3 density tricks

Three simple, clear tricks to understand density!

Safe­ty pre­cau­tions



  • shot glass­es;
  • tint­ed hot wa­ter;
  • tint­ed cold wa­ter;
  • col­or­less cold wa­ter;
  • veg­etable oil;
  • soap foam;
  • sheets of pa­per.

Step-by-step in­struc­tions

  • Fill one shot glass with tint­ed hot wa­ter, and a sec­ond with tint­ed cold wa­ter. Cov­er the shot glass with hot wa­ter with a piece of pa­per. Quick­ly in­vert the shot glass and care­ful­ly place it atop the shot glass with cold wa­ter. Re­move the pa­per bar­ri­er. The liq­uids won't mix! If you re­peat this ex­per­i­ment with the cold wa­ter on top, the liq­uids will mix and the so­lu­tion will be­come ho­mo­ge­neous.
  • Try the ex­per­i­ment again with tint­ed wa­ter and veg­etable oil. Cov­er the shot glass full of wa­ter with a piece of pa­per and in­vert it over the shot glass full of oil. Re­move the pa­per and watch as the liq­uids change places: the wa­ter sinks, the oil ris­es.
  • Fill one shot glass with tint­ed wa­ter, and the oth­er with soap foam. Cov­er the shot glass full of wa­ter with a piece of pa­per and in­vert it over the shot glass with soap foam. Re­move the pa­per and ob­serve as they change places: the wa­ter sinks, and the foam ris­es.

Process de­scrip­tion

The unit­ing fac­tor be­tween all three tricks is a dif­fer­ence in den­si­ty. Den­si­ty is a phys­i­cal quan­ti­ty ex­pressed by the ra­tio of body weight to the vol­ume it oc­cu­pies. The first demon­stra­tion in­volves the same liq­uid (wa­ter) at dif­fer­ing tem­per­a­tures. As hot wa­ter is less dense than cold wa­ter, the liq­uids don’t mix even when there is no bar­ri­er be­tween them. How­ev­er, if these liq­uids are in­vert­ed, they mix as the less dense liq­uid ris­es to the top. The sec­ond vari­a­tion on this ex­per­i­ment demon­strates that oil is less dense than wa­ter. The molec­u­lar char­ac­ter­is­tics of oil and wa­ter pre­vent them from mix­ing, so the oil freely pass­es through the lay­er of wa­ter and ris­es to the sur­face. The third vari­a­tion in­volves a process sim­i­lar to that of the sec­ond. Soap foam con­sists of gas bub­bles sur­round­ed by a thin shell of a soap so­lu­tion. The air that is in its com­po­si­tion is less dense than wa­ter, and there­fore wa­ter dis­places it at the bot­tom of the struc­ture.