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No matter where you are, or what you are doing,
things are changing all around you.
Whether it's plants growing or logs burning,
or even if you're just eating food, the materials
around you are changing in size, shape, and
structure; releasing or absorbing energy,
and creating new substances.
In chemistry, most changes are divided into
two categories: physical changes, and chemical
changes.
A physical change is something that changes
the physical properties of an object or substance
- like size, shape, or form.
Cutting a piece of cloth is an example of
a physical change.
Even though the cloth changes shape and size
when it is cut, it does not create a new substance.
It's still cloth!
Even if the pieces of cloth are sewn together
and made into a shirt, only physical changes
have occurred.
The shirt is still made of the same substance
as the original cloth, just in a new form.
Another type of physical change involves changing
states of matter, and water is a great example
of this.
Ice melts and becomes water, and water evaporates
to become steam, but no matter its form, the
water is still water.
One way we can tell this is a physical change
is that the change can be undone.
Water vapor can condense back into liquid
water, and liquid water can be frozen back
into ice without changing to a different substance.
In fact, many physical changes can be undone.
For example, we know that dissolving sugar
into water is a physical change because even
though the sugar and the water are mixed together,
they do not combine and create a new substance.
They are still just sugar and water, and they
can be separated from each other.
Not all mixtures are as simple to separate
as sugar and water.
When you mix butter, sugar, flour, and eggs,
there is not a practical way to separate them
again, but it is still only a physical change.
The chemical change occurs when you put your
mixture in the oven and add heat.
Baking has many of the indications of a chemical
change.
The mixture in the oven absorbs heat.
It also changes color and produces an odor.
Gases are released, which is what helps the
baked goods to rise.
Finally, this change cannot be undone - you
can't unbake a cake.
Chemical changes occur when the particles
of two or more substances are rearranged to
form a new substance.
For example, when you burn wood, the wood
is transformed into a new substance - ash.
Just like in baking, this change cannot be
undone.
Also like the cookies, the wood changes color,
releases gases - smoke - and produces an odor.
Unlike in baking, this chemical change releases
heat and gives off light.
Chemical changes take place all around us.
Metal rusting, fireworks exploding, fruit
ripening, and digesting food are all examples
of chemical changes!
Here are some clues to look at if you're not
sure whether something is a chemical or physical
change.
Chemical changes always form a new substance.
They may absorb or release heat.
Sometimes a chemical change will cause a change
in color, produce an odor, create a sound,
release gases, or produce light.
Chemical changes usually cannot be undone.
Physical changes do not form a new substance.
A physical change may change the shape or
size of an object.
It may change the state of a substance's matter.
Many physical changes can be undone in some
way.
In a physical change, you end up with the
same substance you started with, even if it
looks different.
I hope you enjoyed learning about physical
and chemical changes today.
Look around and see if you can identify the
physical and chemical changes around you!
Goodbye till next time!
