The first four carboxylic acids

1. Methanoic acid is $HCOOH$.
2. Ethanoic acid is ${CH}_{3}COOH$.
3. Propanoic acid is $C_2{H}_{5}COOH$.
4. Butanoic acid is $C_3{H}_{7}COOH$.
5. Each differs from the next by ${CH}_2$, so they are a homologous series.

The functional group is -COOH

1. Every carboxylic acid contains the group $-COOH$.
2. That group is a carbon joined to an oxygen by a double bond and to an $-OH$ group.
3. It always sits at the end of the carbon chain.
4. It is what gives the family its characteristic reactions.
5. The $-OH$ inside it is what releases the hydrogen ion in solution.

Drawing the structures

1. The end carbon shows four bonds: two to one oxygen, one to the other oxygen, and one to the chain.
2. The second oxygen carries a hydrogen, completing the $-OH$.
3. Methanoic acid has a hydrogen in place of the carbon chain.
4. Ethanoic acid has a ${CH}_3$ group attached to the $-COOH$.
5. Counting two oxygens in the group is the usual check on the structure.

Their solutions show typical acidic properties

1. A carboxylic acid dissolves in water and releases hydrogen ions.
2. Its solution therefore has a pH below 7 and turns litmus red.
3. It reacts with a metal to give a salt and hydrogen: $$ethanoic\; acid+magnesium\to magnesium\; ethanoate+hydrogen$$
4. It reacts with a carbonate to give a salt, water and carbon dioxide.
5. It reacts with an alkali to give a salt and water, which is neutralisation.