Uses of aldehydes and ketones
NCERT Class 12 Chemistry Chapter 8 Section 8.5
Quick answer
Aldehydes and ketones serve as solvents and starting materials for polymers, medicines and other products. Formaldehyde is used in formalin, which NCERT gives as a 40% solution (beyond the NCERT text, commercial formalin is about 37% by mass: Wikipedia, “Formaldehyde”), and in phenol-formaldehyde resins and glues; acetaldehyde is a starting material for acids and esters. Acetone and butanone are solvents, while selected compounds such as benzaldehyde and vanillin have fragrance or flavour applications in suitable formulations.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 8 “Aldehydes, Ketones and Carboxylic Acids”, Section 8.5 “Uses of Aldehydes and Ketones”, pages 243–244.
Open the official NCERT chapter (PDF, 4.4 MB, ncert.nic.in) (opens in a new tab)
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What are aldehydes and ketones used for?
Uses of aldehydes and ketones
In the chemical industry aldehydes and ketones serve as solvents, as starting materials and as reagents for making other products (NCERT page 243). The best known is formaldehyde; NCERT says:
Formaldehyde is well known as formalin (40%) solution used to preserve biological specimens and to prepare bakelite (a phenol-formaldehyde resin), urea-formaldehyde glues and other polymeric products.
NCERT Chemistry Part II (Class 12), Section 8.5, page 243
In plain words: formaldehyde preserves specimens and builds plastics and glues, acetaldehyde is a stepping stone to acetic acid and its esters, acetone dissolves things, and some aldehydes and ketones contribute characteristic aromas in appropriately formulated products.
Formalin and bakelite
- Formaldehyde (methanal) is sold as formalin, which NCERT calls a 40% solution, used to preserve biological specimens (NCERT page 243). Beyond the NCERT text: commercial formalin holds about 37% formaldehyde by mass, or about 40% reckoned per volume of solution (Wikipedia, “Formaldehyde”). In an exam, give NCERT’s 40%.
- It is a raw material for bakelite, a phenol-formaldehyde resin, for urea-formaldehyde glues and for other polymers (NCERT page 243).
- Acetaldehyde (ethanal) is listed by NCERT as a starting material for acetic acid, ethyl acetate, vinyl acetate, polymers and drugs (NCERT page 243). This is a list of chemical applications, not a ranking of present-day manufacturing routes or market shares.
Solvents, perfumes and flavours
- Acetone (propanone) and ethyl methyl ketone (butanone) are common industrial solvents (NCERT page 243).
- Benzaldehyde is used in perfumery and in the dye industry (NCERT page 243).
- Many aldehydes and ketones, for example butyraldehyde, vanillin, acetophenone and camphor, are known for their odours and flavours (NCERT page 243).
| Compound | Main use (NCERT page 243) |
|---|---|
| formaldehyde | formalin; bakelite, glues |
| acetaldehyde | acetic acid, esters, drugs |
| benzaldehyde | perfumes, dyes |
| acetone, butanone | solvents |
| vanillin, camphor | odours, flavours |
These uses concern specified formulations and grades. Do not taste laboratory chemicals or assume that a fragrance chemical is suitable for food; camphor, in particular, is not a general food ingredient.
Solved examples
Example 1: Formaldehyde in formalin
Taking NCERT’s 40% formalin as 40% formaldehyde by mass, how many moles of HCHO are in 75 g of formalin? (C = 12, H = 1, O = 16.)
Answer: Mass of HCHO = 40% of 75 g = 0.40 × 75 = 30 g. M(HCHO) = 12 + 2 + 16 = 30 g mol−1, so the amount is 30/30 = 1.0 mol (NCERT page 243; the mass percentage reading of “40%” is an assumption, as NCERT does not say which percentage). Beyond the NCERT text, commercial formalin, a saturated solution, holds about 37% formaldehyde by mass, or about 40% reckoned per volume of solution (Wikipedia, “Formaldehyde”).
