Primarily due to polarity

Properties of Water

Intro to water

•Molecule

•two hydrogen atoms and one oxygen atom

•v-shaped triangular molecule

•hydrogen bonds

•Polarity

•properties

*Intro to WaterWater 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 bondCohesionAdhesionSurface tensionCapillary 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)

H2O is Polar

•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 is Polar

Water is a universal solvent

•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.

“Like dissolves like”(due to its polarity)

Water (polar) + Styrofoam (non-polar)

Acetone (nonpolar) + Styrofoam (non-polar)

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

Forces due to polarity

• 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

Surface Tension

•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 tensionThere the water behaves like an flexible sheet allowing denser objects to “sit” on the surface.

Surface tension (cohesion)

•Can be seen as water droplets form

•Helps insects walk across water

Adhesion•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.

*Adhesion

*Can be seen as water droplets form on the spider web (another polar surface)

Form spheres & hold onto plant leaves

*Capillary action

*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)Polarity-Like dissolves like-Heat capacityWater acts as a magnet-Trade notebooks with your partner write one constructive comment in your partner’s notebook. Hand them back their notebook( 1min)

Warm up: 2/25/13

• 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.swf

• 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 ionsb.Ions for ionic and molecules for covalent

compoundsc.Ions for covalent and molecules for ionic

Checking for understanding

The dissolution of KNO3 and C3H6O (acetone) in water occurs:a.Molecules by molecules for both KNO3 and

C3H6Ob.Ion by ion for KNO3 and molecule by

molecule for C3H6Oc.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 2/25/13For the following reaction

• identify the products as insoluble or soluble.Na2CO3 + CaCl2 CaCO3 + 2NaCl

•Classify the type of reactionActivity

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

Gra

ms

solu

te/1

00 g

H2O

NaClO3

KNO3 KBr

NaCl

TemperatureTemperatureGra

ms

of

solu

te p

er

10

0 g

of

wate

r

How much solute will dissolve?

Gra

ms

solu

te/1

00 g

H2O NaClO3

KNO3 KBr

NaCl

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.

How much solute will dissolve?

•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.

Gra

ms

solu

te/1

00 g

H2O

Saturated

•What is the solubility of KNO3 at 60 oC in 200 g of H2O?

•How many grams of KBr can dissolve in 300 gr of H2O at 100 oC?

G

ram

s so

lute

/100

g H

2O NaClO3

KNO3 KBr

NaCl

Temperature

*How much solute will dissolve? *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.

Gra

ms

solu

te/1

00 g

H2O

Unsaturated

Gra

ms

solu

te/1

00 g

H2O

How much NaClO3 would you have to add to 100 g of water at 50oC to make a saturated solution?

50o

Look at the intersection.

Approx. 140 – 142 g

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 questions1) 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.