Same atoms, new partners

In the picture

1. 1Balance pointer: the mass stays level before and after the reaction
2. 2Sealed flask: nothing enters or escapes, so all the mass is still counted
3. 3Starting molecule: the reactants, made of atoms joined in a certain way
4. 4Arrow: the atoms break apart and regroup during the reaction
5. 5Product molecule: a new substance built from the very same atoms

Examples

• A burning candle in a sealed jar weighs the same before and after burning.
• An effervescent tablet dissolving in a closed bottle keeps the same total mass.

Quiz: Same atoms, new partners (Age 13)

1. 1

   A chemical reaction happens inside a sealed flask. How does the total mass of the contents compare before and after?

   • It gets smaller because the starting substances disappear
   • It gets bigger because new substances form
   • It stays the same
2. 2

   What happens to the atoms of the starting substances during a chemical reaction?

   • They are destroyed and new atoms are created for the products
   • They break apart and regroup with new partners
   • They stay joined exactly as before and the substance does not change
3. 3

   Why is the total mass of the products equal to the total mass of the reactants?

   • Because the reactants only change shape but stay the same substance
   • Because the products are always lighter atoms that balance the heavier ones
   • Because the products are made of the very same atoms, just rearranged

The answers, each with its explanation, are in the app. This quiz is free there while the lesson is in the free week.