IN THIS ARTICLE YOU WILL LEARN ABOUT . . .
Chemical and Physical Properties
Types of Energy
Endothermic and Exothermic Reactions
Chemistry is the study of matter, that is, anything that takes up space and has
mass.Mass (a measure of the number of particles in an object) should not be
confused with weight, which is the inﬂuence of gravity on mass. Because
gravitational forces can differ, an object that has the same mass on Earth as
it does on the moon will have a lower weight on the moon because there is
less of a gravitational pull on the moon. Because matter has mass and takes
up a certain volume (space), the density for any variety of matter can be cal-
culated using the equation: D =m/V. Units typically used to calculate density
are grams (for mass) and milliliters (mL for volume). Sometimes the unit
cubic centimeters (cm3) is used instead of milliliters. These volumes are
equivalent. You will see an exception to the general rule when calculating the
density for gases later in this book.
PROBLEM: What is the density of a solid cube that has a length of 2.0 cm
on each side and weighs 6.0 grams?
Solution: Because the length of each side of this cube is 2.0 cm, the vol-
ume will be the length × width × height. This is (2.0 cm)(2.0 cm)(2.0 cm)
or 8 cm3. The mass is 6.0 grams. Substitute into the equation D =m/V and
you get D = 6.0 grams/8.0 cm3. The density of this solid is 0.75 grams/cm3.
Think about this: If you had 1,000 grams of feathers and 1,000 grams of the
metal lead, which one would you use to ﬁll your pillow? Even with both sam-
ples having the same mass, it is hard to imagine having much fun at a pillow
ﬁght if the pillow were ﬁlled with lead!
A substance can be deﬁned as any variety of matter with identical properties
and composition. Substances are classiﬁed as either elements or compounds.
Elements cannot be broken down chemically, whereas compounds can be
broken down chemically. An element is made up of a particular atom, the
basic building block of matter. Compounds are formed from the bonding of
two or more elements. Consider the reaction: CH4 + 2O2 → 2H2O + CO2. The
reaction shows the elements carbon, hydrogen, and oxygen in different com-
pounds. The chemical equation also shows how the compounds change over
the course of the reaction. Although the compounds on the left of the arrow
are not the same as those to the right of the arrow, the elements in the reac-
tion are still carbon, hydrogen, and oxygen.
Mixtures are the results of the combination of elements and/or com-
pounds. In a mixture:
• The substances are not chemically combined (each substance retains its
• The ratios of substances can vary.
• The substances can be separated into the original elements and/or com-
Mixtures can be classiﬁed as homogeneous (the same throughout) or het-
erogeneous (not the same throughout). An example of a homogeneous mixture
is homogenized milk. You do not have to shake milk before using it because
all samples of homogenized milk will be the same. Solutions are homoge-
neous mixtures of a solute dissolved in a solvent and can be represented by a
substance followed by the symbol (aq) to show that the substance has formed
a homogeneous mixture with water (an aqueous solution). An example of a
heterogeneous mixture is a mixture of sand in water. The sand is sure to set-
tle to the bottom of the container no matter how much you stir the mixture.
PROBLEM: Classify the following as elements, compounds, or mixtures:
salt water, water, argon, methane, and iron.
Solution: Salt water is a homogeneous mixture of water and NaCl. Water
is a compound made up of hydrogen and oxygen. Argon is an element.
Methane is a compound made up of hydrogen and carbon. Iron is an
Think about this: One simple mixture to consider is a salad. You can vary the
amounts/ratios of the vegetables you put in a salad. If you do not like a particu-
lar vegetable, that vegetable can easily be pulled out of the mixture. A salad is a
heterogeneous mixture. Each portion you eat is likely to contain a different com-
bination of vegetables. Finally, the vegetables in a salad retain their properties.
Imagine a carrot and a piece of lettuce combining to form a new substance!
- CHEMICAL AND PHYSICAL PROPERTIES
All substances have physical and chemical properties. Physical properties are
the observable and measurable properties of substances. These include phase (solid, liquid, or gas), color, odor, density, boiling or melting point. Chemical
properties are the properties observed when a substance reacts with other
substances. Chemical changes result in substances with different physical
properties. For example, when iron (a gray, solid metal) reacts with oxygen
gas (an odorless, colorless gas) the result is iron oxide or rust (a solid that is
orange-red in color). You could also note the changes in the density, melting
points, and boiling points of the iron, oxygen, and iron oxide.
PROBLEM: Classify the following as physical or chemical changes: burning
a piece of paper, smashing a piece of chalk, melting an ice cube, and the rust-
ing of an iron nail.
Solution: Burning paper changes the paper’s chemical composition.
Smashing chalk is a physical change; the chalk is still the same chalk,
only in smaller pieces. Melting an ice cube does not change the compo-
sition of the water; it’s a physical change. Rusting an iron nail is a chem-
ical change; the iron is now an iron oxide.
