The Properties of Water
|Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes|
|Exercises 1-4 take approximately 2 hours.|
Water is everywhere. It's in the air we breathe. It's in our sink faucets, and
it's in every cell of our body. Water is an unusual substance with special
properties. Just think about the wonder of water:
|1.||How does water rise from the roots of a redwood tree to
the very top?|
Several forces collaborate to make this possible. Adhesion of water to the wood and cohesion of water molecules with one another play a critical role in the climbing process. The continual flow of water into the roots and evaporation of water from the leaves maintain an upward flow.
|2.||How do insects walk on water?|
Due to cohesion of the water molecules, the water surface is firm enough to support the insect's weight.
|3.||Why does ice float rather than sink?|
Most elements or molecules are closer together in the solid state than in the liquid state; therefore, the solid is more dense and falls to the bottom of its corresponding liquid. Water is different. In water, the molecules actually move apart as freezing occurs, to form a regular latticework. of H2O molecules. As a consequence, ice is less dense than water and floats on top of it, a characteristic very important for the maintenance of life on earth.
|4.||Why do people become seriously ill, or die, if they go
without liquid for a week or so?|
The human body carefully maintains a particular salt and sugar content in the blood and lymph , keeping the concentrations of these substances within a very narrow range. When humans are deprived of water, this balance becomes increasingly difficult to maintain.
|5.||How would life in a lake be affected if ice sank and
lakes froze from the bottom up?|
Many organisms survive the winters in the water under the ice. If lakes froze from the bottom up, these organisms would be much more likely to die.
The lesson begins with questions about water for students to ponder, both to stimulate students' curiosity and wonder and to provide a framework for the lesson. You might ask your students to generate additional things they wonder about with respect to water. Two overarching goals of this lesson are to 1) encourage students to interpret the behavior of water and other substances in terms of the interactions among their molecules and 2) to introduce a number of ideas that are critically important in understanding living things.
|Once you have completed this exercise you should be able to:|
|1.||Describe the polarity of a water molecule and explain how that polarity affects the properties of water.|
|2.||Explain why water climbs the inside of a thin glass capillary but not a thin plastic capillary.|
|3.||Explain why water climbs a paper strip.|
|4.||Describe a system whereby the components of a water-based substance might be separated and discuss how this separation occurs.|
|5.||Explain why oil and water don't mix.|
|6.||Predict whether a substance, based on its hydrophilic and/or hydrophobic properties, will dissolve into water or oil.|
|Water covers about three fourths of the surface of the earth? It
is ubiquitous. It is also one of the simplest yet most important molecules in
living systems. It makes up from 50 to 95 percent of the weight of living
organisms. The cytoplasm of a cell is a water-based solution that contains a
variety of ions, salts, and molecules which make life 'happen.' Water is
literally involved in every facet of life.|
The simplicity of the water molecule belies the complexity of its properties. Based on its small size and light weight, one can predict how it should behave, yet it remains liquid at a much higher temperatures than expected. It also boils and freezes at much too high, or low, of a temperature for a molecule of its size. Many of these unexpected properties of water are due to the fact that water molecules are attracted to each other like small magnets (cohesion). This attraction results in turn from the structure of the water molecule and the characteristics of the atoms it contains.
Each molecule of water is made up of two atoms of hydrogen connected to one atom of oxygen, as shown below. This is summarized in the familiar formula, H2O.
