Dalton's Atomic Theory of Matter was developed in an attempt to explain the Law of Conservation of Mass, the Law of Definite Proportions, and the Law of Multiple Proportions. Remember that a law is a brief statement summarizing observations, and a theory attempts to explain the why behind what we observe. According to the Law of Conservation of Mass, in a chemical reaction, matter is neither created nor destroyed. This is not an intuitive statement. If you've ever sat around a campfire, you know that before you light the fire, you have a pile of wood, and afterwards, all you can see that's left is a much smaller pile of ashes. So, in order to validate this law, very carefully controlled experiments had to show that mass is conserved even in combustion reactions. We can apply the law of conservation of mass to the reaction between sodium metal and chlorine gas. If exactly 7.7 g of sodium metal reacts with 11.9 g of chlorine gas to produce sodium chloride, or table salt, then according to the conservation of mass, exactly 19.6 g of sodium chloride would be produced. That is, the sum of the masses of the reactants equals the sum of the masses of the products. Our next law applies to different samples of the same compound. According to the Law of Definite Proportions, all samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements. So, if we're talking about pure water, no matter where we obtained this sample of water and no matter how big the sample is-- it could be 1 milliliter or 1000 gallons-- the sample will have an 8 to 1 ratio of oxygen to hydrogen by mass. Our third law applies to samples of two different compounds made up of the same elements. According to the Law of Multiple Proportions, when two elements, let's say A and B, form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers. So, for example, let's say I have a sample of the compound carbon dioxide, which is CO2, and a sample of carbon monoxide, CO. If my sample of carbon dioxide was just big enough to have 1 g of carbon in it, the sample would have 2.67 g of oxygen in it. For my sample of carbon monoxide, if it was just big enough to have 1 g of carbon, the sample would have 1.33 g of oxygen in it. So, if I take the ratio of my mass of oxygen in carbon dioxide to my mass of oxygen in carbon monoxide, it should be a small whole number. And it is. My ratio is 2. That means I have twice as much oxygen in my sample of CO2 compared to carbon monoxide, CO. Next, we'll look at Dalton's Atomic Theory, which attempts to explain these observations.