What describes a process that occurs in definite proportions resulting in new substances with different characteristics?

A process that occurs in definite proportions and results in new substances with different characteristics is best described as a chemical change. In a chemical change, the original substances, known as reactants, undergo a transformation during a chemical reaction to form new substances, referred to as products. This transformation involves breaking and forming chemical bonds, leading to changes in properties such as color, odor, and state of matter.

This process is characterized by the fact that the reactants combine in specific ratios, which are defined by the law of conservation of mass and can be represented by balanced chemical equations. For example, when hydrogen gas reacts with oxygen gas to produce water, a specific ratio of these gases is required. The resulting water has distinct properties that are different from those of the individual gases.

In contrast, physical changes, mechanical changes, and thermal changes do not involve the formation of new substances. Physical changes may alter the state or appearance of a substance without changing its chemical composition, such as melting ice into water. Mechanical changes typically refer to alterations in shape or size due to external forces, and thermal changes generally involve the transfer of heat or changes in temperature, which also do not create new substances. Therefore, the definition provided in the question aligns precisely with the characteristics of a