How is Acetaldehyde converted into Lactic acid?

Hint: Acetaldehyde is also known as Ethanal. Lactic acid preparation from acetaldehyde is also an industrial method of its preparation. Lactic acid is a natural acid found in an animal's body.

Complete step by step solution:
Racemic (a mixture composed of dextrorotatory (d-) and laevorotatory (l-) forms of a particular compound in the equal proportion, i.e. 1:1 ratio) lactic acid is synthesised in industries by the reaction of acetaldehyde with hydrogen cyanide. Then the resultant lactonitrile is hydrolysed. When hydrolysis is performed by hydrochloric acid, ammonium chloride is formed as a by-product of the reaction.
\[C{H_3}CHO\xrightarrow{{HCN}}C{H_3}CH(OH)CN{H_2}O/{H^ + } \to C{H_3}CH(OH)C = O - OH\]

Additional information
Lactic acid is an important byproduct of anaerobic respiration in animals. Extreme Lactic acid accumulation in the muscles can lead to fatigue and tired muscles.

Some important processes stated in the answer and their meanings:
i.) Racemic- It refers to a mixture of both dextrorotatory and laevorotatory forms or isomers of a particular compound found in equal ratio or equal amount i.e. 1:1.
ii.) Hydrolysis- It refers to a chemical process which is the decomposition involving the splitting of a bond and the addition of the hydrogen cation(\[{H^ + }\]) and the hydroxide anion (\[O{H^ - }\]) of water. Hydrolysis reactions are the exact reverse of condensation reactions in their mechanism of action.
iii.) Fermentation - it is the chemical process which involves breaking down of glucose with the action of bacterias and microbes without the presence of air(i.e. anaerobically).
iv.) Dextrorotatory - In simple terms, it refers to the ability of a compound to rotate polarised light in a clockwise manner.
v.) Laevorotatory - In simple terms, it refers to the ability of a compound to change the direction of polarised incident light to the left or in anti-clockwise direction.

Note: Lactic acid can also be prepared by fermentation of carbohydrates by the action of bacteria and other microbes. As a matter of fact, Lactic acid was predominantly produced by fermentation before the reaction of acetaldehyde was known.