Chemistry is deﬁned as the study of matter, but energy plays an important
role in chemistry. Energy is deﬁned as the ability to do work. Energy is con-
served, that is, it is not created or destroyed. This means that the amount of
energy lost by one system is always equal to the amount of energy gained by
another. Energy can also be converted from one form to another. For exam-
ple, a toaster or a hairdryer converts electrical energy into heat energy.
The units used for measuring amounts of energy are the joule or the calo-
rie. Most people are more familiar with the term calorie. This should not be
a problem because the simple relationship between the two units is that one
calorie is equal to 4.18 joules. This ratio is helpful in setting up problems that
ask for a conversion of one unit to another.
- TYPES OF ENERGY
Energy is found in many forms. As mentioned above, energy can exist as heat
or electricity. Other forms of energy are sound, light, chemical energy, and nu-
clear energy. Probably the two most important forms of energy in chemistry
are potential energy and kinetic energy. Potential energy is stored energy. A good
example is someone holding up a hammer, ready to strike a nail. The hammer
has the potential of falling onto the head of the nail. As the hammer is swung
downward and moves through the air, the potential energy is converted to
kinetic energy, or moving energy. Because all of the energy is conserved, the
potential energy stored in the hammer is turned into kinetic energy. As the
hammer is lifted to strike the nail again, the movement of the hammer upward
becomes the potential energy stored for the next strike on the head of the nail.
One thing to remember in chemistry is that nature prefers a lower energy state.
This rule will be noted again and again in this chemistry review.
- ENDOTHERMIC AND EXOTHERMIC REACTIONS
Energy may also be absorbed or released in a reaction. When more energy is
released than absorbed, the reaction is said to be exothermic.When more energy is absorbed than released, the reaction is said to be endothermic. A potential
energy diagram can be used to graph these changes as in Figure 1.1.
Exothermic—more energy is
released than absorbed.
A + B → C + D + Heat
Endothermic—more energy is
absorbed than released.
A + B + Heat → C + D
Notice that energy is always absorbed and released in a reaction. The relative
amounts are what cause the reaction to be endothermic or exothermic. Also,
it takes energy to start the reaction. This is called the activation energy (Ea).
Finally, take note of the difference in the energy of the reactants and prod-
ucts. The change in the energy of the reactants or products is called the heat
of reaction. This is designated by the symbol ∆H. This symbol stands for the
change in heat energy or enthalpy. The simple way to remember how to cal-
culate the change in enthalpy is to use this mnemonic device:
∆H = potential energy of the products minus the
potential energy of the reactants
∆H = PEP − PER (delta H is equal to “pepper”)
When ∆H has a positive value, the reaction is said to be endothermic (enters
heat). When ∆H has a negative value, the reaction is said to be exothermic
PROBLEM: In a reaction the potential energy of the reactants is 150 joules/
mole and the potential energy of the products is 400 joules/mole. What is the
heat of reaction for this reaction? Does this demonstrate an endothermic or
Solution: Use PEP − PER! ∆H = 400 joules/mole − 150 joules/mole =
+250 joules/mole. Because the sign is positive the reaction is endothermic.
1. Which substance can be decomposed chemically?
2. Which units could be used to express the
amount of energy absorbed or released during a chemical reaction?
(A) Degree and gram
(B) Torr and mmHg
(C) Gram and liter
(D) Calorie and joule
(E) Meter and cm3
3. Which sample represents a homogeneous
(E) None of the above
4. A book is lifted off of the ﬂoor and placed on
a table that is one meter above the ﬂoor. The
(A) gained sound energy
(B) lost chemical energy
(C) gained potential energy
(D) gained kinetic energy
(E) lost nuclear energy
5. Which statement is incorrect regarding
(A) Energy can be given off in a reaction.
(B) Energy can be gained in a reaction.
(C) Energy cannot be created or destroyed.
(D) Energy can take various forms.
(E) Energy has mass and takes up space.
6. What is the mass of an object that has a
density of 13 g/mL and a volume of 10 mL?
(A) 1.3 g/mL
(B) 0.77 g/mL
(C) 1.3 g/L
(D) 130 g
(E) 130 g/L
7. Which sentence below is incorrect?
(A) Salads are heterogeneous mixtures.
(B) NaCl(aq) is a homogeneous mixture.
(C) Milk is a homogeneous mixture.
(D) Sand and water make a heterogeneous
(E) Pure iron is a heterogeneous mixture.
8. Which type of change is different from the
(A) Baking a potato
(B) Rusting of an iron nail
(C) Burning a piece of paper
(D) Melting an ice cube
(E) Ignition of propane
9. Which of the following is not a physical
(D) Boiling point
(E) Reactivity with oxygen
10.Which substance cannot be decomposed
11. The study of matter is called
1. (A) 2. (D) 3. (B)
4. (C) 5. (E) 6. (D)
7. (E) 8. (D) 9. (E)
10. (B) 11. (A)