Chemical changes come in many forms, but we teach that there are certain clear signs that can indicate that a chemical change has taken place. Those signs, while not always absolute proof of a chemical change, provide an excellent way to categorize a few examples of chemical changes. In no particular order, save for the order that we will impose upon this article, those signs of chemical changes are: change in physical characteristics (color, odor, or taste), evolution of a gas, change in temperature, emission of light, and formation of a precipitate.
Some physical characteristics, like size, shape, or state of matter can be changed easily without affecting the identity of a chemical. Other characteristics, like color, odor, or taste tend to be fairly specific - when they change, the odds are good that a chemical change has taken place.
Common color changes that represent chemical reactions include acid-base indicators which change color depending on whether a hydrogen atom is attached or not. Autumn leaves change colors as chlorophyll breaks down (a chemical process). Iron turns reddish as it rusts - also a chemical process. Not all color changes are due to chemical reactions though. For instance, copper (II) sulfate exists as a white powder, until water is added. Depending on the amount of water, either a blue powder, or a blue solution forms. The color change is only a physical one, as water either coordinates with, or dissolves the chemical - both are physical processes.
Odors are often a part of chemistry. Some chemicals have pleasant scents; methyl salicylate has the smell of wintergreen, and perfumes contain mixtures of pleasingly fragrant oils. Other chemicals have harsh, or unpleasant odors. Sulfur compounds in particular are notorious for their stink. Mercaptans (a class of sulfur compounds) account for the foul smell in gas leaks, not to mention skunk spray. Hydrogen sulfide accounts for the smell in rotten eggs. While smell alone is not an indicator of a chemical change, since the chemicals may have smelled in the first place, a change in smell is a good sign. If no one has ever mentioned it to you before, it’s a bad idea to mix chlorine bleach with ammonia-based cleaners. Each has a characteristic odor to start, but the mixture reacts to produce hydrogen chloride, which has a very sharp odor to it, that you probably won’t notice because when the HCl hits your eyes, nose, throat and lungs, it forms hydrochloric acid by combining with the moisture in those areas, and begins to burn quite a bit. Reputedly, chemists who generated acidic cyanide mixtures in a chemical reaction noticed a faint smell of almonds. Since hydrogen cyanide gas is highly toxic, it isn’t a smell most chemists are eager to investigate.
Chemists mostly gave up tasting chemicals a long time ago. A few centuries ago it was common practice, and everyone knew that lead salts are nice and sweet. Since they’re also toxic, tasting chemicals at random is largely considered a no-no now. Of course, the flavor industry has to use tasting, and they’re very aware that each new compound they synthesize is the result of a chemical reaction, and has a unique taste. The average person is also aware of a chemical change involving taste - spoiled milk. The aging of wine also results in a flavor change. In both these cases, biological processes are the source of the chemical change.
Some chemical changes result in the evolution of a gas. This is different from when a liquid is heated to boiling. In this case, a chemical reaction produces a new chemical that is a gas to begin with, rather than changing the state of a chemical that was already present. It isn’t always visibly apparent, but odds are that if heat has not been applied, and the bubbles are forming at a specific spot (like the surface of an electrode, as water is being hydrolyzed, or on the surface of a marble slab that has had acid dripped on it), a chemical change is taking place. Referring back to odors - the generation of a new smell also indicates the presence of a new gas.
Changes in temperature aren’t always proof of a chemical reaction. For instance, dissolution of sodium hydroxide in water is only a physical change, but can generate enough heat to burn. Dissolution of sodium nitrate, on the other hand, will cool the mixture significantly. Ignoring cases involving the enthalpy (heat) of solution however, temperature changes (when heating/cooling was not applied from outside) tend to indicate a chemical reaction. Reactions can be exothermic - giving off heat. Any combustion reaction is exothermic - producing heat at such an extreme rate that it converts the surrounding air to a plasma. Iron rusting is actually exothermic as well, but occurs so slowly under normal conditions that you don’t notice. Speed the reaction up under laboratory conditions though, and you can feel the heat. Fast enough, and you end up simply burning the iron, and learning that “rusting” and combustion are both the same oxidation reaction. Sometimes people only state that the evolution of heat is a sign of a chemical change. That overlooks the fact that some chemical changes are endothermic - they take in heat, lowering the temperature. A favorite demonstration is the reaction between barium hydroxide octahydrate and ammonium nitrate in solution. This reaction becomes cold enough to freeze water on the outside of the flask. (It also produces a new and smelly gas - ammonia.)
Emission of light can be a sign of a chemical change. Understanding that light is produced when electrons are first excited by an input of energy and then relax, releasing that energy as a photon of light, the emission of light is just an indication that energy is being produced by some means. The energy can be provided as heat, as is the case with a flame, or it may come as a direct energy transfer from a chemical reaction, when it is called chemiluminescence. Glow sticks offer an example of chemiluminescence. Not all light is the sign of a chemical change however. Incandescent lightbulbs, for instance, use a current passed through tungsten ( a resistor) to generate heat, which excites the electrons in the tungsten, in turn leading to light production. Since the tungsten stays tungsten, no chemical change has taken place. (At that high temperature, the tungsten does eventually react with oxygen that gets into the lightbulb over time, making the tungsten filament brittle, and eventually causing it to burn out.)
Formation of a precipitate is a sign of a chemical reaction, as long as it isn’t simply the result of a phase change from liquid to solid. These can be some of the most colorful reactions to watch, when two clear (or colored) solutions are mixed, producing a solid precipitate. Lead salt precipitates are especially intense in color. Lead nitrate and sodium iodide (two clear solutions) will react to give a bright yellow precipitate. It’s a dramatic crowd pleaser, and also a good example of why they made lead-based paints for a long time. Unfortunately, brilliant colors and a sweet taste (remember - lead salts are sweet) were too much for small children to ignore, which is why toxic lead-based paints have largely disappeared from use.
Those are just a few basic examples of chemical changes. The world (and the lab) is full of them, and knowing what to look for, you can probably find hundreds in a day. Give it a try!