The souring of milk is classified as a chemical change because it results in the production of sour-tasting lactic acid. Both physical and chemical changes closely relate to physical and chemical properties.
A chemical change occurs at a molecular level. For a change to qualify as chemical a new substance has to be formed and energy is either absorbed or given off. A physical change is characterized with states of matter and energy. The molecules are rearranged but the internal structure is not affected in any way.
Common examples of chemical changes include: the rusting of iron (iron oxide is formed), burning gasoline (carbon dioxide and water is formed) and rising of bread (carbohydrates are converted into carbon dioxide). Examples of physical changes include: any change in phase or state (sublimation, condensation, vaporization, freezing and melting), breaking glass, melting ice, crushing a can and dissolving sugar in water. In general, a change in texture, color and shape may constitute a physical change.
There is typically some form of evidence a new substance was created after a chemical change has taken place, such as sound, odor, gas production, heat absorption or production, color change or light. Despite the fact that the end and start materials of a physical change may appear different, their base molecular composition is similar.