisture from its surface
Answer(b) remove the layer of magnesium oxide on its surface — that dull coating protects the metal underneath and would keep the ribbon from catching.
- practice — not a real PYQ
The reaction 2Mg + O2 giving 2MgO is best described as which one of the following?
- (a)A decomposition reaction
- (b)A displacement reaction
- (c)A combination reaction
- (d)A double displacement reaction
Answer(c) A combination reaction — two reactants join to form a single product, and because oxygen is added it is also an oxidation.