water. penny lab water has several important water properties that we will explore this week. title...
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Water
Penny Lab
• Water has several important water properties that we will explore this week.
• Title your lab report “Penny Lab” and add it to your table of contents.
• First observation: How many drops of water can one penny hold?
Hypothesis
• Question: How does soap affect the amount of water one penny can hold?
• If…then…
Materials
• 2 pennies
• 2 dropper
• 1 beaker with water
• 1 beaker with soapy water
Experimental Design
• We are going to test if the amount of soap makes a difference
• Each group will have a different amount of soap in their soapy water
• We will compile class data and see what our results are
White Board• How much soap did you have in your
soapy water?
• What was your hypothesis?
• What happened? Was your hypothesis supported or rejected?
• Draw a bar graph to compare your averages or tap water and soapy water.
• Answer one post lab question (teacher assigns)
Completing the Lab
• Draw your graph in your notebook.
• Look at class data. Describe if an increase in soapy water effected surface tension of water.
• Answer post lab analysis questions in your notebook. Write in complete sentences.
• Clean up your area
Water Race
• 3 drops of water• 1 toothpick• Who will be the
quickest at each table?
Challenge Round!
• Who will be our classroom champion??
Water
• What were some initial observations about your drop of water?
• What were some observations you made while you were in the race?
Properties of Water• Polar molecule• Cohesion and
adhesion• High specific heat• Density – greatest
at 4oC• Universal solvent of
life
Polarity of Water• Two hydrogen atoms form single
polar covalent bonds with an oxygen atom.
• This gives water more structure than other liquids
• Because oxygen is more electronegative, the region around oxygen has a partial negative charge.
• A water molecule is a polar molecule with opposite charges on each end of the molecule.
Draw a water molecule with charges.
• Water has a variety of unusual properties because of attractions between these polar molecules.– The slightly negative regions of one molecule are
attracted to the slightly positive regions of nearby molecules
– They form a hydrogen bond.– Each water molecule
can form hydrogen bonds with up to four neighbors.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.1
HYDROGEN BONDS• Hold water molecules
together
• Each water molecule can form a maximum of 4 hydrogen bonds
• The hydrogen bonds joining water molecules are weak– about 1/20th as strong as
covalent bonds.
• They form, break, and reform with great frequency
• Extraordinary Properties that are a result of hydrogen bonds.– Cohesive behavior
– Resists changes in temperature
– High heat of vaporization
– Expands when it freezes
– Versatile solvent
• Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.– Water has a greater surface tension than most other
liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.
– Water behaves as if covered by an invisible film.
– Some animals can stand, walk, or run on water without breaking the surface.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.3
Cohesion vs. Adhesion
Organisms Depend on Cohesion
• Cohesion is responsible for the transport of the water column in plants
• Cohesion among water molecules plays a key role in the transport of water against gravity in plants
• Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.
Hydrogen bonds hold the substance together, a phenomenon called cohesion
Exploring pH
• What do you already know?
Exploring pH
• What is pH measuring?
• What does the H stand for?
• Think of the water molecule
• If we take away an H+, what is left?
Data TableSolution pH [H+] pOH [OH-]
Tap water
Distilled water
Milk
Salt
Sugar
PowerAde
Soda
Lemon Juice
Bleach
Corn Syrup
Vinegar
Glass cleaner
Dish detergent
Hand soap
White Board
• How can you organize these numbers?
• Draw a solution and put them in order
• What do you think is happening?
Soil at a high pH makes hydrangea flowers pink
Soil at a low pH makes hydrangea flowers blue
pH Key Concepts
pH doesn’t just tell us if a solution is neutral, an acid or a base
It also tells us:
the concentration of H+ ions in the solution in moles/L
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
pH and [H+]pH Key Concepts
pH doesn’t just tell us if a solution is neutral, an acid or a base
It also tells us:
the concentration of H+ ions in the solution in moles/L
which is expressed as a power of 10
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
pH and [H+]pH Key Concepts
Power of 10
A negative exponent means the number is less than 1
pH Key Concepts
Definition of pH: pH = –log[H+]
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
Do not forget the “–” sign!
logarithm: in base 10, a number A derived from another number B such that 10B=A.
The number 7 is the
logarithm of 0.000 000 1
pH and [H+]pH Key Concepts
Dissociation of water:
H2O(l) H+(aq) + OH–(aq)
[H+] and [OH–] are related
pH for bases
[H+][OH-] = 1 x 10-14
Becomes:pH + pOH = 14
pH Key Concepts
Household chemical
Acid or base pH
ammonia base 11
bar soap base 10
baking soda base 8.5
soda water acid 4
vinegar acid 3
lemon juice acid 2
pH range
pH can be less than 0 for stronger acidsgreater than 14 for stronger bases
pH Key Concepts
Measuring pH
The color of red cabbage juice at different pH
You can’t measure pH by just looking at a solution, or measuring its density
or temperature, but you can measure pH indirectly by:
- performing a chemical reaction with a solution of known pH
- using a chemical that changes color at different pH values
(pH indicators)
pH Key Concepts
Measuring pH
You can’t measure pH by just looking at a solution, or measuring its density
or temperature, but you can measure pH indirectly by:
- performing a chemical reaction with a solution of known pH
- using a chemical that changes color at different pH values
(pH indicators)
pH Key Concepts
You can’t measure pH by just looking at a solution, or measuring its density
or temperature, but you can measure pH indirectly by:
- performing a chemical reaction with a solution of known pH
- using a chemical that changes color at different pH values (pH indicators)
- measuring the electrical properties of the solution
Measuring pH
Acids and bases conduct electricity
pH and conductivity (flow of electricity) are related
a pH meter
pH Key Concepts
Most biochemical reactions involve solutes dissolved in water.
