In a chemical reaction atoms are not created or destroyed; they are only rearranged. So the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass.
Sometimes the mass on a balance seems to change. If a gas escapes from an open flask, the mass goes down. If a metal reacts with oxygen from the air, as when magnesium burns, the solid gets heavier because oxygen atoms have joined it. In a sealed container the mass stays exactly the same. Balanced symbol equations show the same number of each type of atom on both sides.
Worked example
12 g of magnesium burns to make 20 g of magnesium oxide. What mass of oxygen reacted?
- Mass of reactants = mass of products.
- 12 g + oxygen = 20 g.
Answer:Oxygen = 20 − 12 = 8 g.
Key words
- conservation of mass
- total mass stays the same in a reaction
- closed system
- nothing can get in or out
- atom
- the smallest particle of an element
- balanced equation
- same number of each atom on both sides
Practise this
New questions every time.
NGSS-referenced: MS-PS1-5