|Atoms are most stable when they have a particular configuration of their outer shells, a concept which will be discussed in future labs. These configurations explain why hydrogen in water will take on a partial positive charge and why oxygen will take on a partial negative charge. These partial charges cause water molecules to 'stick' to each other like magnets. The 'stickiness' in this particular case is due to 'hydrogen bonding'. In this case, hydrogen bonding involves the attraction between the positively charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another water molecule. As no electrons are actually shared however, hydrogen bonds are much weaker than covalent bonds - they easily break and easily form again.|
Surface Tension & Adhesion
|1a||Drop Behavior - Water on Penny|
|To Do||1.||Obtain a medicine dropper and a small (10 ml) graduated cylinder. Make sure the dropper is clean.|
|2.||Drop water into the graduated cylinder with the dropper, counting each drop.|
|3.||How many drops, of the size produced by your medicine dropper, are in each cubic centimeter (cc) of water? (1 cubic centimeter = 1 milliliter)? __________ drops|
|Results||4.||Conversely, how much water is in each drop? (divide 1cc by the number of drops) __________ cc. per drop, average.|
|To Do||5.||Now, let's see how many drops of water you can you place on the surface of a penny before it overflows.|
|Results||6.||How many drops do you predict?|
|To Do||7.||Drop water from the dropper onto a penny, keeping careful count of each drop. Draw a diagram below showing the shape of the water on the penny after one drop, when the penny is about half full, and just before it overflows.|
|Results||8.||How many drops were you able to place on the surface of the penny before it overflowed? __________ drops|
|Interpret||9.||If the number of drops is very different from your prediction,
explain what accounts for the difference.|
Students' predictions will typically underestimate this effect so they will likely be surprised. The students will find that they can put a surprising number of water drops on the head of a penny (typically 30 - 40) and they will observe how the water "piles up" on the penny.
|10.||Explain your results in terms of cohesion|
This exercise provides a demonstration of the 'stickiness' of water caused by cohesion. The effect results from the formation of a weak hydrogen bond between the oxygen of one water molecule (which carries a partial negative charge) and the hydrogen of another water molecule (which carries a partial positive charge). The many weak bonds that are formed are additive in strength, working in unison to bind water together. The water molecule is polar because of its partial charges.
|1b||Effects of Detergent|
|To Do||1.||With your finger, spread one small drop of detergent on the surface of a dry penny.|
|Predict||2.||How many drops do you think this penny will hold after
being smeared with detergent, more, less, or the same as before?
Students will observe that a penny with detergent will hold many fewer drops than a penny without detergent. This occurs because detergent is an amphipathic molecule, being polar (charged) at one end and non-polar at the other. This exercise demonstrates the disruptive effect of detergent upon the cohesiveness of water.
|Question||3.||Specifically, how many drops do you think it
|To Do||4.||Using the same dropper as before, add drops of water to the penny surface. Keep careful count of the number of drops, and draw the water on the penny after one drop, about half full, and just before overflowing.|
Figure 5. Drawing of Drops on a Penny with Detergent|
|Results||5.||How many drops were you able to place on the penny before it overflowed this time? __________ drops|
|Question||6.||Did the detergent make a difference? Describe the effect of the
Because of the amphipathic structure, one end of the detergent molecule forms hydrogen bonds with water molecules, but the other end does not. When water hydrogen bonds with detergent, it is not bonding to other water molecules. As a result, the overall forces binding the liquid together are greatly diminished.
|7.||What does the detergent do to have this effect on
The detergent is similar in structure to phospholipid molecules in our bodies which make up cell membranes. Phospholipids are also amphipathic, having a polar phosphate group at one end and two non-polar fatty acids at the other. The 'oil / water', 'polar / non-polar' distinction is a very important factor in the structure and behavior of living cells.
|Interpret||8.||Explain how detergents act as cleaning agents, considering the cohesion among water molecules and the affects
Detergent molecules act as cleaning agents by surrounding oil droplets with their non-polar ends and interacting with water at their polar ends. Thus they can "pick up" oil off your hands and carry it away on the water flowing into the sink.
|1c||Drop Shape on Glass and Wax Paper|
|Question||1.||What will be the shape of a drop of water on (a) a piece of wax paper and (b) a glass slide. Draw the shape of the drop you expect on each surface:|
|2.||Why did you predict as you did? What assumptions are guiding your
It is a good idea to ask students to discuss their predictions and the reasons for making them. This prompts the students to identify and examine their underlying assumptions about how water behaves.
