Before a chemical compound can form, there must be a chemical reaction. The change that must occur to form a compound is a chemical change. During a chemical reaction, chemical bonds are formed in order to create a new compound. When a new compound is formed, it has a unique set of chemical properties.
Any substance that is created without a chemical change is simply a mixture. A mixture will still share the various properties of its constituents. In fact, you can separate all the components of the mixture using a particular separation technique. On the contrary, when a chemical change occurs, you cannot separate its components using any physical means.
A perfect example of a chemical change is when hydrogen molecules combine with oxygen molecules to give water. While a chemical change automatically creates a chemical bond, this bond can be ionic or covalent.