Transcript Properties of Water

Properties of Water
Primarily due to polarity
Intro to water
•Molecule
•two hydrogen atoms and one oxygen atom
•v-shaped triangular molecule
•hydrogen bonds
•Polarity
•properties
*
Water is a molecule made up of two hydrogen
atoms and one oxygen atom. It has the formula
H2O. When oxygen and hydrogen combine (H-O-H)
they form a v-shaped triangular molecule. While
water molecules are electrically neutral, the
oxygen atom holds a small negative charge and the
two hydrogen atoms hold small positive charges.
Water molecules are attracted to each other,
creating hydrogen bonds. These bonds determine
almost every physical property of water and many
of its chemical properties too. Scientists believe
this unusual electrical balancing, called polarity,
gives water some of its remarkable properties
* Terms to know by the end of the lesson
Polarity
Hydrogen bond
Cohesion
Adhesion
Surface tension
Capillary action
Water Physical Properties (review)
•At what temperature in Celsius does water
start boiling?
•At what temperature in Celsius does water
start melting?
•At what temperature in Celsius does water
start freezing?
•What is the density of water?
Water has a high specific heat capacity
• Water has the ability to absorb a lot of heat with a
relatively small increase in temperature
• Water has one of the highest specific heat capacity
This allows marine organisms to avoid drastic
temperature fluctuations in the seawater.
This allows orchards grown on the coast to
survive hot summers and severe winters.
http://www.youtube.com/watch?v=qeDZQ9-gsjY
Water “acts” like a magnet
Water molecule is
V-shaped - BENT
Thinking critically
If matter expands when heated,
and contracts when cooled, why
does ice expand (increase in
volume) when water freezes?
When water freezes, it goes from a mixed up liquid
state where all these V's are just sliding around
each other, to an ordered crystalline solid state
where all the V's have to connect with each other
in nice orderly solid shapes.
The closest and easiest solid crystal shape for something that exists
as a 104.5 degree V is a hexagonal (really tetrahedral in 3D) crystal.
Think of it as a flat hexagonal snowflake shape, but it really goes in
three dimensions. The water molecules want to do this because to
them it "feels" nicer--that is: they feel less strain and they can get
into a lower energy state by getting into this nice orderly
hexagonal crystal.
“Why ice floats “
Critical Thinking:
If ice were more dense than liquid water, how
would this impact the survival of the marine
life?
(The floating layer of ice insulates the liquid
water below, so that it wouldn’t freeze – this
makes the ocean environment to easier to live
in)
•As we know – water is neutral
(equal number of e- & p+ = zero Net Charge)
•But because the O atom is more
electronegative than the H atoms – electrons
spend more of their time nearer the oxygen.
(O atom attracts more than its “fair share” of
electrons).
•This gives water a slight overall charge.
The oxygen end “acts” negative.
The hydrogen end “acts” positive.
•This charge is called polarity
Oxygen “pulls” closer to it creating
positive and negative sides of the
polar molecule.
•Water can dissolve more substances than any
other solvent.
•Give examples of substances that water can
dissolve.
•The dissolving power of water is very important
for life on Earth. Wherever water goes, it carries
dissolved chemicals, minerals, and nutrients that
are used to support living things.
Water (polar) + Styrofoam (non-polar)
Acetone (nonpolar) + Styrofoam (nonpolar)
“Like dissolves like”
(due to its polarity)
Hydrogen Bonds
(Formed between a highly Electronegative atom of a
polar molecule and a hydrogen
(O-H)
Opposites
attract
This model to show the attraction
between H and O – hydrogen bonding
•
•
Cohesion
Adhesion
Cohesion (water is sticky)
Attraction between particles of the same
substance ( why water is attracted to itself)
•Results in Surface tension (a measure of the
strength of water’s surface)
Produces a surface film on water that allows
insects to walk on the surface of water
•Inside a drop of water polar water molecules
attract to each other in a random fashion
•At the surface of the drop, water does not
attract to the air
•A unified layer of molecules at the surface
creates surface tension
There the water behaves like an flexible sheet
allowing denser objects to “sit” on the
surface.
Surface Tension
• Can be seen as water
droplets form
• Helps insects walk across
water
Surface tension
(cohesion)
•Attraction between two different
substances.