|To Do||3.||Perform the experiment. Place several drops of water on each surface and draw the results below.|
|Interpret||4.||Compare your predictions with your observations and explain.|
On wax paper water binds to itself and not to the non-polar waxy surface, forming a little ball due to the cohesiveness and the resultant surface tension of water molecules. Some students will predict this effect (especially if they have polished cars or had similar experiences), but many will not. The effect is explained by hydrogen bonding among polar molecules and by the spontaneous separation of polar and non-polar molecules.
|5.||Can you explain the differences in drop behavior in terms of adhesion - that is, the formation (or absence)
of hydrogen bonds between molecules of
different types? Which molecules?|
Exercise 1c [Drop Shape on Glass and Wax Paper] demonstrates the interaction of water with polar (glass) molecules and non-polar (wax paper) molecules. Water will bind via hydrogen bonds to the glass slide and spread out in a flat puddle; the attraction between polar molecules of different types like this is called adhesion. In contrast, the water is not attracted to the wax paper.
The Climbing Property of Water
|Review||1.||Water moves to the tops of tall trees due to capillary action combined with root pressure and evaporation from the stomata (openings) in the leaves. Water will also climb up paper, and often the migrating water will carry other molecules along with it. The distance traveled by these other molecules will vary with their mass and charge.|
|Predict||2.||How fast do you think water would climb a strip of absorbent paper
about one-half inch wide? |
about one inch per ____________________ (time)
|To Do||3.||Obtain a 50 ml graduated cylinder, and tear off a strip of chromatography paper that is just long enough to hang over the side of the cylinder (inside) and reach to the bottom.|
Figure 6. 50 ml Graduated Cylinder with Chromatography Paper & Ink|
|To Do||4.||Run the paper strip along the edge of a scissors to take the curl out of it.|
|5.||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.|
|6.||Put 10 ml of water into the graduated cylinder and place the strip
of paper in the cylinder so that the bottom end is immersed in water and the
drop of ink is just above the surface of the water. Fold the paper over the
|7.||Note the starting time below.|
|Results||8.||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.|
|Question||9.||How did the ink change? Glue the paper onto the page here, and
label each color on the strip.|
With a water-soluble ink, such as that in a Vis-a-Vis pen, the dye in the ink is dissolved in the water. That is, the dye breaks up into its component polar molecules, and each molecule of dye is surrounded by and perhaps hydrogen-bonded with water molecules - an effect often described as the cage effect. The surprise in this experiment is that black Vis-a-Vis ink contains molecules of several different colors (this is not true of all black inks). Students will observe bands of colors formed as the water rises up the paper.
|Interpret||10.||How do you explain the results? Your explanation should involve
capillary action, polar molecules and hydrogen bonding.|
Hydrogen bonding and polarity also account for the ability of water to climb up a piece of paper hung vertically, telling us that, like glass, paper contains polar molecules. The flow of water up paper or through a polar tube is called capillary action. When water moves up paper or through a gel, the molecules that are dissolved in it are carried along with the water. Each type of molecule moves at a different rate depending upon its mass and its charge. Since the different types of molecules move at different rates and over different distances, the molecules are sorted into bands of similar molecule types. In this case, the different types of molecules are revealed by their different colors. This fascinating effect is widely used in biological research for separating, purifying, and identifying components in a substance. More often than not in research studies, the molecules themselves are not colored and dyes must be added in order to distinguish the bands.
Cohesion of Water
|3a||Water & Oil|
|To Do||1.||Put 8 ml of water into a 10 ml graduated cylinder.|
|Predict||2.||What will happen if you add cooking oil? (Predict by choosing a,
b, c, d, or e below)|
a. the oil will float on top of the water
b. the oil will sink to the bottom of the water
c. the oil will dissolve in the water
d. the oil will become mixed up with the water
e. other (what?)
