Phy vs Chem depth

Photo by Steve Jurvetson

A pure substance is something that has
consistent and definable physical and
chemical properties including melting
point, boiling point, density, color, and
most importantly: chemical formula.
What are the 2 types of substances?

If the substance
is an element,
the smallest
particle is an

ATOM.

Sulfur

Sulfur Atom

Sodium Chloride

Formula Unit

Covalent compounds are

made of MOLECULES.

Water

Water Molecules

Ionic compounds are made of

SALTS which form crystals.

Sodium Chloride

Salt Crystal

There are 7 elements that exist in nature as

diatomic molecules.
di – means two

Diatomic means 2 atoms.

Every substance has a definite ratio of
elements. Understanding this is crucial
to much of chemistry and especially to
telling the difference between physical

and chemical changes.

CO2 = __C : __O

1

2

H2O = __H : __O

2

1

NH3 = __N : __H

1

3

Determine the ratio of

elements in each substance

2

3

1

2

1

2

1

1
1

1

1

3

1

4

1

1

1

1

1

1

3

1

1

3

Physical Properties

Physical properties like size, shape or
temperature can be changed without
changing the identity of the
substance.

Changing a physical property like the

size, shape or temperature of a

substance is known as a
Physical Change

Multiple Choice

Select the none physical characteristic

• Size

• Shape

• Temperature

• Reaction type

Understanding Physical Changes:

Changing Size and Shape

There are many different words that mean a change
to size and shape. It is important that you
understand what these words mean.

Stretch

Compress

Shatter

Tear

Break

Sugarr

Dissolution

Dissolution is just a physical change. When a substance like
sugar dissolves, it gets broken apart by the water into
individual molecules that are too small to see, but it is still
just a change in size and shape.

Understanding Physical Changes:

Changing Temperature

Both of these sample are water. Their temperature is
determined by their surroundings, not their chemical
formula. Changing temperature is a physical change.

Multiple Choice

A change in temperate is a physical change when...

• It only involves one type of substance

• It involves two or more substances

Understanding Physical Changes:

Changing Temperature

When the temperature changes enough, substances will
change their phase because heating matter causes it to
expand and cooling matter causes it to contract. This is a
direct result of changing their temperature and is does not
change the identity of a substance.

It is important that you know the vocabulary

that describes each phase change.

When solids become
liquids, we say that they
have ________.

When liquids ______,
they have become solids.

melted

freeze

Multiple Choice

A phase change is a chemical reaction

• True

• False

When liquids become
gases, we say that they
have ___________.

When gases become
liquids, we say that they
have_________.

evaporated

condensed

Multiple Choice

When a liquid becomes a gas

• Evaporate

• Condensate

• melt

• freeze

When solids become
gases, we say that
they have _________.

When gases become solids, we say that
they have __________.

sublimed

deposited

Photo by Steve Jurvetson

Multiple Choice

When solids become gas

• Sublime

• deposited

• freeze

• evaporate

When substances are placed together, they
may or may not react with one another. If
substances do not react when they are placed
together, then they create a ________.

mixture

Sulfur and Iron

can be

separated based on their

.

Iron being removed with a magnet.

When substances are placed together and
they exchange or share electrons, then they
have undergone a ________________.

chemical change

This will cause the elements to be joined
together in new ratios.

The elements have rearranged!

Chemical properties allow us to predict the reactions
a substance will undergo. However, once a reaction
has happened, there are several signs that a chemical
change has taken place.

Change in Color

A precipitate forms

Signs of a chemical change:

A precipitate is a solid that forms when two
aqueous solutions are placed together.

Signs of a chemical change:

It may appear that the
color of the solution is
changing. But if you
allow the precipitate
time to settle to the
bottom, it is easy to see
that it is a solid and the
solution is still clear.

Precipitate settled at
the bottom of the
test tube.

Bubbles

Signs of a chemical change:

If bubbling occurs when substances are
placed together, then it is likely a chemical
change has occurred.

Light and/or Heat are given off

Signs of a chemical change:

Photo by Dinshaw Dadachanji

While there are many signs that a
chemical change has occurred, there is
only one sure way to tell:

This means that an element or compound

is formed with a different formula than

any of the original reactants.

Some things you should simply recognize as
chemical changes:

Burning/

Combustion

Acid Rain
Damage

Rusting/
Corrosion

Photosynthesis

Food

Spoilage

Corrosion involves things
like oxygen and acid rain
disintegrating materials
and changing their
chemical composition.

Corrosion actually forms
new substances, so it is a

Chemical Change

(s) = _________________

(l) = _________________

(g) = _________________

(aq) = _________________

Important Facts:

solid
gas

liquid
aqueous

Aqueous means that it has been dissolved in WATER.

Determine of the changes on your

notes are physical changes and which

are chemical changes based on the
ratio of elements in each substance

before and after the change.