Physical Science 9
Unit 3: Get a Charge Out of Matter
#4D
Resolving Dissolving (How substances dissolve)
Three Different Kinds of Substances
All atoms have both + and - charges. What does this mean? At the macroscopic level, it
means you can sometimes see matter attract or repel. At the microscopic level, it means all
matter is made of plus and minus charges. Yet, different substances have different electrical
properties. Some substances conduct electricity when dissolved in water, and some do not. A
material's electrical properties also affect what it will dissolve in, how hard it will be, and how
high the melting and boiling points will be. A microscopic view of matter helps explain these
differences because it lets us understand the charges in that matter.
Polar Covalent Molecules
You were introduced to the idea of a polar molecule in the last activity. How did a stream of
water interact with a positively charged object? A negatively charged object? Water bent
toward both. That means water has characteristics of both positive and negative charges.
Figure 4.1. To the right is a simplified sketch of a polar
particle, such as water. Individual atoms are not shown in
this simple sketch. This particle is a dipole because it has
two, oppositely charged ends.
Many particles have some parts that are more positive and other parts
that are more negative. These particles are called polar covalent
molecules. The covalent refers to the type of bond, a covalent bond,
that holds the atoms in the molecule together. Remember that polar
refers to having a part of the molecule that is somewhat positive and a
part that is somewhat negative.
Figure 4.2. To the left is a sketch of a water molecule that shows
the atoms that are bonded together. You can see that the side
with hydrogen atoms (H) is more positive (+). The side with the oxygen atom (O) is
more negative (-).
Substances made of polar covalent molecules do not conduct electricity even if they are
liquids or are dissolved in liquids. In order for electricity to flow, individually charged particles
(+ or -) must be free to move. A polar covalent molecule has one end that is + and one end
that is -, but the charges can’t leave the molecule. So even though the molecule itself is able
to move around, the charges on the molecule are not able to move freely (because + and charges are on the same particle). Polar molecules do not conduct electricity.
Substances that are made of polar molecules easily dissolve in liquids that are also made of
polar molecules. Why do think this is? What would make polar molecules mix well with other
polar molecules?
Nonpolar Covalent Molecules
Molecules in the same category as baby oil and paraffin wax are not like water. Baby oil and
paraffin wax are made of molecules with different electrical properties from water. What does
that mean? It means molecules of baby oil and paraffin wax do not have plus and minus ends
like water molecules do. Instead, we can think of their positive and negative charges as being
balanced and spread evenly throughout the molecule. Molecules such as baby oil and paraffin
are typically called nonpolar covalent molecules.
Figure 4.3. To the left is a simplified sketch of a nonpolar particle.
Individual atoms are not shown. Notice the particle is net neutral.
Nonpolar covalent molecules DO NOT dissolve in water. Why not? Nonpolar
covalent molecules are neutrally charged. Therefore, the positive and negative sides of water
molecules do not interact with (attract or repel) nonpolar molecules. Nonpolar covalent
molecules DO mix well with other nonpolar molecules. As a general rule, substances with
similar electrical properties will mix with each other. The rule is sometimes stated “like
dissolves like”.
Figure 4.4. To the right is an atom level sketch of a nonpolar molecule,
C2H6. Black are carbon atoms and white are hydrogen atoms. Since
hydrogen and carbon atoms have similar electrical properties no part of
the molecule is significantly more positive or negative.
Nonpolar covalent molecules do not conduct electricity even when they are liquids or dissolved
in other liquids. This is because the microscopic charges they have are balanced and spread
evenly throughout the molecule, making the molecule neutral. These
microscopic charges stay in the molecule and can’t move freely
through the liquid. When charges are not moving freely, no
electricity flows.
Ionic Compounds
The third type of substance is made of particles in which the atoms
are held together by an ionic bond, not a covalent bond. The atoms
each have their own net charge; they are called ions. Notice in
Figure 4.5, how the positively and negatively charged atoms (ions) in
the solid are bonded together by electrostatic attraction (pulling
force). They cannot move about freely like particles in a solution.
Instead, this attractive force locks the ions in place, resulting in a
highly organized microscopic structure of solids, named a crystal
lattice. This is an important feature of ionic compounds: ions
form extended structures, crystals (for example, salt crystals),
where each ion is either positively or negatively charged; and the
attraction between the charges hold the crystal together. These ions
do not form covalently bonded molecules. This means that there is
no set number of atoms in an ionic compound; the crystals grow as
long as there are more positive and negative ions around. In Figure
4.5, the ions in the solid crystal lattice are not free to move from one wire of the conductivity
meter to the other. When charges are not moving, no electricity flows.