Example 2: Acetaldehyde to acetic acid
NCERT names acetaldehyde as a starting material for acetic acid. What mass of acetic acid can 22 g of acetaldehyde give at most, by 2CH3CHO + O2 → 2CH3COOH?
Answer: M(CH3CHO) = 24 + 4 + 16 = 44 g mol−1, so 22 g is 0.50 mol, which gives 0.50 mol of acetic acid, M = 24 + 4 + 32 = 60 g mol−1: 0.50 × 60 = 30 g (reasoned; the use is from NCERT page 243, the equation is ours).
Common mistakes to avoid
Common mistake
Calling formalin pure formaldehyde
Formalin is an aqueous formaldehyde solution, NCERT’s 40% solution (commercially about 37% by mass), the form in which it is used to preserve specimens.
Common mistake
Mixing up the solvents and the flavour compounds
Acetone and ethyl methyl ketone are industrial solvents; butyraldehyde, vanillin, acetophenone and camphor, a mix of aldehydes and ketones, are valued for their odours and flavours.
Quick revision summary
- Formaldehyde: formalin (NCERT: 40%, without a basis; commonly about 37% by mass) preserves biological specimens; makes bakelite, urea-formaldehyde glues and other polymers.
- Acetaldehyde: starting material for acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.
- Benzaldehyde: perfumery and dyes. Acetone and ethyl methyl ketone: industrial solvents.
- Butyraldehyde, vanillin, acetophenone and camphor: odours and flavours.
Formula sheet
- Formalin: NCERT 40% solution (commercially about 37% HCHO by mass)
- Bakelite: a phenol-formaldehyde resin
- Industrial solvents: acetone, ethyl methyl ketone
Questions students ask
What is the difference between formalin and formaldehyde?
Formaldehyde is HCHO, a gas at room temperature; formalin is its aqueous solution. Beyond the NCERT text, a common commercial stock contains about 37% formaldehyde by mass and 10–12% methanol to limit polymerisation (Wikipedia, “Formaldehyde”). Concentrations and stabilisers vary, so use the label. “10% formalin” normally means one part stock diluted to ten parts total, about 3.7–4% formaldehyde, not 10% pure HCHO. Much of the formaldehyde in water is hydrated rather than present as free HCHO molecules.
How does formalin preserve biological specimens?
Formaldehyde reacts with nucleophilic sites in proteins and other biomolecules and can form cross-links that stabilise tissue structure, a process called fixation. Amino groups on amino-acid residues are among these sites; a protein is not simply a single amine, and fixation is more complex than one Schiff-base equation. Its antimicrobial action also slows decomposition. The preservation use is described by NCERT and the US National Cancer Institute.
Why is formalin dangerous?
Formaldehyde vapour irritates the eyes and respiratory tract, and formaldehyde is classified as a human carcinogen by IARC and the US National Toxicology Program, as summarised by the National Cancer Institute. Formalin therefore requires the laboratory’s controlled ventilation and handling procedure. An odour or irritation threshold is not a safe-exposure limit, and absence of an odour does not establish safe conditions.
Why is acetone used as a solvent?
Acetone is miscible with water and dissolves many organic substances, making it useful in cleaning and coating formulations. Its carbonyl oxygen accepts hydrogen bonds from water, while its methyl groups also interact with organic solutes. Its relatively low boiling point, 329 K in NCERT’s table, allows rapid evaporation. Solubility depends on the material, and acetone is flammable; the general solvent use does not mean that every plastic or coating is compatible.
Practice questions
Try each question first, then open the answer to check your working.
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Q1. What is formalin, and give two uses of formaldehyde.
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Formalin is an aqueous formaldehyde solution; NCERT’s answer is a 40% solution (page 243), and commercially it is about 37% by mass (Wikipedia, “Formaldehyde”). Formaldehyde is used to preserve biological specimens and to make bakelite (a phenol-formaldehyde resin) and urea-formaldehyde glues (NCERT page 243).
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Q2. Name two ketones used as industrial solvents and one ketone known for its flavour.
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Acetone and ethyl methyl ketone are solvents; acetophenone (or camphor) is known for its odour and flavour (NCERT page 243).