Quick answer: Denatured alcohol is industrial ethanol made unfit to drink by adding poisonous substances such as methanol, with a dye that colours it blue for easy identification. Methanol is dangerous because the liver oxidises it to methanal, which coagulates protoplasm, and it damages the optic nerve, causing blindness. Ethanol made from sugarcane can be blended with petrol as a cleaner fuel.
What is denatured alcohol?
Ethanol () is widely used in industry as a solvent and in making medicines, paints, varnishes and perfumes. To stop industrial ethanol from being misused for drinking:
- Poisonous substances such as methanol (and sometimes pyridine, which has a bad smell) are added, so it is unfit to drink.
- Dyes are added to colour it blue, so it can be recognised easily.
This mixture is called denatured alcohol.
NCERT point: Denatured alcohol is ethanol that has been made unfit for drinking by adding poisonous substances like methanol, and coloured blue with dyes so it can be identified.
Why is methanol poisonous?
Methanol () looks and smells very similar to ethanol, which is why illicit ("spurious") liquor containing it causes poisoning. Even small amounts can cause death.
Step 1: Oxidation in the liver. The liver oxidises methanol to methanal (formaldehyde, ).
Step 2: Damage to cells. Methanal reacts rapidly with the components of cells and causes the protoplasm to coagulate, much as an egg coagulates when it is cooked.
Step 3: Blindness. Methanol poisoning also affects the optic nerve, causing blindness.
Beyond NCERT: Methanal is further converted to formic acid (methanoic acid). Its build-up makes the blood too acidic and is the main cause of the severe damage, including to the optic nerve.
| Alcohol | Formula | Effect on the body |
|---|---|---|
| Ethanol | Slows down the nervous system; harmful in large amounts or with regular use | |
| Methanol | Highly poisonous even in small amounts; can cause blindness and death |
Ethanol as a fuel
Sugarcane is a very efficient plant for turning sunlight into chemical energy. Molasses from sugarcane can be fermented by yeast to make ethanol.
- Ethanol burns with a clean flame and contains oxygen in its own molecule, so it burns more completely than petrol and produces less soot and carbon monoxide.
- It is made from plants, so it is a renewable fuel. The carbon dioxide released was taken from the air by the growing crop, though energy is still used in farming and processing.
- Ethanol is blended with petrol to make a cleaner fuel, often called gasohol. India has been rolling out E20 petrol, which contains 20% ethanol.
Summary table
| Term | What it is | Key point |
|---|---|---|
| Ethanol | Solvent, fuel and the alcohol in drinks | |
| Methanol | Poisonous; oxidised to methanal in the liver | |
| Denatured alcohol | Ethanol + methanol + blue dye | Unfit to drink; for industrial use |
| Gasohol | Ethanol blended with petrol | Cleaner, partly renewable fuel |
Exam tips
- Define denatured alcohol with both parts: poisonous substances added and blue dye for identification.
- For methanol poisoning, write the NCERT chain: methanol → methanal in the liver → coagulation of protoplasm; also affects the optic nerve → blindness.
- Use the "egg is coagulated by cooking" comparison; it is the standard NCERT explanation.
- When writing the combustion equation for ethanol, check the balancing: 1 ethanol + 3 oxygen gives 2 CO₂ + 3 H₂O.
Common mistakes
- Saying denatured alcohol is safe to drink in small amounts. It is poisonous.
- Writing that methanol is oxidised to ethanoic acid. It is oxidised to methanal (and then formic acid).
- Confusing methanol and ethanol formulas: methanol is CH₃OH (one carbon); ethanol is C₂H₅OH (two carbons).
- Saying ethanol produces no carbon dioxide when it burns. It does produce CO₂, but the carbon came from the crop.
Frequently Asked Questions
What is denatured alcohol?
Denatured alcohol is ethanol made unfit for drinking by adding poisonous substances such as methanol, and coloured blue with dyes so it can be identified easily.
Why is methanol added to industrial alcohol?
Methanol is added to make the alcohol poisonous, so that ethanol meant for industry is not misused for drinking.
Why does methanol cause blindness?
Methanol is oxidised in the liver to methanal, which damages cells by coagulating protoplasm. Methanol poisoning also affects the optic nerve, which leads to blindness.
Why is ethanol considered a cleaner fuel?
Ethanol contains oxygen in its molecule and burns more completely, producing less soot and carbon monoxide than petrol. It is also made from plants, so it is renewable.
What is gasohol?
Gasohol is a fuel made by blending ethanol with petrol. India uses E20 petrol, which contains 20% ethanol.