What is the main difference between dna and rna? dna contains ribose sugar, and rna contains deoxyribose sugar instead. dna contains cytosine, and rna contains guanine instead. dna contains deoxyribose sugar, and rna contains ribose sugar instead. dna contains adenine, and rna contains guanine instead.

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.