Figure 4.5 shows charges in solid sodium chloride (table salt). The charged atoms
are called ions. Solids made of ions are called ionic compounds. Solid sodium
chloride does not conduct electricity because charged ions that are locked in place
are not mobile.
Ionic Compounds Dissolve in Polar Liquids
Solid ionic compounds such as sodium chloride dissolve in water. Imagine an ionic crystal in
water. Figure 4.6 (below) shows what that might look like. Find the plus and minus charges
on the water molecules. The positively charged ends of water molecules attract negative
chloride ions. The negatively charged end of water attracts positive sodium ions. Water
molecules pull ions free from their crystal lattice. Soon sodium and chloride ions are pulled
into the liquid and become part of the solution.
Figure 4.6
Once in solution, water molecules surround each ion. The result
is a solution of dissolved sodium chloride (dissolved sodium
ions and dissolved chloride ions in water).
Now these ions are free to move about in the solution along
with the water molecules. As a result, the solution conducts
electricity. Why does the solution conduct electricity? The
answer is because now there are charged particles, positively
charged sodium ions and negatively charged chloride ions, free
to move in the solution. When charges can move freely,
electricity can flow through a material. Thus, ionic
compounds in solution are strong conductors.
As a general rule, substances with similar electrical properties
to water molecules dissolve in water. The rule is sometimes
stated “like dissolves like.” However, there is a limit to how
much of an ionic compound can dissolve in water. Can you think of a reason why you can’t
dissolve an unlimited amount of ionic salt crystals in water?
Not all Bonds Make Compounds
In the previous two activities, we have only considered compounds: chemical bonds between
different kinds of atoms, like oxygen and hydrogen bonding to make water molecules or
sodium and chlorine bonding to make salt crystals. While some bonds only produce
compounds, we know that not all bonds are between different kinds of atoms.
Oxygen gas, O2, is a diatomic element, not a compound. It is a molecule with a bond
between two atoms of the same element. The bond between the atoms in O2 is a covalent
bond (ionic bonds produce crystals, covalent bonds produce molecules). Do you think this is a
polar or nonpolar molecule? It is nonpolar because the atoms are identical and have identical
electrical properties. One side does not become more negative than the other.
Figure 4.7 To the right is an atom level sketch of an oxygen molecule
Metallic Bonds: a special case
Metal atoms can also bond with other metal atoms to form a different kind of extended
structure. When metal atoms bond with each other, the atoms become more positive by freely
sharing part of their negative charge with their neighboring metal atoms.
The attraction between the positive metal atoms and freely flowing
negative charges help hold all the metal atoms together. These negative
charges can move all over the metal object because they are not fixed on
any one atom. This type of bond is a metallic bond. Can you think of any
properties of a metal that these freely moving negative charges can help
explain?
Figure 4.8 To the left, observe that Negative charges move freely around partly
positive metal atoms
Physical Science 9
Unit 3: Get a Charge Out of Matter
#4D
Resolving Dissolving (How substances dissolve)
3 different categories of matter
Type of particles
Polar Covalent
Molecule
Nonpolar
Covalent Molecule
Ionic Compound
covalent
covalent
ionic
Oppositely charged
ends; one end is net
+ and the other net no
Evenly distributed
charge; molecule
is net neutral
no
Each atom has a
net charge, + or -.
These are ions.
yes
No (or weakly)
No
Strong
Yes or partially
no
yes
Bonds that hold atoms
together
How + and – charges are
arranged on particles
Charges able to move freely
as a liquid or in solution
Able to conduct
electricity
Able to dissolve in water
Polar molecules and ionic compounds are more / less electrically similar to each other than
they are nonpolar molecules.
Physical Science 9
Unit 3: Get a Charge Out of Matter
#4D
Resolving Dissolving (How substances dissolve)
3 different categories of matter
Type of molecule
Bonds that hold atoms
together
How + and – charges are
arranged on particles
Charges able to move freely
as a liquid or in solution
Able to conduct
electricity
Able to dissolve in water
Polar molecules and ionic compounds are more / less electrically similar to each other than
they are nonpolar molecules.