•Water will form hydrogen bonds with
other surfaces such as glass, soil, plant
tissues, and cotton
•Adhesion Causes Capillary Action, which
gives water the ability to “climb”
structures.
* Can be seen as water droplets
form on the spider web
(another polar surface)
Form spheres & hold onto
plant leaves
*
*
* We know that gravity is ALWAYS pulling on objects with
mass
* Yet water can move up a paper towel with relative ease
– How can this happen?
* Because the positive and negative charges in the paper
attract the polar water molecules (adhesion)
* This property of adhesion is called capillary action.
Explain the following water properties in both
words and drawings(2 min)
PolarityLike dissolves likeHeat capacity
Water acts as a magnetTrade notebooks with your partner write one
constructive comment in your partner’s
notebook. Hand them back their notebook(
1min)
•
How do heterogeneous and homogeneous mixtures differ?(Left Side)
Salt solution and Oil in water
2 min - Quick Write: Based on your prior knowledge about mixtures, explain why a
salt solution is classified as a homogeneous mixture and a mixture of oil and water is
said to be a heterogeneous mixture.
White boards activity:
Homo- or Hetero - ?
COKE
AIR
STEEL
A JAR OF PENNIES
Coke (mixture of CO2 gas, water, sugar, caramel color, food flavorings)
Air (mixture of O2 and N2)
Steel (alloy – mixture of Fe and C)
A jar of pennies and nickels
• Notes
By the end of the day you will know:
• Solute
• Solvent
• Solution
• Soluble
• Insoluble
• Immiscible
• Separation
• Solvation
• Factors affecting the rate of dissolution
Next notes…
Complete the sentences.
When you put salt into water it______. The salt dissolves
because it is ________in the water. The substance which
dissolves is called the ______. The substance that does the
dissolving is called the _______. When something dissolves
you get a _______.
Water is a ________. Salt is a ________.
When a substance does not dissolve it is________
Dissolution of salt in water
http://www.nsf.gov/news/special_reports/chemistrynow/chem_
water.jsp
• Animation - Dissolution of an Ionic compound (results in
dissociation) and a covalent compound (no dissociation)
http://www.chem.ufl.edu/~itl/4411/react/dissolution_of_cmpds.s
wf
• Animation – Strong electrolyte (complete dissociation into ions) –
many ionic compounds – conduct electricity - (+ strong acids and
bases – will cover in more detail in Bundle 11)
http://www.chem.ufl.edu/~itl/4411/react/04M05AN1.MOV
• Animation – Weak electrolyte (partial dissociation) – weak acids
and bases (if a conductivity meter is used - light is not as bright
(dim) compared to a strong electrolyte)
http://www.chem.ufl.edu/~itl/4411/react/04M05AN2.MOV
• Animation – Non-electrolyte ( no dissociation)
- no disruption of
its molecular (covalent) structure – does not conduct electricity
http://www.chem.ufl.edu/~itl/4411/react/04M05AN3.MOV
The dissolved components of ionic and
covalent compounds are:
a.All ions
b.Ions for ionic and molecules for covalent
compounds
c.Ions for covalent and molecules for ionic
The dissolution of KNO3 and C3H6O (acetone)
in water occurs:
a.Molecules by molecules for both KNO3 and
C3H6O
b.Ion by ion for KNO3 and molecule by
molecule for C3H6O
c.Ion by ion for C3H6O and molecule by
molecule for KNO3
Based on your observations of the dissolution process on
the animation, how do you think substances get
dissolved?
a.Each solvent molecules gets separated from other
molecules and is surrounded by ions in ionic substances
or molecules in covalent substances
b.Each ion in covalent substances and each molecule in
ionic substances gets separated from other molecules
or ions and is surrounded by solvent molecules
c. Each ion in ionic substances and each molecule in
covalent substances gets separated from other
molecules or ions and is surrounded by solvent
molecules.
Warm up 2/25/13
For the following reaction
• identify the products as insoluble or soluble.
Na2CO3 + CaCl2  CaCO3 + 2NaCl
•Classify the type of reaction
Activity
Sketch the process of dissolution of calcium
chloride. Find a partner - compare each other’s
drawings – make corrections if needed(4min)
Factors that Affect the Rate of
Dissolution
Pre-assessment:
If you wanted to dissolve a substance
in water as quickly as possible what
could you do?
The three methods to increase
the rate of dissolving for a
solid are?
