Two centuries ago, scientists weighed substances before and after a reaction, and they found rules that never fail. Here we learn those two laws, and the idea of the atom that Dalton built from them. We also learn how heavy an atom is, in a unit made for it.
3 lessons, about 1 h 15 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabIn a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants is equal to the total mass of the products.
In a pure compound the elements are always present in the same ratio by mass, whatever the source of the compound or the way that it was made. This is the law of constant proportions, found by Joseph Proust. Water is 1 part hydrogen to 8 parts oxygen by mass. Ammonia is 14 parts nitrogen to 3 parts hydrogen, carbon dioxide 3 parts carbon to 8 parts oxygen, and magnesium oxide 3 parts magnesium to 2 parts oxygen. The law tells a compound from a mixture, and it can be used to find the mass of each element in a given mass of a compound.
Dalton said that matter is made of atoms that are not made or destroyed in a reaction, and that combine in fixed whole number ratios. An atom is extremely small. Its mass is measured in atomic mass units, u.
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