• There are two important quantitative proprieties of aqueous solutions.
– 1. Concentration
– 2. pH
Concentration of a Solution
• Molecular weight – sum of the weights of all atoms in a molecule (daltons)
• Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular weight in daltons.
• Avogadro’s number – 6.02 X 1023
– A mole of one substance has the same number of molecules as a mole of any other substance.
MolarityThe concentration of a material in solution is called its molarity.
A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water.
Calculate a one molar solution of sucrose, C12H22O16.
C = 12 daltons
H = 1 dalton
O = 16 daltons
12 x12 = 144
1 x 22 = 22
16 x 11 = 176
342For a 2M solution?
For a .05 M solution?
For a .2 M solution?
• Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other.– The hydrogen atom leaves its electron behind and is
transferred as a single proton - a hydrogen ion (H+).– The water molecule that lost a proton is now a
hydroxide ion (OH-).– The water
molecule with the extra proton is a hydronium ion (H3O+).
Dissociation of Water Molecules
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Unnumbered Fig. 3.47
• A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:
– H2O <=> H+ + OH-
• This reaction is reversible.
• At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.
• In pure water only one water molecule in every 554 million is dissociated.
– At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) .
Acids and Bases• An acid is a substance that
increases the hydrogen ion concentration in a solution.
• Any substance that reduces the hydrogen ion concentration in a solution is a base.– Some bases reduce H+ directly by
accepting hydrogen ions.
• Strong acids and bases completely
dissociate in water.
• Weak acids and bases dissociate only partially and reversibly.
pH Scale• The pH scale in any aqueous solution :
– [ H+ ] [OH-] = 10-14
• Measures the degree of acidity (0 – 14)
• Most biologic fluids are in the pH range from 6 – 8
• Each pH unit represents a tenfold difference (scale is logarithmic)– A small change in pH actually indicates a
substantial change in H+ and OH- concentrations.
ProblemHow much greater is the [ H+ ] in a solution with pH 2 than in a solution with pH 6?
Answer:
pH of 2 = [ H+ ] of 1.0 x 10-2 = 1/100 M
pH of 6 = [ H+ ] of 1.0 x 10-6 = 1/1,000,000 M
10,000 times greater
Buffers• A substance that eliminates large sudden
changes in pH.• Buffers help organisms maintain the pH of
body fluids within the narrow range necessary for life. – Are combinations of H+ acceptors and
donors forms in a solution of weak acids or bases
– Work by accepting H+ from solutions when they are in excess and by donating H+ when they have been depleted.
Acid Precipitation• Rain, snow or fog with more strongly acidic than pH of
5.6• West Virginia has recorded 1.5• East Tennessee reported 4.2 in 2000• Occurs when sulfur oxides and nitrogen oxides react
with water in the atmosphere– Lowers pH of soil which affects mineral solubility
– decline of forests– Lower pH of lakes and ponds – In the Western
Adirondack Mountains, there are lakes with a pH <5 that have no fish.
Salt Water vs. Fresh Water
• Boiling Point
• Freezing Point
• Gummy Bear Osmosis
Freezing Point
• What happens to salt water after 1 hour in the freezer?
• Independent variable?
• Dependent variable?
• Control?
• Hypothesis: If… then…
Gummy Bear Osmosis
• Read through handout
• Define roles for each group member and write them down
• Measurements and Lab set up
• Day 2 Friday
• Day 3 Monday
• Lab report will be due Tuesday beginning of class
Ice Cream Composition
Freezing Point
• What happened to salt water after 1 hour in the freezer?
• Was it what you expected?
• Analyze the results
• Exit Ticket: Draw a sketch of salt water molecules
Boiling Point
• What happens to salt water after 1 hour in the freezer?
• Independent variable?
• Dependent variable?
• Control?
Moderates Temperatures on Earth
• What is kinetic energy?• Heat?• Temperature?• Calorie?• What is the difference
in cal and Cal?• What is specific heat?
Celsius Scale at Sea Level
100oC Water boils
37oC Human body temperature
23oC Room temperature
0oC Water freezes
Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature.
Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for:
1. Prevention of temperature fluctuations that are outside the range suitable for life.
2. Coastal areas having a mild climate
3. A stable marine environment
Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to
change its temperature by 1oC.
Evaporative Cooling• The cooling of a
surface occurs when the liquid evaporates
• This is responsible for:– Moderating earth’s
climate
– Stabilizes temperature in aquatic ecosystems
– Preventing organisms from overheating
Density of Water• Most dense at 4oC• Contracts until 4oC• Expands from 4oC to
0oC
The density of water:
1. Prevents water from freezing from the bottom up.
2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.
3. Makes transition between season less abrupt.
– When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.
– As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.
– Ice is about 10% less dense than water at 4oC.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.5
Solvent for Life• Solution
– Solute– solvent
• Aqueous solution• Hydrophilic
– Ionic compounds dissolve in water
– Polar molecules (generally) are water soluble
• Hydrophobic– Nonpolar compounds