Transcript Properties of Water Important to Life

Properties of Water
Directions
 Read information about the properties of water and
carry out the experiments using the materials at your
lab station.
 Answer all numbered questions in red on a separate
piece of paper. Make sure that paper has a proper
heading.
Vocabulary
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Polar bond
Hydrogen bond
Cohesion
Surface tension
Adhesion
Specific heat
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Density
Solution
Solvent
Solute
pH
Acid
Base
Part I: Structure of Water
Write this heading on your paper and then answer all of
the questions for this section under that heading!
Structure of Water
(–)
 This is the structural
formula of water.
A. Draw a picture of
water.
B. What is the chemical
formula of water?
(–)
O
H
(+)
H
(+)
Structure of Water
ACTIVITY
 Build a water molecule using the molecular model
pieces in the box. (blue is O and white is H)
Structure of Water
(–)
(–)
• The Hydrogen and Oxygen
in water are bonded
covalently.
O
H
(+)
• HINT: definition of
covalent!
H
(+)
C. How many electrons are
shared to form a covalent bond?
D. Why does oxygen form 2
bonds and hydrogen form one
bond?
Structure of Water
(–)
(–)
 However, as the atoms share their
electrons they do not do so evenly.
O
H
(+)
E. Which atom is larger? O or H?
 The oxygen in water hogs the
electrons and is therefore slightly
negative.
H
(+)
 This results in the two hydrogen
atoms becoming slightly positive.
 Add the + and – charges to your
picture in A.
Structure of Water
(–)
(–)
 Water is POLAR.
 Hint: what is polarity?
 Polarity contributes to emergent
properties of water
O
H
H
Batteries have a
+ and – pole!
(+)
(+)
F. What is polarity?
G. Why is water considered polar?
Earth has a north
and south pole!
Hydrogen Bonds
 One water molecule tends to bond
to another water molecule by
hydrogen bonding.
(–)
Hydrogen bond
 The H+ on one molecule is attracted
to the O- of another.
(+)
H
O
(+)
(–)
H
(–)
(+)
(–)
(+)
 Each water molecule can form a
max. of 4 hydrogen bonds with 4
other water molecules
 A single hydrogen bond is weaker
than a single covalent bond.
However, groups of hydrogen
bonds are very strong
 Responsible for the “stickiness” of
water molecules
H. Why do hydrogen bonds form between water molecules?
I. Draw 4 water molecules connected by hydrogen bonds.
Part II: Water is the Universal
Solvent
Write this heading on your paper and then answer all of
the questions for this section under that heading!
Water is the ‘universal solvent’
 Solution = liquid consisting of uniform mixture of two or more
substances
 Two parts of a solution:
 Solvent = liquid (dissolving agent)
 Solute = substance dissolved
 (EX: Water = solvent; Koolaid powder/sugar = solutes; Kool-aid =
solution)
 To make a solution you need to remember the rule - “like
dissolves like”
 Since water is polar it will dissolve ions and polar substances
 Since water is polar it will NOT dissolve anything nonpolar
Non polar molecules are hydrophobic
 Oil is an example of a nonpolar
molecule. It does not form hydrogen
bonds.
 Like dissolves like, therefore, oil
(nonpolar) does not dissolve in water
(polar).
 Nonpolar molecules are called
hydrophobic or 'water hating’ because
they do not mix with water.
A. Briefly explain what a solution is and its parts.
B. You make instant coffee in the morning by adding a
tablespoon of coffee to a hot cup of water. Label the
solution, solute and solvent in this scenario.
C. Why doesn’t oil and water mix?
ACTIVITY
 Using the pipettes provided add 2ml of water to a
test tube.
 Gently add 2 ml of cooking oil to the same tube by
tilting the test tube of water slightly and letting the
oil run slowly down the inside of the test tube.
D. Describe what happened.
ACTIVITY
 Using the pipettes provided add 2ml of cooking oil to
a DIFFERENT test tube. Gently add 2 ml of water to
the same tube by tilting the test tube of oil slightly
and letting the water run slowly down the inside of
the test tube.
E. Describe what happened.
ACTIVITY
 Add a few drops of food coloring to each test tube.
 Place your finger over the test tube and gently shake to mix the
oil and water.
F. How does the food coloring act in each test tube? Does it mix
into the oil? Into the water?
G. Explain why water molecules and oil molecules separate from
each other.
 Your explanation should discuss polar and non-polar molecules,
and the effects of polarity on the interactions between water
molecules, and hydrogen bonding.
Part III: Cohesion and Adhesion
Write this heading on your paper and then answer all of
the questions for this section under that heading!
