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Course: High school chemistry > Unit 3
Lesson 1: Representing chemical reactionsPhysical and chemical changes
Physical and chemical processes can be classified by the changes occurring on the molecular level. In general, chemical processes involve changes in chemical bonds, while physical processes involve changes only in intermolecular forces. Created by Sal Khan.
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- Can a chemical and physical reaction happen at the same time on something?(7 votes)
- how does chemical change effect physical change(4 votes)
- A physical change isn't always affected by a chemical change or vise versa. However, a chemical change may produce energy that might affect the physical properties of a material. For example, wood burning (chemical) generates heat, which boils water (physical) or a detonated bomb (chemical) may crack glass windows (physical).(2 votes)
- Can a chemical and physical change happen at the same time?(3 votes)
- can a chemical make a physical change ?(1 vote)
- How can I notice the water vapor formed by burning propane?(1 vote)
- A physical change isn't always affected by a chemical change or vise versa. However, a chemical change may produce energy that might affect the physical properties of a material. For example, wood burning (chemical) generates heat, which boils water (physical) or a detonated bomb (chemical) may crack glass windows (physical).(1 vote)
Video transcript
- [Instructor] So what we have
are three different pictures of substances undergoing
some type of change. And what we're gonna
focus on in this video is classifying things as
either being physical changes or chemical changes. And you might already have
already thought about this or seen this in a previous science class, but when we talk about a physical change, we're talking about where
there could be a change in properties but we're
not having a change in the actual composition
of what we're talking about. While in a chemical change, you actually do have a
change in composition, how the different constituent
atoms and elements match up or connect or bond to each
other might be different. So my first question to
you is pause this video, and we have some ice melting here, we have some propane
combusting or burning here, and we have some iron rusting here. And I want you to think about which of these are physical changes and which of these are
chemical changes and why. All right, now let's first
think about this water, this ice melting. And if we wanted to write it
in fancy chemical language or chemistry language, we
could write this as H2O going from its solid form to
H2O going into its liquid form. Now we don't have a change in composition. In either state, whether you're looking
at this liquid water here or whether you're looking
at the solid water there, you'll see a bunch of water molecules. Each oxygen is still
bonded to two hydrogens and so you're not having
a change in composition, and so this over here
is a physical change. And if we kept heating that water up and it started to vaporize, that would also be a physical change. Whereas it turns into water vapor, you have your intermolecular
forces being overcome but the covalent bonds between the oxygens and the hydrogens, those aren't breaking
or forming in some way. So once again, when you
go from ice to water, physical change. From water to vapor, or you
could say from liquid to gas, that is also going to
be a physical change. One general rule of thumb, when you think about what's
going on on a microscopic level, and this is a general rule of
thumb, it doesn't always apply and we'll think about an
edge case in a little bit, is when you're overcoming
intermolecular forces, that tends to be a physical change. But if you have chemical
bonds forming or breaking, that would be a chemical change. Now let's think about what's
going on here with the propane. If you were to write the
chemical reaction here, it would be propane, C3H8 in gas form. It needs oxygen to combust, so for every mole of propane, we have five moles of
molecular oxygen in gas form. And then when it combusts, you're going to produce three, for every one mole of propane and five moles of molecular oxygen, you're gonna produce three
moles of carbon dioxide gas, and four moles of water
in vapor form as well. And so what you actually have is the bonds in those
molecules are actually breaking and then reforming. So you don't just have
physical change going on here, you have chemical change. Chemical change. One way to think about it, you had propane here before, C3H8, after the reaction, you no
longer have the propane here. What you actually see as
fire, which is fascinating, this is just very hot gas. And that very hot air that you're seeing, and there's going to be
some carbon dioxide in there and there's going to be
some water vapor in there, the reason why it's getting so hot is because this releases a lot of energy. Now let's think about what's
going on here with this iron. If I were to write this
as a chemical reaction; for every four moles of iron in solid form plus three moles of
molecular oxygen in gas form, and that would just be the
ambient oxygen around this iron, it is going to produce two
moles of iron oxide as a solid. And that's what you see
there in the orange, that is the iron oxide. So notice this reaction
is forming new ionic bonds in that ferrous oxide. And to undergo the reaction, we had to break the metallic
bonds of the solid iron and the covalent bonds
in the molecular oxygen. So anytime we are breaking and
making these chemical bonds, we have a chemical change.