Name_________________________________________Date_________________Class______________Score_____
Active Chemistry 2.1 – Chemtalk
Introduction
In this investigation you used electricity to decompose
water into two gases. You learned that the gases were
different because they reacted differently to the
burning and glowing splints. Since water is referred to
as H2O, a first guess would be that hydrogen (H) and
oxygen (O) were produced in the experiment. The test
for hydrogen is a small explosion when exposed to a
burning splint. The test for oxygen is the re-ignition of a
glowing splint. If you look back on the results of the
experiment, you find that
the volume of the hydrogen
gas was twice as much as
the oxygen gas. There was
twice as much hydrogen as
oxygen; that’s where the “2”
comes from in the molecular
formula H2O.
_________________________________________
Hydrogen and oxygen are elements. An element is any
material that cannot be broken down into simpler
materials by chemical means. You are probably familiar
with many elements like hydrogen, oxygen, zinc, gold,
or helium. Other elements like strontium and beryllium
are more exotic and less likely to be familiar to you.
Every kind of matter you observe in your everyday life is
made up of the chemical elements. There are only a
little more than a hundred different kinds of chemical
elements. This is an amazing discovery of chemistry –
everything you observe in the world is made of different
combinations of only one hundred elements. Chemistry
is the study of the properties of these elements, how
these elements combine, the new properties of these
combinations, and the energy changes that result from
these combinations.
_________________________________________
Elements are represented by symbols. The periodic
table shows that the symbol used for each element is
one or two letters to represent the name. It’s easier to
write O than to write oxygen. It’s easier to write H than
to write hydrogen. The symbols come from many
different sources. However, the same symbols are used
for each element in all the countries in the world.
_________________________________________
When different elements combine, they form new
substances called compounds. These compounds have
entirely new characteristics unlike the characteristics of
the elements that make them. It is like combining the
letters of the alphabet to make words – twenty-six
letters can be combined to make thousands of different
words. Water is an example of a compound. A water
molecule, H2O, is composed of two atoms of hydrogen
and one atom of oxygen. In this activity, you used
electricity to decompose molecules of water into the
elements hydrogen and oxygen. This process is called
electrolysis. You observed that oxygen gas made a
glowing splint burst into flame, and that hydrogen gas
was explosive. However, to extinguish a burning splint,
you could use liquid water. The compound has very
different characteristics from the elements from which
it is made.
Compounds are represented by chemical formulas. A
chemical formula shows the symbols of the elements
that are combined to make the compound. For
example, as you discovered in this activity, the chemical
formula for water is H2O, because water molecules are
a combination of hydrogen and oxygen atoms. If there
is more than one atom of an element in a molecule, a
subscript is added after the symbol indicating how
many atoms of that element there are. Two hydrogen
atoms combine with one oxygen atom to produce one
molecule of water, which is why there is a little 2 under
the H in H2O. You could call H2O dihydrogen monoxide
but you use the common name water. In chemistry,
there are common names for some familiar chemical
compounds.
Examples of Some Chemical Formulas
Compound
Common Name
Chemical Formula
Calcium carbonate
Carbon dioxide
Hydrochloric acid
Sodium hydrogen
carbonate
Chalk
Dry ice
Muriatic acid
Baking soda
CaCO3
CO2
HCl
NaHCO3
Sodium chloride
Sulfuric acid
Table salt
Battery acid
NaCl
H2SO4
From the table of chemical formulas, you can see that
carbon dioxide (CO2) is a compound of carbon and
oxygen. There are 2 atoms of oxygen for every 1 atom
of carbon. Sodium hydrogen carbonate (NaHCO3) is a
compound of sodium, hydrogen, carbon, and oxygen.
There are three atoms of oxygen for every atom of the
other elements. Also, there are a total of three atoms in
the carbon dioxide formula and a total of six atoms
sodium hydrogen carbonate.
Also, notice that the gases are written in the elements’
diatomic form. Conservation of mass and elements
states that the number of hydrogen atoms must be
equal before and after the reaction. In addition, the
number of oxygen atoms must be equal before and
after the reaction. In the previous equation, you have
one oxygen atom before the reaction and two oxygen
atoms after the equation. You can fix this problem by
“balancing the equation”.
2H2O(l) + energy
2H2(g) + O2(g)
You now have 2 water molecules before the reaction (4
hydrogen atoms and 2 oxygen atoms). You also have 4
hydrogen atoms and 2 oxygen atoms after the reaction.
You balanced the equation because you know that mass
and elements must be conserved. By balancing the
equation, you found that in electrolysis 2 water
molecules decompose into 2 molecules of hydrogen gas
and 1 molecule of oxygen gas. This is why you saw that
there was twice as much hydrogen gas as oxygen gas in
the two test tubes (hydrogen in one tube, oxygen in the
other).
_________________________________________
_________________________________________
Many elements appear as single atoms in nature. There
are seven common elements that exist in diatomic (two
atoms) form. They are: Hydrogen (H2), Oxygen (O2),
Nitrogen (N2), Fluorine (F2), Chlorine (Cl2), Bromine (B2),
and Iodine (I2).
To generate the gas to fill the empty eggshell in this
activity (the demonstration), zinc was placed in
hydrochloric acid. Zinc is an element. Hydrochloric acid
(HCl) is a compound of hydrogen and chlorine. The
reaction of zinc and hydrochloric acid created a gas.
Given the explosion you observed, you can guess that
the gas that was produced was hydrogen. The hydrogen
gas came from the hydrogen in the hydrochloric acid.
You can write this reaction as an equation:
For example, hydrogen may be found as a single atom
in a compound like HCl and oxygen may be found as a
single atom in H2O. When hydrogen is not combined
with any other element, it always appears as H2, two
hydrogen atoms combined together.
_________________________________________
You can write an equation that summarizes the
electrolysis of water:
H2O(l) + energy
H2(g) + O2(g)
Notice that the letters “g” and “l” are used. The “g”
means gaseous state and the “l” means liquid state.
HCl (aq) + Zn (s)
ZnCl2 (aq) + H2 (g)
The sub letter “s” means that the substance is in the
solid state. The sub letters “aq” stands for aqueous,
meaning that the substance (HCl) is dissolved in water.
The zinc combined with the chlorine and hydrogen was
released. Once again, conservation of mass and
elements requires you to balance the equation:
2HCl (aq) + Zn (s)
ZnCl2 (aq) + H2 (g)