Jan 6, 2015
This Christmas break, I have been mucking around with science
experiments for my kids. Here is the first of a few easy
experiments you can try at home.
The following video shows you how to make a home-made lava lamp. It
is very simple - grab a cup (or bottle or vase or flask), fill it
about a third full of water and two thirds full of oil. The oil
floats on the water as it has a lower density. Add some food
colouring (you can do this at the start directly to the water, or
after you have added the oil - this has the added benefit of
showing you that the food colouring does not dissolve in the oil,
so it drops through the lower density oil to the water below).
You may already have Alka Seltzer in
your medicine cabinet - it is an over-the-counter pain reliever
containing Citric acid and
Sodium
bicarbonate (also known as baking soda). Split the alka seltzer
tablets into about 4 bits, and add them to make your lamp. The
reaction that is occurring is:
Citric Acid + Sodium bicarbonate → Sodium citrate + Carbon dioxide
+ Water
C6H8O7 + 3NaHCO3 →
Na3C6H5O7 +
3CO2 + 3H2O
Or more simply:
3H++ 3HCO3- →
3CO2 + 3H2O
Note, if you don't have alka seltzer, you can try baking soda, but
add some vinegar to the original water mix, as vinegar is acidic
and provides the H+.
When you add the alka seltzer to the water, the citric acid and
sodium bicarbonate start to dissolve, which allows the reaction to
start and is why they don't react in solid form in the tablet.
The reaction produces gaseous carbon dioxide, which has a lower
density than both water and oil, and hence rises through the
layers. When the gas bubbles exit the water into the oil, they trap
and pull up a small amount of water with them. When the bubbles
reach the surface, they burst and the water falls back through the
oil. It's worth looking at this closely, as you will observe
coloured water droplets that don't have quite enough CO2
stuck to, or dissolved within, them to make it all the way to the
surface, and so they float around, buffeted from side to side by
other, more vigorously moving, droplets.
Here's some more
over at CSIRO.