Atoms are rearranged, so the total mass cannot change
Definition
Conservation of mass
The principle that the total mass of the products of a reaction equals the total mass of the reactants, because no atoms are created or destroyed.
- A reaction rearranges the atoms that are present, and it neither creates nor destroys any of them.
- Every atom in the reactants ends up somewhere among the products.
- The total mass of the products therefore equals the total mass of the reactants.
- The mass of any one substance does change, because substances are used up and formed.
- A balanced equation is that same statement written in symbols.
Key Idea
- Mass is conserved whenever every substance stays inside the container.
- A change in the balance reading means something has crossed the boundary, and it is almost always a gas.
A closed system: a precipitate is a change of form, not of mass
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed.
- A closed system is sealed, so nothing can enter it or leave it.
- Mixing two solutions in a stoppered flask keeps every product inside.
- In a precipitation reaction, two solutions produce an insoluble solid.
- Lead nitrate solution and potassium iodide solution give a yellow solid: Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \rightarrow \text{PbI}_2(s) + 2\text{KNO}_3(aq)Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)
- The flask looks cloudy afterwards, but the balance reads exactly the same.
- The atoms now in the solid were already present in the two solutions.
Example
- Before: two clear solutions with a combined mass of 50.0 g50.0\ \text{g}50.0 g.
- After: a yellow precipitate suspended in solution, with a mass of 50.0 g50.0\ \text{g}50.0 g.
A reaction that takes in a gas gains measured mass
- A non-enclosed system lets matter cross its boundary, so a gas can enter or escape.
- Magnesium burning in an open crucible takes in oxygen from the air: 2Mg(s)+O2(g)→2MgO(s)2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)2Mg(s)+O2(g)→2MgO(s)
- Those oxygen atoms join the magnesium and stay in the product.
- The magnesium oxide left behind therefore weighs more than the magnesium did.
- Nothing has been created, because the extra mass came out of the air.
Common Mistake
- The extra mass comes from the air, so an answer that does not mention oxygen is unfinished.
- A sealed container changes the observation, because the oxygen supply inside it runs out.
A reaction that gives off a gas loses measured mass
- Calcium carbonate reacts with hydrochloric acid and releases carbon dioxide: CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)
- The gas escapes from the open flask into the room.
- The balance reading falls while the reaction runs, and steadies once it finishes.
- The carbon dioxide still has mass, so nothing has been destroyed.
- Running the same reaction in a sealed flask would show no change in mass at all.
Note
- Naming the gas is the answer, because saying mass was lost or gained does not say where it went.
- Mass is still conserved, once the gas out in the surroundings is counted in.
Explaining a mass change
- Decide first whether the system is closed or non-enclosed.
- For a closed system, state that nothing enters or leaves, so the total mass is unchanged.
- For a non-enclosed system, name the gas and say whether it entered or escaped.
- Link that gas to the direction in which the balance reading moved.
- Finish by stating that mass is still conserved once the surroundings are included.
Self review
- What does the law of conservation of mass state?
- Why does a precipitation reaction in a sealed flask show no change in mass?
- Why does magnesium oxide weigh more than the magnesium it was made from?
- Why does the flask get lighter when calcium carbonate reacts with hydrochloric acid?
- Why is mass still conserved in both of those open-flask reactions?
