Name the reagent you would add to an unknown compound to test if it contains a carbonyl group.

Draw the skeletal formulae of :

pentan-$2$-one

Draw the skeletal formulae of :

pentan-$3$-one

Draw the skeletal formulae of :

pentanal

Ethanol can be made from ethanal using sodium tetrahydridoborate (III) as a reducing agent. Give the formula of sodium tetrahydridoborate (III). 

$$\begin{gathered} \mathrm{A}: {\mathrm{NaBH}}_2 \\ \mathrm{B}: {\mathrm{NaBH}}_4 \\ \mathrm{C}: {\mathrm{NaBH}}_3 \end{gathered}$$

(Enter your correct answer as $\mathrm{A}, \mathrm{B} \mathrm{or} \mathrm{C}$)

Ethanol can be made from ethanal using sodium tetrahydridoborate (III) as a reducing agent. The reaction mechanism proceeds in a similar way to the steps in the reaction of ethanal with $\mathrm{HCN}$, but the initial attack is by the ${\mathrm{H}}^{+}$ion instead of the ${\mathrm{CN}}^{-}$ion. The intermediate then gains an ${\mathrm{H}}^{+}$ion from a water molecule to form the product, ethanol.

Using your knowledge of the reaction between ethanal and $\mathrm{HCN}$, suggest the name of the mechanism to reduce ethanal. Then describe the mechanism as fully as you can, using curly arrows to show the movement of electron pairs.