The fermentation of milk is considered a chemical alteration since it results in the creation of lactic acid, which has a sour flavour. There is a strong relationship between physical and chemical changes and physical and chemical attributes.
A chemical transformation occurs on the molecular level. To qualify as chemical, a change must involve the formation of a new substance and the absorption or emission of energy. States of matter and energy are used to describe a physical transition. The molecules are rearranged without affecting their intrinsic structure.
Examples of common chemical changes include the corrosion of iron (iron oxide is produced), the combustion of gasoline (carbon dioxide and water are produced), and the rising of bread (carbohydrates are converted into carbon dioxide). Any change in phase or condition (sublimation, condensation, vaporisation, freezing, and melting), breaking glass, melting ice, crushing a can, and dissolving sugar in water are examples of physical changes. Changes in texture, colour, and shape may, in general, constitute a physical change.
After a chemical transformation, a new material is generally accompanied with evidence, such as sound, odour, gas production, heat absorption or production, colour change, or light. Despite the fact that the materials at the beginning and finish of a physical transformation may appear different, their molecular structure is similar.