Mass seems to change only because a gas is not being weighed
Balanced symbol equation
A balanced symbol equation has the same number of atoms of each element on both sides of the equation.
- Mass is always conserved in a chemical reaction, so any apparent change in mass has a physical explanation.
- In a non-enclosed system, a gas can move into or out of the reaction container.
- The balance only weighs what is in the container, so a gas entering or escaping makes the reading appear to change.
The atoms are never lost; the balance reading changes only because a gas that carries mass is not sitting on the balance.
Mass increases when a gas is one of the reactants
Reactant
A reactant is a substance that is changed or used up during a chemical reaction.
- When a gas from the air is a reactant, its atoms become part of the solid product.
- The solid product therefore weighs more than the solid you started with.
- The extra mass gained is exactly the mass of the gas that reacted.
When magnesium burns, 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO, the solid gains mass because oxygen from the air is now locked into the magnesium oxide.
Mass decreases when a gas is one of the products
Product
A product is a substance formed during a chemical reaction.
- When a gas is a product and escapes into the air, the substances left behind weigh less.
- The mass lost is exactly the mass of the gas that escaped.
Heating calcium carbonate, CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3→CaO+CO2, leaves a lighter solid because the carbon dioxide gas drifts away and is no longer weighed.
A sealed container keeps the total mass constant
- In a sealed (closed) system, no gas can enter or escape.
- The balance reading then stays the same, which shows the mass really is conserved.
- To explain an apparent mass change, name the gas involved and say whether it entered from the air or escaped into it.
- Never write that atoms are made or destroyed, as that contradicts conservation of mass and scores zero.
Reading the balance tells you which way the gas moved
- A rise in mass means a gas has been taken in from the air and added to a solid.
- A fall in mass means a gas has been given off and has left the open container.
- Why can mass appear to change in a non-enclosed system?
- Explain why a piece of magnesium gains mass when it burns in air.
- Explain why heating a metal carbonate leaves a lighter solid.
- What would the balance show if the same reaction happened in a sealed flask?
