The universal language of chemistry
A chemical equation化学方程式 uses chemical formulas to describe a reaction. Whatever language they speak, chemists everywhere read the same equation the same way. In a single line it tells you what reacts, what forms, under what conditions, and how much of each substance is involved.
Symbols in an equation
| Symbol | Meaning |
|---|---|
| + | Between reactants: “and” or “reacts with.” Between products: “and” |
| → | “Yields” or “produces.” (Chinese middle-school textbooks often write an equals sign instead, with the same meaning.) |
| Text above the arrow | Reaction conditions, such as heat (Δ), electricity, or a catalyst |
| (s) (l) (g) (aq) | State symbols: solid, liquid, gas, aqueous (dissolved in water). Standard in English textbooks and on the AP exam |
| ↑ ↓ | Gas given off / precipitate formed. Common in Chinese textbooks, rare in English ones |
Three ways to read an equation
Take (relative atomic masses: H = 1, O = 16):
- Substances: hydrogen and oxygen react to form water.
- Particles: every 2 hydrogen molecules react with 1 oxygen molecule to form 2 water molecules. The particle ratio equals the coefficient ratio: 2 : 1 : 2.
- Mass: every 4 parts by mass of hydrogen react with 32 parts of oxygen to form 36 parts of water. The mass ratio is coefficient × formula mass: 4 : 32 : 36 = 1 : 8 : 9.
Notice that 4 + 32 = 36: the total mass of the reactants equals the total mass of the products. That’s the conservation of mass from the last lesson.
Example 1: Burning hydrogen
Use the tool below to see that both the particle ratio and the mass ratio are fixed. Drag the slider to scale up: every mass grows, but the ratio and the equal totals on both sides never change.
| Particle ratio | 2 HHHH | 1 OO | 2 HHOHHO |
|---|---|---|---|
| Relative mass (coefficient × formula mass) | 2 × 2 = 4 | 1 × 32 = 32 | 2 × 18 = 36 |
| Actual mass (g) | 4 | 32 | 36 |
Mass ratio: 4 : 32 : 36 = 1 : 8 : 9
- Read the substances. Hydrogen and oxygen react to form water.
- Read the particles. Every 2 hydrogen molecules react with 1 oxygen molecule to form 2 water molecules.
- Read the masses. Every 4 parts by mass of hydrogen react completely with 32 parts of oxygen to form 36 parts of water. Doubled, that’s 8 g of hydrogen and 64 g of oxygen forming 72 g of water.
Example 2: Burning phosphorus
Relative atomic masses: P = 31, O = 16.
| Particle ratio | 4 PPPP | 5 OOOOOOOOOO | 2 PPOOOOOPPOOOOO |
|---|---|---|---|
| Relative mass (coefficient × formula mass) | 4 × 31 = 124 | 5 × 32 = 160 | 2 × 142 = 284 |
| Actual mass (g) | 124 | 160 | 284 |
Mass ratio: 124 : 160 : 284 = 31 : 40 : 71
- Read the substances. Phosphorus and oxygen react to form diphosphorus pentoxide.
- Read the particles. Every 4 phosphorus atoms react with 5 oxygen molecules to form 2 molecules of diphosphorus pentoxide.
- Find the mass ratio. P: 4 × 31 = 124; : 5 × 32 = 160; : 2 × 142 = 284. The mass ratio is 124 : 160 : 284 = 31 : 40 : 71.
- Check. 124 + 160 = 284, so the totals on both sides match.
Common mistakes
Mistake 1: Reading it like a math problem. Don’t read “+” as “plus” or “→” as “equals.” Read as “hydrogen and oxygen react to form water.”
Mistake 2: Treating the coefficient ratio as the mass ratio. Hydrogen and oxygen have a coefficient ratio of 2 : 1, but their mass ratio is 4 : 32 = 1 : 8. Coefficients give the particle ratio. For the mass ratio, multiply by the formula masses.
Mistake 3: Mixing up the state of a substance. In , water at room temperature is a liquid, so it gets (l), not (g). State symbols describe each substance as it actually is in the reaction.
Quick check
Which is the correct way to read ?
A reads the symbols as math. C should say 2 hydrogen molecules and 1 oxygen molecule. D treats the coefficient ratio as a mass ratio. The mass ratio is 4 : 32 : 36.
In , what is the mass ratio of C, , and ? (C = 12, O = 16)
The mass ratio is 12 : 32 : 44, which simplifies to 3 : 8 : 11. B forgets that the formula mass of is 2 × 16 = 32.
Summary
- A chemical equation describes a reaction with chemical formulas. Read “+” as “and” and “→” as “yields” or “forms.”
- Three ways to read it: which substances react, the particle ratio (= the coefficient ratio), and the mass ratio (= coefficient × formula mass).
- The total mass of the reactants equals the total mass of the products, which is conservation of mass.
On to AP (for later)
Equations on the AP exam always use the state symbols (s), (l), (g), and (aq), and you’ll often translate between equations and particle diagrams (topic 4.3). In Stage 2 you’ll learn about the mole. The coefficient ratio then becomes the mole ratio, which is the core of AP stoichiometry (topic 4.5).