The Poisonous nature of Carbon monoxide (CO) is due to its
- (a)insolubility in water
- (b)ability to form a complex with haemoglobin
- (c)ability to reduce some metal oxides
- (d)property of having one sigma bond
Correct — B, ability to form a complex with haemoglobin. Haemoglobin carries oxygen around the body by binding it reversibly at the iron atom in each of its four haem groups. Carbon monoxide binds at that same iron site, but with an affinity of the order of two hundred times greater than oxygen's, and it lets go far more slowly. The product, carboxyhaemoglobin, is a stable complex that can no longer pick up or deliver oxygen. So even a small concentration of carbon monoxide in the air progressively takes haemoglobin out of service, and the victim suffocates while surrounded by perfectly breathable oxygen. That is the whole mechanism of carbon monoxide poisoning, and the reason it is so dangerous is compounded by the gas being colourless, odourless and non-irritating, so there is no warning.
- (a)insolubility in water — Carbon monoxide is indeed only sparingly soluble in water, but solubility has nothing to do with its toxicity. Nitrogen is far less soluble still and is harmless; the danger comes from what the molecule does at the iron of haem, not from how it behaves in water.
- (c)ability to reduce some metal oxides — This is a genuine and important property of carbon monoxide — it is the actual reducing agent that converts iron oxides to iron inside a blast furnace. But it is a piece of industrial chemistry, not a physiological one, and it plays no part in why the gas harms the body.
- (d)property of having one sigma bond — Doubly wrong. The carbon-oxygen linkage in CO is a triple bond, made of one sigma and two pi bonds, so the description itself is inaccurate. And in any case the count of sigma bonds in a molecule tells you nothing about whether it is a poison.
Carbon monoxide is produced whenever a carbon fuel burns with too little oxygen — a charcoal brazier in a closed room, a faulty gas geyser, a petrol engine idling in a shut garage, a smouldering fire. Because it is colourless, odourless and tasteless, it gives no warning of its presence, and the early symptoms of poisoning, headache, dizziness and nausea, are easy to mistake for something ordinary. Treatment rests on the same chemistry that causes the harm: the binding is reversible, so giving the patient pure oxygen, or oxygen under pressure, drives the carbon monoxide off the haem sites and restores the blood's carrying capacity.
The booklet prints capitals in the middle of the sentence — 'The Poisonous nature of Carbon monoxide (CO)' — and the text is reproduced here exactly as it appeared. Nothing in the chemistry turns on it. What the item is really testing is whether a candidate can tell a relevant property from an impressive one: options (a), (c) and (d) are all statements about carbon monoxide, and (c) is even a fact worth knowing, but only one of the four is about the interaction with the body. That is the habit to build for toxicity questions generally — ask what the substance does inside an organism, not what it does in a beaker. It is worth carrying two neighbouring cases in the same mental file, since examiners rotate among them: carbon dioxide is not poisonous in this sense but suffocates by displacing oxygen from the air, and cyanide kills by blocking the enzyme cytochrome oxidase inside cells rather than by occupying haemoglobin.
- Carbon monoxide binds to the iron of haemoglobin to form carboxyhaemoglobin.
- Its affinity for haemoglobin is roughly two hundred times that of oxygen, and it dissociates far more slowly.
- Blood loaded with carboxyhaemoglobin cannot carry oxygen, so the tissues are starved of it.
- The gas is colourless, odourless and non-irritating, so it gives no warning.
- It forms wherever carbon fuels burn with an insufficient supply of air.
Three of the four options are statements about the molecule; only one is a statement about the harm.
- Choosing a true but irrelevant property; every wrong option here is about the molecule rather than about the poisoning.
- Confusing carbon monoxide's toxicity with carbon dioxide's suffocating effect, which works by displacing oxygen from the air.
- Assuming a strong-smelling gas is the dangerous one; carbon monoxide is entirely odourless.
NDA and UPSC both ask why carbon monoxide is dangerous, and reliably plant a true-but-irrelevant property among the options.
Excessive release of the pollutant carbon monoxide (CO) into the air may produce a condition in which oxygen supply in the human body decreases. What causes this condition ?
- (a) When inhaled into the human body, CO is converted into CO₂
- (b) The inhaled CO has much higher affinity for haemoglobin as compared to oxygen
- (c) The inhaled CO destroys the chemical structure of haemoglobin
- (d) The inhaled CO adversely affects the respiratory centre in the brain
Answer(b) The inhaled CO has much higher affinity for haemoglobin as compared to oxygen
The same mechanism, keyed by UPSC eight years earlier, and note the distractor it rejects — carbon monoxide does not destroy haemoglobin, it occupies it, which is why the damage is reversible with oxygen therapy.
- practice — not a real PYQ
The compound formed when carbon monoxide binds to haemoglobin is called
- (a)oxyhaemoglobin
- (b)carboxyhaemoglobin
- (c)methaemoglobin
- (d)carbaminohaemoglobin
Answer(b) carboxyhaemoglobin — a stable complex at the haem iron that can no longer carry oxygen.
- practice — not a real PYQ
A patient with carbon monoxide poisoning is given pure oxygen because
- (a)oxygen destroys carbon monoxide chemically in the lungs
- (b)the binding is reversible, and excess oxygen displaces carbon monoxide from haemoglobin
- (c)oxygen increases the solubility of carbon monoxide in blood
- (d)oxygen neutralises the acidity caused by the gas
Answer(b) the binding is reversible, and excess oxygen displaces carbon monoxide from haemoglobin — which is why the treatment works.