# Which of the listed reactions evolve a combustible gas...?

## $\text{A. magnesium hydroxide + hydrochloric acid} .$ $\text{B. magnesium carbonate + hydrochloric acid} .$ $\text{C. magnesium oxide + hydrochloric acid} .$ $\text{D. magnesium metal + hydrochloric acid} .$

Apr 2, 2017

$\text{Option B}$

#### Explanation:

We can write out the individual stoichiometric equations for $A$, $B$, $C$, and $D$:

$\text{A:}$

$M g {\left(O H\right)}_{2} \left(s\right) + 2 H C l \left(a q\right) \rightarrow M g C {l}_{2} \left(a q\right) + 2 {H}_{2} O \left(l\right)$

$\text{B:}$

$M g C {O}_{3} \left(s\right) + 2 H C l \left(a q\right) \rightarrow M g C {l}_{2} \left(a q\right) + {H}_{2} O \left(l\right) + C {O}_{2} \left(g\right) \uparrow$

$\text{C:}$

$M g O \left(s\right) + 2 H C l \left(a q\right) \rightarrow M g C {l}_{2} \left(a q\right) + {H}_{2} O \left(l\right)$

$\text{D:}$

$M g \left(s\right) + 2 H C l \left(a q\right) \rightarrow M g C {l}_{2} \left(a q\right) + {H}_{2} \left(g\right) \uparrow$

At 1st year university level I would expect you to know these reactions. Now ONLY $\text{B}$ and $\text{D}$ result in gas evolution. However, for $\text{D}$, the gas evolved is dihydrogen, which is certainly combustible:

${H}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow {H}_{2} O \left(g\right) + \Delta$

And hydrogen gas typically ignites with a pop. On the other hand, the gas evolved in $\text{B}$ is non-combustible carbon dioxide, which WOULD extinguish a lit taper.