Physical & Chemical Changes

A physical change involves a change in the phases of matter, whereas a chemical change involves a change in chemical bonds. 

Understanding Mixtures

Concept: Homogeneous vs. Heterogeneous Mixtures

2m
Video Transcript

Last time we met, we said that Chemistry is the study of matter. Matter is anything that has mass and takes up space. Basically matter is everything around us: tables, the books that we're reading, TV, our cars, even ourselves. Everything is matter.
Now, we're going to say that most matter consists of basically mixing different things together. We're going to mix pure substances together, different elements mixed together to give us these mixtures. We're going to say that these mixtures can be broken down into two categories. We're going to say blank mixtures have no indistinguishable parts.
This first type of mixture basically means when I mixed them together, I can't tell which part is which. A good example here is if we have red bull and vodka. When we mixed these two things together, they give us this pale yellow solution here. I can't tell which part is red bull, I can't tell which part is vodka. All I know is that it taste okay, that's all I really know.
We're going to say that this type of mixture is known as a homogenous mixture. I can't tell the different parts from one another. Everything mixes well together. We're going to say homogenous mixture mixed together uniformly. We're going to say it’s a uniform mixture. All the parts mixed perfectly together.
The next type of mixture does have distinguishable parts. We've all heard this before, oil and water don't mix. That's because they form a heterogeneous mixture. I can tell just from looking which part is water, which part is oil. This is basically a non-uniform mixture. 

Physical & Chemical Changes

Concept: Physical vs. Chemical Changes

2m
Video Transcript

We also talked about Chemistry not only being the study of matter, but the changes that matter undergoes. We're going to say that this is broken down into two types of category. We're going to have our physical changes and our chemical changes.
Now, in the first blank, we have their changes in the form of the substance, but not its chemical composition. Basically, we're going to say that these are physical changes. What do I mean by not a change in its chemical composition? For example, I have ice—which is solid water and then I just leave it out and it melts. It goes from solid water to liquid water.
At the end of the reaction, it's still water. It began as water. It's still water at the end. I didn't change its chemical makeup. Once I change its chemical makeup or chemical composition, it should stop being water. It should change into something entirely different.
Now the second type of change creates new substances with different properties and different chemical compositions, so we say that this is chemical. For example, I have that liquid water and I super heat it. That little triangle means heat, so I super heat the water, that ice, heat it so much so, then now it becomes H2 gas and O2 gas. It becomes hydrogen gas and oxygen gas.
It started off as water, but now it's no longer water. It's hydrogen gas and oxygen gas which are very different from water. As a result, because they're so different from water, they're going to have different properties, different melting points, different boiling points, different densities. That's what we mean by a chemical change. 

In a physical change there is a change in matter as it goes from one physical state to another. In a chemical change atoms rearrange themselves to create a new substance. 

Example: Which of the following represents a physical change? 

a.  Alkanes burn spontaneously.

b.  The sublimation of CO2.

c.  2 H2 (g) +  O2 (g) ----> 2 H2O (g)

d.  The rusting of a car. 

4m

Physical & Chemical Changes Additional Practice Problems

Which of the following statement(s) is/are  TRUE?

   i. Helium is less dense than air is an example of a physical property.

   ii. The melting of ice is a chemical change.

   iii. The burning of wood is a physical change.

A) i only

B) ii only

C) iii only

D) i and ii

E) ii and iii

Watch Solution

Classify the following as physical or chemical change:

a) Melting a sugar cube   

b) Iron that rusts

c) Burning a sugar cube

d) Dissolving salt in water 

Watch Solution

Circle the corresponding letter to indicate whether the following is a chemical (C) or physical (P) change. 

a) Rusting of a bicycle

b) Vinegar has a pungent smell

c) Evaporation of water                    

 

Watch Solution

Indicate whether the following is a physical or chemical change.

a. Over time, liquid water in an open container evaporates and becomes a vapor in the air
b. Gasoline burns in air when exposed to spark or high heat.
c. After cutting a piece of sodium in air, the surface of the metal changes from a highly shiny surface to a dull gray color

Watch Solution

Which of the following are examples of a chemical change?

a. Copper building materials develop a green patina over time.

b. a match burns

c. ethanol evaporates

d. Both A and B are examples of chemical change.

e. All of the above are examples of chemical change.

Watch Solution

Which of the following is a physical change?

A. Milk sours when left out of the refrigerator
B. Wood is burned on a stove
C. Tums (containing CaCO3) neutralize stomach acid
D. Sugar burns when heated on a skillet
E. Butane in a lighter evaporates

Watch Solution

Which of the following is a chemical change?
A. Sugar dissolves in water
B. Solid CO2 is shattered with a hammer
C. Ice undergoes fusion to form liquid water
D. Sugar burns when heated on skillet
E. Butane in a lighter evaporates

Watch Solution

Which of the following is a chemical change?

a. oil burns on a heating element.

b. water forms dew on a leaf.

c. Ice undergoes fusion to form liquid water.

d. Solid ice is shattered.

e. Propane evaporates from a grill.

 

Watch Solution

A chemical change

A) occurs when methane gas is burned.

B) occurs when paper is shredded.

C) occurs when water is vaporized.

D) occurs when salt is dissolved in water.

E) occurs when kool-aid is stirred into water.

Watch Solution

A physical change

A) occurs when iron rusts.

B) occurs when sugar is heated into caramel.

C) occurs when glucose is converted into energy within your cells.

D) occurs when sand is filtered from salt water.

E) occurs when propane is burned for heat.

Watch Solution

Which of the following are examples of a chemical change?

A) coffee brewing

B) water boiling

C) leaves turning color in the fall

D) salt dissolves in water

E) None of the above are chemical changes.

Watch Solution

Which of the following are examples of a chemical change?

A) Copper building materials develop a green patina over time.

B) a match burns

C) ethanol evaporates

D) Both a and b are examples of chemical change.

E) All of the above are examples of chemical change.

Watch Solution

Which of the following are examples of physical change?

A) dew forms on a blade of grass

B) a halloween light stick glows after shaking

C) an egg solidifies during cooking

D) a hydrogen balloon explodes when contacted with a flame

E) None of the above are physical changes.

Watch Solution

Which of the following are examples of physical change?

A) sugar is dissolved in water.

B) coffee is brewed.

C) dry ice sublimes.

D) ice (solid water) melts.

E) All of these are examples of physical change.

Watch Solution

A chemical change occurs when ________.

A. glucose is converted into energy within cells

B. water vaporizes

C. ethanol evaporates

D. dew forms on a blade of grass

E. salt is dissolved in water 

Watch Solution

Which of the following describes a physical change?

A. Condensation forms on a windshield

B. A light stick glows after shaking

C. An egg solidifies during cooking

D. A firecracker explodes when ignited

E. Leaves turn color in the fall 

Watch Solution

Which of the following is a chemical change?

a. Sugar dissolves in water

b. Solid CO2 is shattered with a hammer

c. Ice undergoes fusion to form liquid water

d. Sugar burns when heated on skillet

e. Butane in a lighter evaporates.

Watch Solution

Each of the following is an example of a chemical change EXCEPT _________.

A. A bicycle rusting

B. Burning toast

C. Chocolate melting

D. Fuel combusting

E. Milk souring

Watch Solution