Conservation of Mass

Open and closed combustion reactions are run; mass is lost in the open system but conserved in the closed reaction.

Equipment

  • 1 L round bottom flask fitted with threaded stopper
  • propane torch
  • ring stand
  • balance
  • birthday candle (supported on a stopper)
  • lighter

Reagents

  • white phosphorus, P4, 0.3 grams

Presentation

  1. For the open system, the balance is tared and the birthday candle is placed on top of it, supported upright by the stopper.
  2. The candle is lit and the mass reading on the balance decreases as it burns.  Mass escapes into the surroundings.
  3. For the closed system, 0.3 g of P4 is placed into the 1 L round bottom flask with air and sealed tightly.
  4. The balance is tared, and an initial reading is made of the flask and its contents.
  5. The flask is moved to the ring stand and the propane torch is used to heat up the P4 to the point that it just begins to react with oxygen in the air inside the flask. The combustion reaction will proceed on its own from this point so the torch is turned off.
  6. Once all of the phosphorus has reacted, the flask is allowed to cool sufficiently and is then placed on the balance to show that its mass is unchanged. 

Details

The birthday candle is simple and makes the point that mass is being lost as the combustion reaction proceeds.  The phosphorus reaction is also a combustion reaction:

P4 + 5 O2 --> P4O10

Some online sources say that some diphosphorus trioxide may also be formed as a less favorable product.  P4O10 is a white fluffy solid that will coat the inside of the flask.  The 0.3 g of P4 that are prescribed in this demo is in slight stoichiometric excess to the amount oxygen that would be present in air, thereby ensuring that all of the oxygen is consumed (although at the end there will be very little P4 left as well).  This reaction decreases the total number of moles of gas in the flask, but is also exothermic, which partially cancel each other.  After the reaction is completed and the contents are cooled, there will be a negative pressure inside the flask.  Be sure to wait until it is properly cooled before putting it onto the balance.

Safety

fire

hot

toxic

Special Safety Notes

  • Like all flames, the birthday candle flame is hot.
  • The propane torch is hot and contains compressed flammable gases and should be treated appropriately.
  • The flask will be hot after the P4 reaction and could burn your hands or damage the balance.  Let it cool before re-massing the system.
  • Be aware during clean up that diphosphorus trioxide (a minor product of this combustion reaction) reacts with water to form toxic phosphine gas.
mass
Birthday candle on a balance -- the open system.
mass2
Round bottom flask on the balance -- the closed system.
mass3
A wider shot showing the torch and ring stand that would be used in the demo.