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Consider a different event. Let us say that you take a matchstick and light it. The matchstick turns into different substances: burnt wood, smoke, and gas.

Remember that the matchstick, wood, smoke, gas, etc are all matter. We can calculate the total mass of this matter before the burning and after the burning.
Without actually performing the experiment, can you say whether the total mass before the burning is equal to the total mass after the burning? (That is, if the mass is conserved in this case.)
  • Yes
  • No