Video Transcript
The table shows the densities of
alkali metals. If the density of kerosene is about
0.80 grams per centimeter cubed, which of the alkali metals cannot be protected
under kerosene?
An alkali metal is any metal in the
leftmost group of the periodic table. So all elements in group one of the
periodic table are alkali metals, except for hydrogen. The alkali metal atoms tend to
react with oxygen in the air and form metal oxide compounds. In the equation shown, the alkali
metal is represented by the letter m. To prevent this oxidation reaction,
alkali metals are often stored in kerosene or paraffin. We need to figure out which of the
alkali metals cannot be protected under kerosene.
In order to do this, we have been
given the densities of alkali metals and kerosene. It is important to remember that
less dense objects float on the surface of a more dense liquid. An example of this is ice floating
on water. For this reason, lithium being less
dense than kerosene would float on its surface. All of the other alkali metals
listed have a greater density than kerosene. Thus, they would be submerged in
the liquid.
If lithium floats on the surface of
kerosene, it is not protected from the air, thus will be oxidized. Generally, paraffin is used to
protect lithium, rather than the kerosene. Therefore, the alkali metal which
cannot be protected under kerosene is lithium.
