Observation-measuring or observing some aspect of nature

Hypothesis-tentative explanation of the observations

Laws-summarize the the results of a large number of observations

Theories-models that explain and give underlying causes for observations and laws

Physical and chemical changes do not destroy nor create matter.

The total amount of matter remains constant.

1. Each element is composed of tiny indestrucable parts called atoms.
2. All atoms of a given element have the same mass and other properties that distinguish them from other atoms and elements.
3. Atoms combine in simple, whole-number ratios to form compounds.

Joseph Proust said that elements combine in fixed proportions to form compounds.

All samples of a given compound have the same proportions of their constituent elements.