Conservation of Mass in Chemical Reactions
Chemical reactions create new substances by rearranging atoms. They do not create or destroy atoms, so the total mass remains constant when all matter is included.
1. The law of conservation of mass
A closed system can exchange energy with its surroundings but does not allow matter to enter or leave. During a reaction, bonds change and atoms form new groupings. Because the number and identity of each type of atom are conserved, the combined mass of all products equals the combined mass of all reactants.
If a reaction releases a gas from an open container, the measured mass of the material remaining in the container may decrease. If a solid combines with oxygen from air, its measured mass may increase. Neither observation breaks the law: matter crossed the chosen system boundary.
2. What a chemical equation shows
A chemical equation is a symbolic statement of a reaction. Reactants are written on the left, an arrow means “produce” or “form,” and products are written on the right. A plus sign separates different substances.
\(2H_2 + O_2 \rightarrow 2H_2O\)
reactants ( ightarrow) products
Optional state symbols give physical states: (s) solid, (l) liquid, (g) gas, and (aq) dissolved in water. Conditions such as heating may be written above the arrow; they are not reactants or products.
3. Coefficients and subscripts have different jobs
Coefficient
A number before a formula multiplies the entire formula. In (2H_2O), the coefficient 2 gives four H atoms and two O atoms in total. Changing a coefficient changes the number of particles, not their chemical identity.