Two oxides, one carbon
When carbon burns in plenty of air we get carbon dioxide, CO₂. When air is short, we get carbon monoxide, CO. Both are colourless gases with no smell, so you cannot sense them.
C + O₂ → CO₂ (plenty of air). 2C + O₂ → 2CO (little air).
Carbon dioxide: properties
- Colourless, with no smell and a slightly sour taste when dissolved.
- About 1.5 times heavier than air.
- Does not burn and does not help burning.
- Dissolves a little in water and makes weak carbonic acid (H₂CO₃), so it turns blue litmus faintly red.
- Turns limewater milky.
- Under pressure it becomes a liquid; solid CO₂ is dry ice, which changes straight to gas at about −78 °C (sublimation).
Uses of carbon dioxide
- Plants use it in photosynthesis to make food.
- Fizz in soft drinks and soda water.
- Fire extinguishers: the heavy gas covers the fire and keeps air away.
- Dry ice keeps ice cream and vaccines cold, and makes stage fog.
- Raising agent in baking: CO₂ bubbles make dough rise.
Carbon monoxide: why it is poisonous
CO forms in incomplete combustion: a car engine in a shut garage, a charcoal fire in a closed room, a badly set gas geyser.
Red blood cells carry oxygen using haemoglobin. CO sticks to haemoglobin far more strongly than oxygen does and does not let go easily. So less oxygen reaches the brain and heart. Signs are headache, dizziness and sleepiness. Move the person to fresh air and call for medical help.
CO burns with a blue flame to make CO₂: 2CO + O₂ → 2CO₂.
Carbon monoxide as a reducing agent
A reducing agent takes oxygen away from another substance. CO does this to metal oxides at high temperature, for example in a blast furnace:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Iron oxide loses oxygen (it is reduced to iron). CO gains oxygen (it is oxidised to CO₂). Both happen together, which is called a redox reaction.
Greenhouse effect
The Earth gets sunlight. The warm ground sends out heat. Gases such as CO₂, methane and water vapour take in some of that heat and send part of it back to the ground. This natural blanket keeps the Earth near 15 °C. Without it, it would be about −18 °C.
Burning coal, oil and gas, and cutting forests, add more CO₂. A thicker blanket traps more heat, so the world gets warmer. This is global warming. Planting trees, saving energy and using clean power help.
Try it: the heavy gas pour
Put a pinch of baking soda and some vinegar in a tall glass. When it fizzes, tip the glass (not the liquid) over a small burning candle, as if pouring water. The candle goes out. The gas you cannot see is heavier than air. Do it with an adult near.
Key formulas and definitions
- C + O₂ → CO₂
- 2C + O₂ → 2CO
- 2CO + O₂ → 2CO₂
- Fe₂O₃ + 3CO → 2Fe + 3CO₂
- CO₂ + H₂O ⇌ H₂CO₃
- Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Worked examples
1. Name one way CO₂ and CO differ in how they are formed.
CO₂ forms when carbon burns in plenty of air. CO forms when carbon burns in little air (incomplete combustion).
2. Why does a CO₂ fire extinguisher stop a fire?
The gas is heavier than air and does not burn. It covers the fire and keeps oxygen away.
3. Why is CO called a silent killer?
It is colourless and has no smell, and it blocks haemoglobin so the body lacks oxygen, so a person may not notice until too late.
4. In Fe₂O₃ + 3CO → 2Fe + 3CO₂, name the reducing agent and what is reduced.
CO is the reducing agent. Fe₂O₃ (iron oxide) is reduced to iron.
5. How many moles of CO are needed for 2 moles of Fe₂O₃?
1 mole of Fe₂O₃ needs 3 moles of CO, so 2 moles need 6 moles of CO.
6. 28 g of CO burns completely in oxygen. How much CO₂ forms? (CO = 28 g/mol, CO₂ = 44 g/mol)
28 g CO = 1 mol. 2CO + O₂ → 2CO₂ gives 1 mol CO₂. Mass = 44 g.
Common mistakes
- Thinking CO₂ is poisonous like CO. CO₂ is not toxic in small amounts; in a closed room a lot of it can still suffocate by pushing out oxygen.
- Saying CO burns with a yellow flame. It burns with a blue flame.
- Mixing up "CO₂ is a reducing agent". It is CO that is the reducing agent.
- Believing the greenhouse effect is itself bad. The natural effect keeps Earth warm; the extra CO₂ is the problem.