Question

What happens when a piece of magnesium ribbon is placed in cold water?
(A) A vigorous effervescence occurs.
(B) Bubbles of gas form slowly on the magnesium.
(C) The magnesium floats on the surface of the water and reacts quickly.
(D) The magnesium glows and a white solid is produced.

Answer 1

Hint: Magnesium is an alkali metal and in general their reaction with water produces hydrogen gas. These metals react very slowly with water.

Complete step by step solution:
-First of all, we will see what the magnesium ribbon is. It is basically a thin ribbon of magnesium metal used for laboratory demonstrations and in the magnesium lamp as well.
Since it is just a form of magnesium, its molecular formula is Mg. It is simply a metallic element with an atomic number of 12 and an atomic weight of 24. It is a lightweight, silvery-white, malleable and ductile metal which can be used in various metallurgical and chemical processes.
-Now let us talk about the reactivity of the magnesium ribbon.
It reacts very slowly with water or moisture and has the ability to release highly flammable gases or vapours of hydrogen. This reaction between magnesium ribbon and water is accelerated due to its exposure to strong acids. We know that burning substances react explosively with water. The magnesium ribbon reacts violently with various compounds like with strong oxidizers. On exposure to these oxidizers, there is a high risk of fire or explosion. Also the magnesium ribbon on reaction with some acids releases highly flammable gases or vapours of hydrogen.
As a conclusion, we can say that magnesium ribbon is a flammable solid and may catch fire on self-heating.
-We should also know that the magnesium ribbon burns very easily in the air to form magnesium oxide in the presence of some external source of energy. The reaction involved is:
$2Mg + {O_2} \to 2MgO$
During this process, magnesium metal gets oxidized to its metal oxide and hence it is an oxidation reaction.
-We have already seen in the above discussion that magnesium ribbon reacts very slowly with cold water. When the ribbon is kept in cold water some bubbles of hydrogen gas form on the surface after some time and the coil of magnesium ribbon starts to float on the surface.

Hence the correct option will be: (B) Bubbles of gas form slowly on the magnesium.

Note: Magnesium ribbon on reaction with hot water gives slightly soluble magnesium hydroxide and hydrogen. This reaction is also very slow.
$Mg + 2{H_2}O \to Mg{(OH)_2} + {H_2}$
But with steam, the magnesium ribbon reacts vigorously and produces intense white light.
$Mg\xrightarrow{{steam}}MgO + {H_{2(g)}}$