Science 10 Chapter 2.2_keyed
Mrs. Steinbrenner
Chapter 2.2 – Explaining Properties of Substances
(pages 56 – 62 in your text)
What to do: Read through section 2.2 in your text. Fill in the missing information and answer
the questions that follow.
In a chemical formula the physical state of a
substance is communicated by a subscript
letter following the chemical formula.
complete the table:
State
Abbreviation
example
(at 21°C)
iron: Fe(s)
water: H2O(l)
helium: He(g)
salt water:
NaCl(aq)
** aqueous means: dissolved in water
solid
liquid
gas
aqueous
(s)
(l)
(g)
(aq)
It is important to know the physical state of a substance because substances have different
properties in different states. For examples, substances have different properties when they are
dissolved in water. To avoid confusion, scientists use subscripts to show the state of a substance
when writing the formula.
Properties of Ionic Compounds
- In a solid state, ionic compounds have regular crystalline shapes with flat sides; the ions in the
crystal are arranged in a regular, repeating pattern. Scientists call this a crystal lattice.
A sodium ion and a chloride ion
form an ionic bond.
The chemical formula NaCl(s) is
refered to as one formula unit.
Formula units do not exist alone;
they form a crystal lattice
structure with other formula units.
- Ionic compounds tend to have fairly high melting points; the attraction between positively and
negatively charged ions in the lattice is strong and extends throughout the crystal. A great deal of
energy is required to break the strong ionic bonds.
Science 10 Chapter 2.2_keyed
Mrs. Steinbrenner
- In a solid state, ionic compounds do not conduct
electricity; ions in the lattice are not free to move.
- When melted or dissolved in water, ionic compound are
electrolytes, they conduct electricity. This property suggests
that melting and dissolving break down the crystal lattice
and allow ions to move freely and carry charge.
Summarize the properties of ionic compounds:
- high melting points
- solid at room temp
- crystalline structure
- dissolves in water
-conducts electricity when
dissolved in water (electrolyte)
- some are colored
** when the ionic compound
is dissolved in water the ions
break free of the crystal lattice
structure. Because the ions
carry an electrical charge, and
can move freely, they can
conduct electricity.
Properties of Molecular Compounds
- Most molecular compounds have fairly low melting points; the attractive forces between
molecules is weak and easy to overcome.
- Once melted, molecular compounds can usually
be heated further without decomposing into other
compounds or elements; the covalent bonds within
molecules must be strong.
- Molecular substances tend to be non-electrolytes;
they do not conduct electricity in any state as pure
substances. Molecular substances are not made of
ions; they do not conduct electricity in any form
because the liquid contains only neutral molecules.
- When dissolved in water, most molecular substances
do not conduct electricity. (Some molecular substances
do conduct electricity; these molecular compounds form ions when they are dissolved in water).
Summarize the properties of molecular compounds:
- low melting point
- can be solid or liquid at room temp.
- do not conduct electricity in solution (non-electrolyte)
- not usually colored.
Science 10 Chapter 2.2_keyed
Mrs. Steinbrenner
Questions:
1. Compare and contrast each term in the pairs below:
a)
crystal la)ce molecule b)
electrolyte non-­‐electrolyte 2. Explain why ionic compounds tend to have high melting points, while molecular compounds
tend to have relatively low melting points. Ionic compounds have high melting points because
the attraction between the ions is so strong. It takes a lot of energy (heat) to overwhelm the
attraction; causing the substance to melt.
3. You have been given two solid substances. One of them is an ionic compound and one of them
is a molecular compound.
a) What are two tests that you could perform to identify the substances as ionic or molecular?
Conductivity test of the substance dissolved in solution (ionic will conduct, molecular does not)
Test the melting point of the substance; ionic compounds have high melting points, molecular
tend to have low MPs.
b) What would you expect to see for an ionic compound, and what would you expect to see for a
molecular compound? Ionic compounds are crystalline, molecular compounds can be in a solid
or liquid state at room temp. Molecular compounds are usually colorless.
Science 10 Chapter 2.2_keyed
A
Mrs. Steinbrenner
B
Use the pictures above as a reference for the following questions:
4. Which image, A or B, represents an ionic compound? How do you know?
“A” represent an ionic compound; it shows the attraction between ions to form the
compound. The formula units are also arranged in a crystal lattice.
5. Which image, A or B, represents a molecular compound? How do you know?
“B” represents a molecular compound; the atoms in the compound are sharing
electrons. There is a low attraction between the molecules.
6. Which compound, A or B, would you expect to have a lower melting point? Why?
There is a low attraction between the molecules in compound “B”, they would
have a low melting point.
7. Which compound, A or B, would conduct electricity when dissolved in water? Why is this?
Compound “A” will conduct electricity when dissolved in water. This is because
charged particles are required for the flow of electricity to occur. When an ionic
compound is dissolved in water the ions are free to move around.