Open or closed: where does the gas go?
Imagine the same gas-producing reaction in two containers. Each complete setup starts at 150 g, including its container. Values are illustrative.
Open system
Matter can enter or leave the system.
Mass on the balance · Escaped gas: 0.0 g
Closed system
Matter cannot enter or leave; energy can.
Mass on the balance · All gas remains inside.
Law of conservation of mass: matter is not created or destroyed in a chemical reaction. Atoms rearrange into new substances. For a closed system, total mass before = total mass after.
In an open system, the measured mass may decrease when gas leaves, or increase when matter enters. Include all transferred matter to account for mass.
Conceptual model, not a lab procedure. Do not seal a gas-producing reaction in a rigid container during a classroom experiment.
What does a chemical formula tell us?
A formula shows which elements are present and their atom counts or simplest ratio. For these molecular examples, it tells us the atoms in one molecule.
Element symbol
One capital letter, sometimes followed by a lowercase letter, identifies an element. H = hydrogen, O = oxygen, Co = cobalt.
Capitalization matters: CO contains carbon and oxygen; Co is cobalt.
Subscript
The small number at the lower right counts atoms of the element just before it. No written subscript means 1.
Coefficient versus subscript
A coefficient is the number in front of a formula. It multiplies every atom count in that formula. An unwritten coefficient means 1.
Hydrogen atoms
Oxygen atoms
Compare: 2H₂O = two water molecules (4 H, 2 O). H₂O₂ = one hydrogen peroxide molecule (2 H, 2 O).
Balance the formation of water
Reactants are on the left of the arrow; products are on the right. The arrow means “react to form.” A plus sign separates substances.
Reactant molecules
Product molecules
Our method: write correct formulas → count atoms → change coefficients → recount → use the smallest whole-number ratio.
Your turn: make the atom counts match
Change only the numbers in the boxes. Use positive whole numbers and aim for the smallest whole-number coefficients.