2. Chemistry
2 Two Types of Substances
• Of the known elements, only about 90 occur
naturally on Earth. The remainder are
synthesized, usually in barely detectable
amounts, in high-energy nuclear experiments.
• Less than half of the 90 naturally occurring
elements are abundant enough to play a
significant role in the chemistry of everyday
stuff.
3. Chemistry
2 Organizing the Elements
• The periodic table organizes elements in a
way that provides a wealth of chemical
information—much more than is evident to
you now. It shows the chemical symbols for
the elements.
• Their symbols usually correspond to their
names in Latin.
5. Chemistry
2 Compounds Are More Than One Element
• A more complete definition is that a
compound is a chemical combination of two
or more different elements joined together in a
fixed proportion with a unique set of chemical
and physical properties.
• More than 10 million compounds are known
and the number keeps growing.
6. Chemistry
2 Compounds Are More Than One Element
• The properties of the compound are different
from the properties of the elements that
compose the compound.
silver + bromine = silver bromide
7. Chemistry
2 Compounds Are More Than One Element
• More than 10 million compounds are known
and the number keeps growing.
• New compounds are discovered and isolated
from natural chemical sources such as plants
and colonies of bacteria and are synthesized
in laboratories for many different uses.
8. Chemistry
2
Compounds Are More Than One Element
• A formula is a combination of the chemical
symbols that show what elements make up a
compound and the number of atoms of each
element.
Compound Formula
Caffeine C8H10N4O2
9. Chemistry
2
States of Matter
• Some substances are described as volatile,
which means that they change to a gas easily
at room temperature.
• Alcohol and gasoline are more volatile than
water.
• Density is the amount of matter (mass)
contained in a unit of volume.
• Styrofoam has a low density or small mass per
unit of volume.
10. Chemistry
2
States of Matter
• Stones have a large
density or a large mass
per unit of volume.
• In science, the density of solids and liquids is
usually measured in units of grams (mass)
per milliliter (volume) or g/mL.
11. Chemistry
2
Chemical Properties
• Chemical properties are those that can be
observed only when there is a change in the
composition of the substance.
• Rusting is a chemical reaction in which iron
combines with oxygen to form a new
substance, iron oxide.
• Inability to react is also a chemical property.
12. Chemistry
2
Chemical Changes
• A chemical property always relates to a
chemical change, the
change of one or more
substances into other
substances.
• Another term for
chemical change is
chemical reaction.
13. Chemistry
2
Chemical Changes
• All matter is made of atoms, and any
chemical change involves only a
rearrangement of the atoms. Atoms do not
just appear. Atoms do not just disappear.
• This is an example of the law of
conservation of mass, which says that in a
chemical change, matter is neither created
nor destroyed. It would be equally correct to
call this the law of conservation of matter.
14. Chemistry
2
Chemical Reactions and Energy
• All chemical changes also involve some sort
of energy change.
• Energy is either taken in or given off as the
chemical change takes place. Energy is the
capacity to do work.
• Work is done whenever something is moved.
15. Chemistry
2
Chemical Reactions and Energy
• Many reactions give off energy.
• For example, burning wood
is a chemical change in
which cellulose, and other
substances in the wood,
combine with oxygen from
the air to produce mainly
carbon dioxide and water.
16. Chemistry
2
Chemical Reactions and Energy
• Energy is also produced and released in the
form of heat and light.
• Chemical reactions that give off heat energy
are called exothermic reactions.
• Chemical reactions that absorb heat energy
are called endothermic reactions.
17. 2
Compound or mixture?
A. sand
B. water
C. juice
18. Chemistry
2
Physical or chemical property?
A. density
B. reactivity
C. color
D. melting point
19. 2 Chemistry
• End of Module 2
• Take Module 2 Test
