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Chemical laws are thoselaws of nature relevant tochemistry. The most fundamental concept in chemistry is thelaw of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinarychemical reaction. Modern physics shows that it is actuallyenergy that is conserved, and that energy and mass arerelated; a concept which becomes important innuclear chemistry.Conservation of energy leads to the important concepts ofequilibrium,thermodynamics, andkinetics.

The laws ofstoichiometry, that is, the gravimetric proportions by which chemical elements participate in chemical reactions, elaborate on the law of conservation of mass.Joseph Proust'slaw of definite composition says that pure chemicals are composed of elements in a definite formulation.^[1]

Dalton'slaw of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction.^[2] Such compounds are known asnon-stoichiometric compounds.

The third stoichiometric law is thelaw of reciprocal proportions, which provides the basis for establishingequivalent weights for each chemical element. Elemental equivalent weights can then be used to deriveatomic weights for each element.

More modern laws of chemistry define the relationship between energy and transformations.

• In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule.
• Transforming one structure to another requires the input of energy to cross anenergy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
• There is atransition state (TS), that corresponds to the structure at the top of the energy barrier. TheHammond–Leffler postulate states that this state looks most similar to the product or starting material which has intrinsic energy closest to that of the energy barrier. Stabilizing this transition state through chemical interaction is one way to achievecatalysis.
• All chemical processes are reversible (law ofmicroscopic reversibility) although some processes have such an energy bias, they are essentially irreversible.

• History of chemistry

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