Which compound is soluble in $H_2O$?

What are the products formed when $(i)$ $HCHO$,$(ii)$ $CH_3CHO$,and $(iii)$ $CH_3COCH_3$ are reacted with $CH_3MgBr$ followed by hydrolysis?

In which of the following reactions does an aromatic aldehyde react with an acid anhydride in the presence of the salt of the corresponding acid to form an unsaturated aromatic acid?

Consider the following reaction:
$H-CHO-(CH_2)_4-OH \xrightarrow{H^+} \text{Cyclic product}$
The above reaction is an example of:

Which pair of reactant compounds may be used to make the given acetal?

When two molecules of acetaldehyde condense in the presence of dilute alkali,it forms