5. When NaCl dissolves in water, which of the following is true? a. the sodium ions are surrounded by positively charged ends of the water molecules, which are the oxygen ends

b. the sodium ions are surrounded by positively charged ends of the water molecules, which are the hydrogen ends

c. the sodium ions are surrounded by negatively charged ends of the water molecules, which are the oxygen ends

d. the sodium ions are surrounded by negatively charged ends of the water molecules, which are the hydrogen ends

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.