This page covers some of the most common mistakes
and misconceptions in the matter and energy unit.
Chemical and Physical Change
Physical changes do not create new substances, chemical changes do.
Students misinterpret this because they see things like making lemondade as making a "new substance"
Chemically however, this is not true. Lemonade is just water molecules, lemon juice molecules, and sugar
molecules mixing around together. The parts are too small to see, but they are still separate in the same way
the fruit in a fruit salad stays separate when mixed.
To be a chemical change molecules must break apart and reform in a new way. This is what is meant by a
"new substance" being created.
To show this in class we will make jello and pour small amounts in a petri dish. When the water evaporates
we are left with a solid film we could grate back into jello powder. The water stays water and the jello stays
jello. We could evaporate the water out to prove this. You can show this easily with salt water or sugar
water as well.
Here are some key ideas that may help you to identfy physical changes:
1. breaking, chopping, shredding or any other synonym for tearing apart something (easy-students usually
get these)
2. Phase changes-any time a substance is going from solid to liquid to gas or back is a physical change.
This is also called a change in the state of matter. (harder- many students start making mistakes here)
3. Melting and dissolving- Anything that is melting or dissolving is going through a physical change.
(tough-separates the men from the boys- dissloving is especially hard because a new substance appears to
form, but it is not chemically a new substance!)
Here are key ideas that may help you identify chemical reactions (chemical change):
1. Fire is a chemical change. If something is burning it is a chemical change.
2. Baking- baking almost always invovles baking soda and/or eggs.
Bakind soda combines with other ingrediaents and releases carbon dioxide gas. That is what makes the air
bubbles that make baked goods light and fluffy. Baking soda molecules break down and combine with
other molecules to make the carbon dioxide.
Proteins- like eggs- become solid when heated. If you think about i,t this is not a phase change. Things are
supposed to melt when heated, not turn solid. Proteins break apart and recombine in a new way. That
causes them to turn solid. This is a chemical change.
3. Color change- this can be tricky- just painting something is not a chemical change. Mixing red and
yellow paint to make orange is not a chemical change. When you put rust red iodine on a starchy potato a
chemical change occurs and the color turns dark bluish purple. That kind of color change tells us that a
chemical change has occurred.
4. Bubbles- this is confusing some students- just mixing air into something by beating it really fast or the
bubbles from boiling liquid are not a chemical change. If I pour two substances together and get bubbles,
then that is likely a chemical change. Hydrogen peroxide is made of two hydrogen atoms and two oxygen
atoms in each molecule. When you pour it on a cut, it breaks down into water (two hydrogen and one
oxygen) molecules and oxygen gas. It is the releasing of the oxygen that causes the bubbles and kills the
germs.
Open and Closed Systems
Students need to understand that the whole equals the sum of its parts.
In a physical change no mass is lost regardless of how it is changed as long as all the parts are retained.
A cookie = the mass of its crumbs
Water = the mass of the water turned to ice= the mass of the water boiled to water vapor
Lemonade = the mass of the water + the mass of the sugar + the mass of the lemon juice
In a chemical reaction the mass will change if it is in an open system. This is because we are adding
something to or taking something away during the reaction.
The vinegar and baking soda in an open bottle lost mass because the carbon dioxide gas that was created
was lost to the air.
Iron nails left to rust will gain mass because the iron combines with oxygen from the air to make rust. This
oxygen was not included in the mass when the unrusted nails were massed (weighed), but if you remass
them after the rusting has occurred they will have gained mass from the extra oxygen molecules.
So how can we say then that the sum of the whole always equals the sum of the parts?
In a closed system we weigh everything that will possible combine with the items and contain any gases
that may be lost. A closed system just means we contain the experiment in something, like a jar.
In class we put a balloon over the bottle we were using to combine the vinegar and baking soda. That way
when we took the mass again the carbon dioxide gas would still be massed.
If you put nails in a damp jar and then take the mass of the whole thing then the mass will not change after
they rust because you already massed the oxygen in the jar before the nails rusted.
Water Cycle
Most students do well on this, but they have to know the definition of each term! Here are some areas that
are more challenging.
Ground water is water under the ground not on it.
Sleet vs. freezing rain
Sleet freezes on the way down- freezing rain is just rain. It freezes on a surfaces after it hits the ground.
Ways to say sleet
freezes as it falls
freezes on the way down
falls through a colder layer of air and freezes
freezes while falling
starts as liquid and freezes before hitting the ground
frozen raindrop
Ways to say freezing rain
freezes on contact
is liquid in the air freezes on the ground
freezes once it hits something
encases tree branches, cars, etc. in a coat of ice
Sleet is not very damaging. Since it freezes in the air it creates a very small ice pelt that just pings off
surfaces it hits. Freezing rain is a liquid when it hits, but freezes to surfaces coating them in layers of ice
that can bring down trees, limbs, and power lines.
Since freezing rain is not falling as anything but rain it is not a separate type of precipitation. We say there
are four types of precipitation: rain, snow sleet, and hail. Freezing rain is just a special type of rain.
Hail
I told the students hail is so much more complicated than the other forms of precipitation. You can almost
identify it from the length of the description! Key points are:
develops in cumulonimbus storm clouds
these clouds are extremely tall
lower part of the cloud is above freezing and the top is below freezing
strong winds called updrafts push raindrops up in to the freezing part of the cloud where they turn to ice
(hail stones)
small hail stones fall to the lower part of the cloud and get wet with a layer of water
hail stones are blown back up and freeze the layer of water
this continues adding layers of ice like an onion
when the hail stone becomes too heavy for the wind to lift it falls to the ground
the stronger the winds in the cloud, the larger the hails stones