In some chemical reactions, atoms combine to form new molecules or compounds, while other chemical reactions cause atoms to break apart from each other or trade places with another atom. Because you can’t see this exchange of atoms, you must look for changes in physical properties to determine if a chemical reaction has occurred.
A change in color often indicates a chemical change. For example, the Statue of Liberty is known for its green color, but the statue is covered in a layer of copper, which is a shiny brown metal. This layer of copper has gone through a series of chemical reactions that cause the copper to turn green. First, the reduction-oxidation, or redox, reaction between copper and oxygen forms copper oxide. Copper also reacts with sulfur in the air, forming copper sulfide, which further reacts with carbon dioxide in the air and hydroxide in water to form the layer of patina that gives the statue its green color.
Changes in temperature might also indicate a chemical change has occurred. Exothermic reactions give off heat while endothermic reactions absorb heat. The thermite reaction between iron oxide and aluminum is an exothermic reaction so extreme that it actually causes the iron product to melt. If you mix barium hydroxide octahydrate and dry ammonium chloride in a beaker and place it on a wood block with water on it, you can observe the chemical change because the reaction is so endothermic, it freezes the water on the block.
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The formation of a precipitate is a sign that a chemical change has occurred. A precipitate is an insoluble solid that emerges from a liquid solution. For example, if you mix the clear solutions of silver nitrate and sodium chloride, silver chloride forms as a precipitate. The formation of precipitates is a fairly obvious sign of a chemical change, because the insoluble solid floats or sinks to the bottom in what was previously a clear liquid solution.
Combustion reactions are notorious for giving off light. For example, phosphorus in the presence of oxygen burns spontaneously, producing a flame. Other reactions can give off light without heat. Light sticks work as the result of a chemical reaction between hydrogen peroxide and phenol oxalate ester; when you break the stick, the peroxide mixes with the ester, producing energy in the form of light.
Some chemical changes produce a gas as a product of a reaction. The electrolysis of water, for example, is a decomposition reaction that breaks water into hydrogen and oxygen gas. You can tell this change has occurred when bubbles of gas arise from the electrodes. In the single replacement reaction between zinc metal and sulfuric acid, zinc sulfate and hydrogen gas are formed. You can tell hydrogen gas is present by lighting a splint and placing it inside the test tube after the reaction occurs; the splint will pop because the flame ignites the hydrogen.