Physical and chemical changes
Physical Properties & Changes
Characteristics like colour and mass are called physical properties, which we can define as
characteristics that can be seen and measured. We can identify physical properties for all
types of matter, including all the ingredients and tools we use to bake a cake. For instance,
a wooden stirring spoon has mass which we can measure. Because mass can be measured, we
know that it's a physical property.
Changes that affect physical properties are called physical changes. With a physical
change we see that a substance changes form but does not transform into a different
substance. For example, to use an egg in our recipe, we need to crack it open. This is a
physical change because the egg is now in two parts, but the egg did not transform into a
chicken, or into any other substance for that matter; it is still an egg. If we were to take
our wooden spoon and grind it down to sawdust, we could see another example of physical
change. The spoon would change form, but it would still be wood.
Reversibility of Physical Changes
Sometimes physical changes can be reversed, and sometimes they cannot. We just looked at
two examples of physical changes that could not be reversed, namely the cracked egg and
the ground-up wooden spoon. We learned from the Humpty Dumpty nursery rhyme that you
cannot put a broken egg back together again. The same goes for the wooden spoon; it would
be impossible to change the sawdust back into a wooden spoon.
But, don't get fooled into thinking that all physical changes are irreversible. In fact, many
physical changes can be reversed. Remember that when a substance changes physically, it is
still the same substance, just in a different form. Water turning into ice is a great example
of a reversible physical change. Water, which we know is made up of two hydrogen
molecules and one oxygen molecule (H2O), can be frozen, but that doesn't change the fact
that it is still H2O. If you let it thaw, you could drink it again.
Chemical Properties & Changes
Let's keep baking so we can learn about chemical properties, which are not as easy to see
as physical properties. Chemical properties are characteristics of a substance that can only
be observed during a reaction. For example, our wooden spoon will burn, but we can only
know that for sure if we light it on fire and watch it burn. Once the wood burns, it
transforms into ash. So, we see that with chemical changes, a substance transforms into a
different substance. In other words, the identity of the object that undergoes a chemical
