Chemical changes are those changes that involve changes in chemical composition of material. In comparison, physical changes, do not involve any changes in the material's composition. An example of chemical change is radioactive decay, where, starting material (such as uranium) decays into products (such as Krypton and Barium).
A knowledge of chemical change is necessary to ensure that we are on the right track for our intended application. For example, nuclear decay is used to generate electricity in a nuclear power plant. Unless we know when the change has occurred (radioactive decay started), we cannot start energy production. In some cases, knowledge of chemical change is necessary to know that we can no longer carry out operation. For example, we may be using a material for its specific properties, say iron pipes. However, formation of rust (which degrades the pipe), is a sign of chemical change and is a sign that we can no longer use the same pipe for the same application (say transmission of water or fluid, etc.).
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