Conservation of Mass: Atoms Rearrange but Are Never Lost

0:00When wood burns, only a little ash is left. Does mass really disappear?

0:06Put two reactants, the substances that react, in a bag: vinegar and baking soda. Then seal it.

0:15Place the bag on a scale. It reads 250 grams.

0:20Tip the vinegar onto the baking soda. They react and make carbon dioxide gas.

0:26The sealed bag is a closed system: nothing can get in or out. The scale still reads 250 grams.

0:35To see why, zoom in on one particle of baking soda and one of acetic acid, the acid in vinegar.

0:43Count the atoms: 3 carbon, 5 hydrogen, 5 oxygen and 1 sodium. That is 14 atoms.

0:52In the reaction, the atoms rearrange into new substances, called products.

0:58The products are water, carbon dioxide and sodium acetate.

1:03Count again: 3 carbon, 5 hydrogen, 5 oxygen and 1 sodium. Still 14 atoms, none made and none lost.

1:14This is the law of conservation of mass: in a chemical reaction, the total mass stays the same.

1:21In the 1700s, Antoine Lavoisier showed this by weighing reactions in sealed containers, like this bag.

1:29Now open the bag. The carbon dioxide escapes, and the reading drops to 246 grams.

1:36Those atoms are not destroyed. They have only moved into the air, off the scale.

1:42That is why burning wood seems to lose mass: its gases go into the air too.

1:48In a chemical reaction, atoms rearrange, but they are never lost, so the total mass stays the same.