Question

Which metal reacts with hot water to give hydrogen?
A) Magnesium
B) Mercury
C) Tin
D) Titanium

Answer 1

Hint: The metals readily react with the water to form the oxides or hydroxides. Alkaline earth metal can react with water. They can react with cold or water. The tin and titanium also reacts with the water but when it is exposed in the form of hot water vapour or steam. Mercury is denser than water hence has no reactivity towards water.

Complete step by step answer:
The alkaline earth metals react with the water to form the hydroxides. The hydroxides of calcium, strontium and barium are slightly soluble in water. The general reaction for the hydroxides of alkaline earth metal is as shown below,
${{\text{M}}_{\text{(s)}}}\text{+2}{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{(l)}}}\to \text{M(OH}{{\text{)}}_{\text{2 (aq)}}}\text{+}{{\text{H}}_{\text{2}}}{{\text{ }}_{\text{(g)}}}$
The reaction of metals with the water depends on the reduction potential of the metal. The reduction potential is the tendency of an electrode to absorb an electron.
Due to intermediate values for the reduction potential the magnesium does not react with the cold water. But it does react with the hot water. The reaction of magnesium with the hot water is as shown below:
$\begin{align}
  & \text{Mg + 2}{{\text{H}}_{\text{2}}}\text{O}\to \text{Mg(OH}{{\text{)}}_{\text{2}}}\text{ + }{{\text{H}}_{\begin{smallmatrix}
 \text{2} \\

\end{smallmatrix}}} \\
 & \text{ OR} \\
 & \text{Mg + }{{\text{H}}_{\text{2}}}\text{O}\to \text{MgO + }{{\text{H}}_{\begin{smallmatrix}
 \text{2} \\

\end{smallmatrix}}} \\
\end{align}$
That is, Magnesium (Mg) reacts with hot water to form magnesium hydroxide and hydrogen gas.
 The tin is stable in water. When tin comes in contact with the steam it forms the tin oxide and hydrogen as shown below:
$\text{ Sn + 2}{{\text{H}}_{\text{2}}}\text{O}\to \text{ Sn}{{\text{O}}_{\text{2}}}\text{ + 2}{{\text{H}}_{\text{2}}}$
Similarly the titanium reacts with the water vapour or steam to generate the titanium hydroxide and hydrogen as shown below:
$\text{ Ti + 2}{{\text{H}}_{\text{2}}}\text{O}\to \text{ Ti}{{\text{O}}_{\text{2}}}\text{ + 2}{{\text{H}}_{\text{2}}}$
Titanium is normally coated with the oxide layer thus it is inactive. However when exposed to steam forms 7y titanium dioxide, and hydrogen${{\text{H}}_{\text{2}}}$.
Let’s consider mercury. Mercury does not react with the water since mercury and water do not mix with each other. This is because the mercury is 5.43 times denser than the water thus mercury sinks at the bottom of the container if water is mixed with the water. Mercury has a large surface tension thus it is possible to see the beads of mercury in the water. But it is a physical change so no reaction takes place.
Here, the titanium and tin react with the water but in its vapour or steam form.However the magnesium reacts with the hot water to generate the hydrogen gas.

Hence, (A) is the correct option.


Note: The reduction potential plays an important role in deciding the reaction of elements with the water. The magnesium does not readily undergo the reaction with the cold water as the other alkaline earth metal even though it has the favourable potential .Because it is coated with the oxide layer. It does not react until the oxide layer is removed or destroyed.