Track quiz
Here are the eight rules. Use the table if you get stuck.
| Idea | The rule |
|---|---|
| Nothing is lost | In a closed system, mass before a change equals mass after. A chemical change only rearranges matter. Matter is neither created nor destroyed in a chemical change. |
| Always the same proportion | A compound's elements are always in the same mass ratio. This holds however much you make. Extra of one element is left over, unused. |
| Why those two hold | Matter is made of tiny atoms. A chemical change only rearranges atoms. Each compound is built from the same atoms in the same count. So its ratio stays fixed. |
| Weighing something you cannot see | By agreement, one carbon-12 atom is fixed at 12 u. Every other atom's mass is compared to that one. This comparison is the atom's mass, in u. |
| Molecular mass | Add each atom's mass, from the formula's counts. For a molecule, that total is its molecular mass. For ions in a ratio, it is one formula unit's mass. |
| When it is not molecules | Some substances are ions, not molecules. Ions pack in one repeating ratio. Add the ratio's masses for one formula unit. A metal joined to a non-metal usually gives ions. |
| Valency | Each element here has one usual combining number. This number is called its valency. Two elements' valencies decide a formula's counts. |
| Writing the formula | Swap the two valencies for counts. Cut the counts down by their shared factor. This gives the simplest formula. Write the symbols, and leave out a count of one. |
A fizzing change runs in a corked flask on a balance. The reading holds. Why?
Reactants weigh 9 g + 3 g. Products are 5.2 g, 4.1 g and one more. It is a closed system. How much is the last product?
27 g of pure water is split. Water is 1 part hydrogen to 8 parts oxygen. How much hydrogen comes out?
Atoms only rearrange in a chemical change. Why does the total mass stay the same?
Hydrogen is 1 u and oxygen is 16 u. What does that say about one oxygen atom?
Ammonia is NH₃. What is its molecular mass?
Calcium chloride is CaCl₂. Its 111 u is the mass of what?
Sodium has valency 1 and oxygen has valency 2. How many sodium atoms join one oxygen atom?
This is Chapter 9 — Atomic Foundations of Matter. Read it with a pencil, and work every sum yourself.
After this comes Sound Waves: Characteristics and Applications. You know how matter carries a count. Next, a push travels through it.