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In the atmosphere carbon dioxide (also called carbonic anhydride: CO₂) is the most abundant form of carbon (Section 4.1). In the lithosphere and hydrosphere carbon is found in the form of carbonates ${(\text{CO}}_3^{2-})$ and bicarbonates (hydrogen carbonates: ${\text{HCO}}_3^{-})$, bound to mono-cations such as sodium Na⁺, potassium, K⁺, ammonium cation, ${\text{NH}}_4^{+}$, or dications such as calcium Ca²⁺ and magnesium Mg²⁺. Carbon dioxide, carbonates and bicarbonates are mineral carbon compounds (Sections 4.3, 4.5 and 4.6). These compounds are described, their uses and their most important properties are presented. This exercise will allow us to introduce fundamental notions about chemical equilibria (Section 4.2).

Natural gas consists mainly of methane (CH₄) and ethane (CH₃CH₃) and other hydrocarbons in small quantities. Depending on the source it may also contain CO₂, nitrogen (N₂), hydrogen sulfide (H₂S) and helium (He). It is very abundant and its combustion contributes to the increase of the CO₂ content in the biosphere. Natural gas is a non-renewable fuel (Section 5.1). It is widely used to produce electricity, heat, chemicals including hydrogen, and hydrocarbons (liquid fuels) and as a fuel for motor vehicles…