Physical and Chemical Properties of Carbon Dioxide

Carbon dioxide is a naturally found oxide of carbon that is present in the atmosphere in trace quantities. It can also be manufactured synthetically by industrial processes because it has several applications in the food and beverage industry, fire extinguishers, life jackets, etc. It is an important component of photosynthesis. It consists of carbon as the central atom, which is covalently bonded to two oxygen atoms by double bonds. Numerous chemical properties of carbon dioxide make it important for everyday applications. We shall study those here in detail.

Physical Properties of Carbon Dioxide

CO2 naturally exists in the gaseous form. Its molar mass is 44gmol-1. It has a melting point of -55.6oC. I.e., it exists in the solid form below this temperature. In the solid form, it is known as dry ice, and it should not be handled with utmost care as it can cause damage to the skin. It is a linear and covalent molecule that is slightly soluble in water. As the temperature increases, the solubility in water decreases.

In gaseous form, it is colourless and odourless. It has a sour taste. It is a dense gas, with a density of 1.977gmL-1. Due to this, it is heavier than air and can displace oxygen. It is acidic in nature and forms a weak acid called carbonic acid when dissolved in water. It can turn blue litmus paper red.

The gas is non-flammable, making it especially useful for applications in fire extinguishers. The presence of oxygen is crucial for combustion to take place. Carbon dioxide in fire extinguishers cuts the supply of oxygen from the fuel source due to which fire is doused.  

O2 + CH4 → CO2 + H2O (combustion reaction) 

In high concentrations, it is toxic to humans and has a pungent odour. When exposed to pure carbon dioxide, the person collapses and eventually dies when they are unable to get oxygen. At low concentrations, however, it is relatively non-toxic and odourless.  

Chemical Properties of Carbon Dioxide

The chemical properties of carbon dioxide are very much influenced by the acidic behaviour of the gas. For example, it reacts with water to form carbonic acid, which is a weak acid. It dissociated partially into hydronium and carbonate ions. 

CO2 + H2O → H2CO3 

H2CO3 ⇆ H+ + HCO3– 

HCO3– ⇆ H+ + CO32-  

It is a reversible reaction. Carbonic acid is a weak acid, dissociating partially in solution in a two-step reaction. 

Since it is acidic, it reacts with basic compounds as per the neutralisation reaction, giving salt and water. 

Reaction of carbon dioxide with sodium hydroxide: 

CO2 + 2NaOH → Na2CO3 + H2O 

In the presence of excess carbon dioxide, the following reaction occurs. 

Na2CO3 + H2O + CO2 → 2NaHCO3 

NaHCO3, or sodium bicarbonate, is an alkali. It is a salt of a weak acid (carbon dioxide) and a strong base (NaOH). NaHCO3 has several uses. It is used in the kitchen as it makes dough fluffy and raises it.  

2NaHCO3 + H+ → Na2CO3 + CO2 + H2O 

In the presence of acid (which is present in food) and heat, baking soda gives the above reaction in which CO2 is evolved. This evolved carbon dioxide makes the dough fluffy.

Reaction of carbon dioxide with calcium hydroxide: 

CO2 + Ca(OH)2 → CaCO3 + H2O 

Calcium carbonate further has many uses, the major being that it is used to produce CO2. Limestone, when heated, releases carbon dioxide, which is used for industrial purposes. 

Reaction with metals: Chemical properties of carbon dioxide involve reaction with metals to form metal oxides, which are basic in nature. 

CO2 + 2Mg → 2MgO + C

Reaction with metal oxides: CO2 reacts with group 1 and 2 metal oxides to form metal carbonates via exothermic reactions. 

CO2 + CaO → CaCO3 

Lime Test 

It is a chemical test performed to detect whether carbon dioxide is evolved in a reaction or not. This test makes use of one of the chemical properties of carbon dioxide, which is that it reacts with calcium hydroxide to form calcium carbonate and calcium bicarbonate.

For the lime test, the gas (carbon dioxide) is allowed to pass through a calcium hydroxide solution, transparent in colour and completely soluble in water. The reaction forms calcium carbonate, which makes the solution cloudy. 

CO2 + Ca(OH)2 → CaCO3+ H2O 

When excess carbon dioxide is passed, then calcium bicarbonate is formed, which gives a colourless solution. 

CaCO3(aq) + CO2 +H2O → Ca(HCO3)2 

Hence, first, the solution turns milky, which soon turns colourless. This confirms that carbon dioxide evolved from the reaction.

Photosynthesis Reaction  

A very important biochemical reaction in which carbon dioxide is involved in photosynthesis. It is the process by which plants process glucose. They inhale carbon dioxide in the presence of sunlight and water to form glucose and oxygen. Hence, carbon dioxide is the basis of the existence of plant life on earth.  

6CO2 + 6H2O + sunlight → 6O2 + C6H12O6 (glucose)

Conclusion 

Numerous physical and chemical properties of carbon dioxide make it fit for everyday and laboratory applications. It exists mostly in the gaseous state, as a colourless and odourless gas. It is non-flammable, and this property makes it useful in fire extinguishers. It is acidic in nature, and its chemical reactions are synchronous with its acidic behaviour. It reacts with basic compounds as per neutralisation reactions. It forms carbonic acid upon reaction with water and turns blue litmus red.