 COHESION
 Water molecules 'stick to each other' (due to
hydrogen bonding)
 Makes water act as if it has invisible “skin”
 Called SURFACE TENSION (how difficult it is to break
surface)
A. define cohesion
 See the ‘skin’ of the
water AKA surface
tension!
ACTIVITY
 Fill the top of a penny with water until a clear bubble
has formed on top but it is NOT overflowing.
B. Can you can float a needle on the water drop on
your penny?
C. What property of water allows you to do this?
ACTIVITY
 Dry off your penny.
 With your finger, spread one small drop of detergent on
the surface of a dry penny.
 Fill the top of a penny again with water until a clear bubble
has formed on top but it is NOT overflowing.
D. Could this penny hold more, less or the same amount of
water?
E. Did the detergent make a difference? Describe the effect
of the detergent.
 ADHESION Water molecules stick to other substances
 Makes water cling to the sides of a cylinder.
F. Define adhesion.
ACTIVITY
 What will be the shape of a drop of water on (a) a piece
of wax paper and (b) a glass slide?
 Perform the experiment. Place several drops of water
on the glass slide and several drops on the wax paper.
G. Draw (and label) the results.
H. Explain the differences in drop shape in terms of
adhesion - that is, the formation (or absence) of hydrogen
bonds between molecules of different types.
 CAPILLARY ACTION
 movement of liquid through a narrow passage or
substance
 Result of cohesion of water molecules to each other
and adhesion to another surface
Makes water ‘travel’
Capillary Action
 Water is pulled to the top
of redwood trees as it
evaporates from leaves at
the top of the tree.
ACTIVITY
 Obtain a graduated cylinder and a strip of chromatography
paper that is just long enough to hang over the side of the
cylinder (inside) and reach to the bottom.
 Place a single small drop of ink from a black vis-a-vis pen on
the paper, about one inch from the bottom, and let it dry
completely.
 Put 30 ml of water into the graduated cylinder and place the
strip of paper in the cylinder so that the bottom end is in
water (just barely). Fold the paper over the top side. Use a
paper clip to hold it to the edge.
I. Note the starting time in your notebook. Watch and note the
time at 5 minute intervals. When the water climbs to the top of
the paper, remove the paper from the water, and let it dry.
Record how long it took for the water to reach the top.
(NOTE: go on to other experiments while you are waiting!)
J. In your notes: Describe what you see on the strip of paper.
Draw the strip, and label each color on the strip.
Part IV: Specific Heat
Write this heading on your paper and then answer all of
the questions for this section under that heading!
HIGH SPECIFIC HEAT - amount of heat a substance needs for a given
increase of temperature
 Takes a lot of energy to raise 1 g of water by 1° C because you must
break Hydrogen bonds
 So… Liquid H2O can absorb large amounts of heat with small changes
in temperature
 So… It heats up more slowly and retains heat longer than
surroundings
 This is important for many reason – for instance in organisms it
helps us maintain a constant temperature (homeostasis)
HIGH HEAT OF VAPORIZATION Takes a lot of energy to convert liquid H2O into vapor
(hydrogen bonding-restricts movement)
 Vaporization (evaporation) is the change from liquid to
gas; Molecules of liquid escape and enter air
 Evaporation of water produces cooling effect
 Again this is important for many reasons – for instance in
some organisms sweat/pant to cool off when hot
(evaporative cooling)
ACTIVITY
 Click this link and complete the exercise.
 Changing Matter
A. As you do describe what happens to the water
molecules as you melt the ice cube.
B. Why does the graph level off during freezing/melting and
then again during condenses/boils?
C. What property of water makes it able to absorb a
great deal of energy before it changes from solid to
liquid to gas? (Hint: what has to break to release water
molecules from each other?)
 Water expands when it freezes (one of few substances that
does this)
 SO . . . ICE IS LESS DENSE THAN LIQUID WATER
Hydrogen bonds keep molecules “at arm’s length” when
freezing so ice floats
 Again this is important for many reasons – for instance it
insulates bodies of water. Lakes freeze from top down;
allowing living things to survive in winter when the lake
freezes
Water
Alcohol
 This blue ice cube floats
in water because ice is
less dense than liquid
water.
 The blue ice cube sinks in
alcohol because ice has a
greater density than
alcohol.
D. Describe the structure of ice vs. liquid water and
explain why ice is less dense than water.
 See this link for help: Why does ice float?
Review
 If you have time watch this quick video to review the
PPT. Make sure to use headphones!
 http://www.youtube.com/watch_popup?v=Jh2qpsZe6
GA#t=32
 http://www.youtube.com/watch?v=DVCYlST6mYQ