${\text{CO}}_2$  is prepared by the two human actions as

Case1: By burning carbon in presence of ${\text{O}}_2,\text{C}+{\text{O}}_2\to {\text{CO}}_2\uparrow$

Case2: By thermal decomposition of ${\text{CaCO}}_{\text{3}},{\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2\uparrow$

Then from these two cases ${\text{CO}}_2$ is formed by following one of the following laws.

The preparation of elements or compounds from different sources should be followed by law of constant proportions or law of definite proportions. Law of constant proportions: By burning carbon in presence of ${\text{O}}_2,\text{C}+{\text{O}}_2\to {\text{CO}}_2\uparrow$  and  By thermal decomposition of ${\text{CaCO}}_{\text{3}},{\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2\uparrow$ Elements always combine in a fixed ratio of their atomic weights  Here the ratio of Carbon and Oxygen is 3 : 8.