Conservation of Mass



Two flashbulbs are shown to weigh the same on a pan balance. One of the bulbs is ignited using a 9V battery. After ignition, the spent bulb is placed back on the balance along side the other bulb. It is seen that they still weigh the same. 

Curriculum Notes 

This demo would be appropriate at the beginning of a general chemistry course or anytime when the law of conservation of mass is discussed.

One day of lead time is required for this project.


This demo readily illustrates the conservation of mass after a chemical reaction, in this case, combustion within a flashbulb. The magnesium wire within the flashbulb is ignited via an electric current, forming magnesium oxide, a white powder.
$\ce{2Mg + O2 -> 2MgO}$2


  • 2 flashbulbs
  • pan balance
  • 9V battery
  • heat resistant glove


Remove plastic pans from pan balance so they don't melt. Place 2 flashbulbs on the balance and zero the balance. Pick up one of the bulbs with a heat resistant glove and touch the tip of the bulb to one of the terminals on the battery and touch the sleeve of the bulb to the other terminal to complete the circuit and cause ignition. The bulb will flash brightly. Place the bulb back on the pan balance to demonstrate that mass was conserved. 

Safety Precautions 

Bulb ignition causes the bulb to be very hot, possibly causing burns if heat resistant glove isn't used.