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CO2 properties

This section of All about CO2 provides data and inputs on a range of properties associated with CO2 - physical, chemical and properties that define how it interacts with diverse environments.

A deeper understanding of CO2’s properties can help professionals, researchers and entrepreneurs come up with innovations and solutions that optimally utilize these properties for reducing CO2 emissions, converting it to valuable products or capturing and sequestering it effectively.

1. What type of chemical bond is present in CO2?

CO2 is considered a simple molecule, and it has covalent bonds. In these types of molecules, while the covalent bonds are strong, the intermolecular forces are weak.

2. Can CO2 be toxic?

If one were to consider toxins as poisons, CO2 is not toxic - if it were, we would not be alive as we have CO2 present in small amounts in the air around us.

Plants use CO2 to make their - and our - food, so that too makes CO2 much less of toxin. 

Studies suggest that at concentration levels of over 50,000 PPM and especially over 75,000 PPM, CO2 can be life-threatening. Well, too much of anything can be bad - prolonged exposure to even high concentrations of oxygen can cause damage to lungs and the eyes.

In Sep 2021, the atmospheric CO2 concentration was about 415 PPM - hardly a level to make us worried about toxicity. Rather, it is CO2's potential as a greenhouse gas that should be of concern to us.

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3. What is the density of CO2 under STP?

1.96 g/l.

Compare this with the density of air - 1.225 g/l. As CO2 is heavier than air, we can expect CO2 to sink below the oxygen molecules in the air.

This implies that the lower altitudes of the atmosphere could have higher levels of CO2 than higher altitudes.

4. What is the density of liquid CO2? Of solid CO2 (dry ice)?

5. Does CO2 have a smell?

At normal conditions and concentrations, CO2 does not have any odour. At high concentrations, it has an acidic odour.

6. Does CO2 have a colour?

CO2 is colourless.

7. Can CO2 dissolve in water?  

CO2 is highly soluble in water. At room temperature, about 90 ml of CO2 can dissolve in 100 ml of water.

Portions of CO2 react with water and form a weak acid, carbonic acid.

One reason carbon dioxide dissolves in water is because of the way its atomic structure is charged. Because of the uneven way in which CO2's carbon-oxygen atoms share electrons, the oxygen ends of a CO2 molecule have a slight negative charge. Water molecules are attracted to these areas, and this results in CO2 dissolving in water.

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8. How is CO2 different from CO?

What a difference one oxygen atom can make!

Laymen are often confused between CO2 and CO, but the two are quite dissimilar.

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9. Why is CO more toxic than CO2?

Carbon monoxide is poisonous, whereas carbon-dioxide is non-toxic.

Carbon monoxide is toxic because it is capable of forming a complex with haemoglobin (carboxy-haemoglobin), which is more stable than the oxygen-haemoglobin complex. The concentration range of 3-4% of carboxy-haemoglobin decreases the oxygen-carrying capacity of blood. This results in headaches, weak eyesight, nervousness, and cardiovascular disorders. An increased concentration may even lead to death.

Carbon dioxide, on the other hand, is not poisonous. It does harm only at concentrations much higher than its current levels.

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10. What is the half life of CO2?

Carbon dioxide has a half life of about 120 years.

Put scientifically, half life is the duration of time required for one-half of the atomic nuclei to decay. Put simply, it is the amount of time for CO2 to be reduced to half its initial amount.

11. Why does CO2 give that fizz to drinks?

You don't see bubbled inside  an unopened soda bottle. This is because the pressure inside the bottle keeps the carbon dioxide dissolved in the liquid.

When you open the can however, you release the pressure and allow the CO2 gas bubbles to free themselves from the liquid and rise to the surface, and that is the fizz you see.

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12. Is there a CO3? CO4? C2O?  

Interestingly, all three - CO3, CO4 and even C2O - "exist", but in the way we understand existence!

13. What happens to CO2 if it is pumped underground?

One important avenue being explored in climate change mitigation efforts is to reduce the amount of CO2 in the atmosphere by storing it underground.

The idea is as follows: Once CO2 has been captured using a carbon capture technology, it’s pressurised and turned into a liquid-like form known as ‘supercritical CO2’. This liquid is transported via pipelines and injected into rocks found in the formations deep below the earth’s surface. This process is termed geological sequestration.

Underground reservoirs store CO2 through solid impermeable rocks that typically surrounds them. Once CO2 is injected into a reservoir, it slowly moves upwards through the reservoir until it meets this layer of impermeable rock, and its movement comes to a stop. This can keep CO2 locked underground for millions of years.

That's a very long period of time. During this duration, CO2 will begin to chemically react with its surrounding environment, resulting in solid minerals that can lock up the CO2 much longer.

All these ensure that geological CO2 sequestration could help in keeping the captured CO2 out of the atmosphere for a very long time, perhaps as long as even 10,000 years!

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14. What happens to CO2 if it is pumped into saline aquifers?  

Saline aquifers are geological formations consisting of water permeable rocks saturated with salt water. CO2 can be injected into the aquifers for sequestration.

Injected CO2 remains in these acquifers owing to a combination of three processes:

Which of the above three plays a dominant role in CO2 sequestration will depend on the reservoir characteristics.

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