•Heat it!
•Crush it!
•Stir it!
How can you achieve the following:
•Increased number of collisions between
solvent and solute
•Agitation
•Increased surface area
•Increased kinetic energy
Notes –Solubility
•the amount of solute that will dissolve in a
given amount of solvent at a specified
temperature and pressure (for gases)
•grams of solute per grams of solvent
•grams of solute per 100ml of solvent
Example: at 25.0 0C, the solubility of sodium
chloride is 35.0 grams per 100 ml of water.
You can dissolve up to 35.0 grams of
sodium chloride in 100 ml of water.
If you add more than 35.0 grams the solid
will simply not dissolve.
Based on solubility we can have three types of solutions:
• Unsaturated – a solution that could dissolve MORE
solute at a specific temperature
• Saturated – a solution that contains the MAX amount
of solute that can dissolve at a specific temp (stable)
Visual evidence: a small quantity of un-dissolved solute
remains in solution
• Supersaturated – a solution that contains more
dissolved solute than a saturated solution. (Prepared
by heating the solvent, adding solute, and cooling
slowly – very unstable)
DEMO. Supersaturated solution of Sodium Acetate
Grams solute/100 g H2O
NaClO3
KBr
KNO3
NaCl
Temperature
Temperature
• A solubility curve
shows the amount of
each solute that will
dissolve in 100g H20 at
each temperature.
• Saturated is on the
line.
• Unsaturated is below
the line.
• Supersaturated is
above the line.
Grams solute/100 g H2O
How much solute will dissolve?
NaClO3
KBr
KNO3
NaCl
Temperature
• A solubility curve shows
• Saturated is on the line.
• Unsaturated is below the
line.
• Supersaturated is above
the line.
Saturated
Grams solute/100 g H2O
the amount of each
solute that will dissolve
in 100g H20 at each
temperature.
• What is the solubility of
• How many grams of KBr
can dissolve in 300 gr of
H2O at 100 oC?
Grams solute/100 g H2O
KNO3 at 60 oC in 200 g of
H2O?
NaClO3
KBr
KNO3
NaCl
Temperature
*
*A solubility curve shows
Grams solute/100 g H2O
the amount of each
solute that will dissolve
in 100g H20 at each
temperature.
*Saturated is on the line.
*Unsaturated is below
the line.
*Supersaturated is above
the line.
Unsaturated
Look at the intersection.
Approx. 140 – 142 g
Grams solute/100 g H2O
How much
NaClO3 would you
have to add to
100 g of water at
50oC to make a
saturated
solution?
50o
Mini Lab: WHAT FACTORS INFLUENCE DISSOLUTION
Surface Area (Solid Solute)
1) Record the time 1 sugar cube needs to dissolve in 200 ml of tap water. ______
2) Crush 1 sugar cube in the mortar & pestle. Record the time the crushed sugar
cube needs to dissolve in 200 ml of tap water. _____________
Temperature (Solid Solute)
3) Obtain 200 ml of warm water from the front lab table. Color one side of the
sugar cube with a Vis-A-Vis marker. Add the sugar cube to the warm water &
record the time the cube needs to dissolve. _____________
4) Obtain 200 ml of cold water from the front lab table. Color one side of the sugar
cube with a Vis-A-Vis marker. Add the sugar cube to the cold water & record the
time the cube needs to dissolve.___________
Stirring (Solid Solute)
5) Place 1 sugar cube in 200 ml of tap water. Record the time the cube needs to
dissolve without stirring.______________
6) Place 1 sugar cube in 200 ml of tap water. Record the time the cube needs to
dissolve while you stir the solution._____________
Homework: Lab analysis questions
1) How do you increase the surface area of a solid?
2) How is surface area of a solid solute related to
dissolution? WHY?
3) How is temperature related to the dissolution of a
solid solute? WHY?
4) What effect does stirring have on the dissolution of
a solid solute? WHY?
Checking for understanding
The Great Soda Mystery!
1. What is the difference
between the hot soda and
cold soda? Why?
2. As temperature _______,
solubility of a gas ______.
3. Does pressure affect the
amount of gas in the soda?
How?
4. As pressure _____,
solubility of gases ______.
•In general solubility of
solids increases with
increasing temperature.
•Solubility of gases in water
decreases with increasing
temperature.