One of the most easily noted indicators of a chemical reaction is a change in color. When two or more substances are mixed together they may react to form a new substance. Even if the two original substances are clear, their reaction may result in a change in color.
In some instances the chemicals involved in a chemical reaction may need to first be dissolved in water to form solutions. Solid compounds break into components that are free to intermix when solutions are blended together. Sometimes when this happens, solid substances are formed. Such solids are called precipitates and may be large clumps, fine particles or anywhere in between.
Another indicator of a chemical reaction is the production of gas. There may be just a few bubbles or a large amount; in either case, the presence of the gas following the mixing of two or more reactants usually indicates a chemical reaction has occurred. The detonation of a hydrogen bomb is a large-scale reaction that produces massive amounts of gase, while the mixing of baking soda and vinegar to create a "volcano" for a school project produces gas on a smaller scale.
Chemical reactions often involve the transfer of heat. Some reactions release heat making the system feel warm or hot to the touch. This is called an exothermic reaction. Other reactions absorb energy. This sort of reaction would leave the system feeling cold to the touch and is known as an endothermic reaction.