GCSE Chemistry (AQA)

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Conservation of Mass

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Quantitative chemistry Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

Conservation of Mass

3:39 Conservation and Changes of Mass in Chemical Reactions
Spec 4.3.1.1
Previous Paper Chromatography Next Changes of Mass in Reactions
  • During a chemical reaction, bonds between atoms in reactants are broken and new bonds form to make products.
  • Example reaction: Two hydrogen molecules react with one oxygen molecule to form two water molecules.
  • Initial bonds between hydrogen and oxygen atoms are broken to form water molecules.
  • New bonds form between each oxygen atom and two hydrogen atoms.
  • Atoms are not created or destroyed during a reaction.
  • Before the reaction: four hydrogen atoms and two oxygen atoms; after the reaction, the same number of atoms remain.
  • The law of conservation of mass states no atoms are lost or made during a chemical reaction.
  • The mass of products equals the mass of reactants; chemical equations must be balanced to obey this law.
  • To balance equations, the number of atoms of each element on both sides must be the same.
  • Example of balancing: Magnesium and oxygen reaction initially unbalanced; adjusted by adding coefficients to ensure equal numbers of each atom on both sides.
  • Visual representation helps to see balanced equations with equal numbers of atoms on both sides.

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