is it true that the compound calcium magnesium carbonate is present in solid state in shells and sedimentary rocks?

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.

gasoline is a mixture. it contains several different kinds of compounds that can be present in a variety of ratios.