Which type of reaction results in the formation of new substances?

The formation of new substances is a defining characteristic of chemical reactions, where the bonds between atoms are broken and formed, resulting in different molecular structures. A synthesis reaction, also known as a combination reaction, involves the joining of two or more simple substances (reactants) to form a more complex substance (product). This process not only rearranges the atoms but also creates entirely new chemical products with different properties compared to the reactants.

In contrast, other types of reactions may involve the breaking or rearranging of bonds without producing new substances. For instance, in a decomposition reaction, a compound breaks down into simpler substances; however, this does not imply the formation of new molecular products from different elements. Redox reactions, while they may involve the transfer of electrons and changes in oxidation states, still typically deal with rearrangements of existing compounds rather than the creation of entirely new substances. A phase change involves transitions between different states of matter (solid, liquid, gas), but it does not alter the chemical identity of the substance itself.

Therefore, synthesis reactions are unique in that they produce new substances, differentiating them from the other types mentioned in the options.