Transcript Chapter 15: Solutions - Vernon Hills High School

Forming Solutions
15.1: Pgs 520 - 527
Objectives
• To understand and describe the
process of dissolving
• To learn why certain substances
dissolve in water and others won’t
• To understand factors that affect the
rate at which a solid dissolves
Important Solution Vocab:
• Solution - a homogenous mixture.
– Solvent - the substance present in the largest
amount.
– Solutes - the other substance or substances.
• Aqueous solutions - solutions with water as
the solvent.
solution
solvent
solute
Solubility of Ionic Substances
• What is occurring when something
dissolves?
•The ionic forces that hold the salt together are
overcome by the attractions between the ions and
the polar water molecules.
Ionic Compounds
• The resulting solution contains ions that can
move about independently!
– Ex: NaCl (s)  Na+ (aq) + Cl- (aq)
Step 1 of Lab
1. Obtain a CLEAN test tube!!!
2. Place 2.2 grams of Na2SO4 into a test tube
3.
4.
5.
6.
and add 10mL of water.
Dissolve the solid: Heat the test tube in a
bunsen burner until you hear a “pop”.
Remove the test tube from the bunsen
burner and stir the contents with a stirring
rod. Continue this process until the solid
dissolves.
Place the test tube into the test tube rack
when the solid is dissolved.
Place 2 ice cubes into a beaker and fill the
beaker with water (You will use this later)
Sit back in your seat.
Factors Affecting the Rate of
Dissolving
• Surface area
– The larger the surface area, the
faster the dissolving will occur
• Stirring will increase the rate of
dissolution
• Temperature
– Dissolving occurs faster at higher temperatures
Solubility of Polar Substances
• Water can also dissolve nonionic
substances
– Examples: sugar, ethanol
•These have polar O-H groups in their structure
that are attracted to the water
Polar OH groups that allow sugar to
dissolve in water!
Substances Insoluble in Water
• Not all substances are soluble in water
– Ex: Petroleum  What happens when it
leaks from an oil tanker in the ocean?
• Nonpolar substances are not
compatible with polar water
molecules
– Ex: Oil and water don’t mix!
An oil layer floating on water.
How Substances Dissolve
• Follow the saying “like dissolves like”
– A solvent usually dissolves solutes that have
polarities similar to its own.
•Ex: Water and Sugar:
– Since water and sugar are both polar, the solute – solvent
interactions formed are similar to the water – water
interactions present in the pure solvent
As we add more salt to the solution, eventually
there won’t be enough water to surround
each ion. What will happen then?
Step 2 of the Lab
• 1 person from your team must place your
test tube into the beaker of ice water and
then return to your seat.
Solution Composition
• Saturated – When a solution contains as
much solute as will normally dissolve at that
temperature.
• Unsaturated – When a solution contains less
solute than can normally dissolve
• Supersaturated – it contains more solute
than a solution will normally hold at that
temperature
http://www.youtube.com/watch?v=HnSg2cl09PI&feature=fvw
http://www.youtube.com/watch?v=aC-KOYQsIvU&feature=related
Solution Composition
• Supersaturated solutions are very
unstable!
– A saturated solution can be forced to
precipitate by adding one crystal of the
solid.
•Precipitation will continue until the solution
reaches the saturation point
Last step of the lab
GENTLY remove the test tube from
the cold water. Add a small
amount of Na2SO4 and observe.
Clean up the lab: All
solution/compounds can be rinsed
down the drain.
Sit back in your seat.
What just happened???
• How would you classify the solution
BEFORE the extra solid was added to
the test tube?
– What does this mean?
• What did the addition of the extra solid
do to your solution?
• How would you classify your solution
now???
Yesterday we covered…
• 4 factors that will increase the rate of
dissolution
Today we will cover…

How temperature not only increases
the rate of dissolution, temperature
also affects the amount that can
dissolve.
Solubility
• Solubility: the amount of a solute that
can dissolve in a given amount of
solvent at a specific temperature
• The solubility of a substance depends
on TWO things:
– The nature of the solute
– The temperature of the solvent
Factors Affecting Solubility
• Solubility is VERY temperature
dependent!
– Ex: 36.2g of NaCl dissolves in 100g
H2O at 25oC, HOWEVER 39.2g NaCl
can dissolve in 100g H2O at 100oC!
• For most substances, solubility
increases as temperature of the
solvent increases
Expressing Solubility
• Solubility is expressed as follows:
g of solute
@ a temperatu
re
g of solvent
36.2 g NaCl
o
@ 25 C
100g H 2O
• Can use solubility like to set-up
proportion problems.
Problem:
•I dissolved 5.2g of a substance
into 16g of water. What is the
solubility of this substance in
units of g / 100g of water?
Solubility Curves
• What substance
has the highest
solubility at
50oC? (exclude KI)
• What has the
lowest at 50oC?
Solubility Curve Lab
• Not all substances are stable
enough to create a
supersaturated solution. Most
substances can only form a
saturated solution.
• We will complete this as a class.
Each pair of students will be
assigned a specific amount of salt
to use and we will collaborate our
data to create a graph.
Directions
• Heat the test tube in hot water on a hot plate.
•
•
•
•
•
Make certain all the solid has dissolved.
Remove the test tube from the hot water and
place a thermometer into the test tube.
Continue to stir the contents slowly until you see
some crystals (solids) begin to form. Record the
temperature for when the solid begins to form in
the data table on the board.
Repeat steps 1-3 in order to record a total of 2
trials.
Calculate and record the solubility of KNO3in units
of “g/100g of water” in the data table.
When you are done, work on textbook homework
and worksheet that is due tomorrow.