|Oil is a hydrophobic or 'water hating' molecule, so called because its chemical
structure does not allow the formation of hydrogen bonds. Therefore, oil does not
dissolve in water. When mixed, the two substances form separate layers, and because
oil is less dense, it sits on top of water.|
|To Do||3.||Gently add 2 ml of cooking oil by tilting the cylinder of water slightly and letting the oil run slowly down the inside of the cylinder.|
Students should observe that added oil tends to float on the water. Sometimes a bubble of oil or all the oil will be trapped below water due to the cohesiveness of water, which forms a boundary that the oil has difficulty penetrating. However, if a glass stirring rod is used to disrupt the interface between the oil droplet and the water, the oil will rise to the top. This illustrates again the powerful effect of polarity, in which polar water and non-polar oil molecules clearly separate from one another and avoid interaction or intermixing. It also shows that oil is less dense than water, meaning that oil weighs less per unit volume (e.g., per cc) than water does.
|To Do||5.||Save this graduated cylinder with its contents and get a clean 10 ml cylinder for the next experiment.|
|3b||Oil & Water|
|To Do||1.||Place 8 ml of cooking oil in a 10 ml graduated cylinder.|
|Predict||2.||What will happen when you add water? (Predict by choosing
a, b, c, d, or e below)|
a. the water will float on top of the oil
b. the water will sink to the bottom of the oil
c. the water will dissolve in the oil
d. the water will become mixed up with the oil
e. other (what?)
|To Do||3.||Gently add 2 ml of water by tilting the cylinder of oil slightly and letting the water run slowly down the inside of the cylinder.|
The added 2 ml of water should collect at the bottom of the graduated cylinder, below the oil. Sometimes the water fails to penetrate the oil initially, but piercing the layers with a stirring rod will allow them to adjust, with the more dense liquid (water) going to the bottom.
|Question||4.||Which is less dense (that is that has less weight per ml.),
oil or water? ____________________ |
Oil is less dense than water.
|Interpret||5.||This characteristic behavior of water and oil is of critical
importance for living things, determining many properties of the cell.
Can you explain how?|
Cell membranes are composed of two layers of amphipathic phospholipid molecules. The fatty acid tails face one another, forming a non-polar membrane interior. The polar phospholipid heads form both the outer surface and the interior surface of the membrane. These polar surfaces interact comfortably with the watery environment inside and outside the cell. Because of this structure, every cell contains both polar and non-polar environments.
|Question||6.||What mechanism causes water molecules and oil molecules to
separate from one another? Your explanation should involve polar and non-polar molecules, the effects of polarity on the molecular interactions, and hydrogen bonding.|
|3c||Water, Oil, and Dye|
|Predict||1.||What will happen if you add a few drops of a water-soluble
dye solution to each of the above graduated cylinders containing water and oil.
Will the dye mix with the water, the oil, or both?|
Students should observe that when a water-soluble dye is mixed with water and oil, it will dissolve into the water layer and not the oil layer.
|To Do||2.||Perform the experiment. Add a few drops of dye to each cylinder.
Use a glass stirring rod to penetrate the interface between each layer, giving
the dye access to both water and oil. How does the dye behave in each cylinder?
Does it diffuse into the oil? Into the water?|
As noted above, if the dye is water-soluble, it will go into the water layer.
|Results||3.||Compare your predictions and results. Explain any differences.
Students should be encouraged to think and talk about the ways in which their observations differed from their predictions. When there is a contradiction, students often assume their mental model is correct and their observations were incorrect. Comparing observations made by all the students in the class helps to build a consensus model of what actually happened. If this consensus model of observations is different from students' predictions, then the students need to think about the assumptions they were making, if those assumptions need to be changed. and if so, how.
|To Do||4.||Stir the contents of each cylinder with a stirring rod and then let it sit.|
|Predict||5.||Will the contents remain mixed? Why do you think so?|
One can stir vigorously and to mix oil and water pretty thoroughly, as with salad dressing. When allowed to sit, however, such mixtures will separate once again, with oil and water each seeking their own layers.
|Interpret||6.||Observe what happens, compare with your prediction, and explain
why it happens. Your explanation should involve polarity, polar and non-polar molecules, solution and hydrogen bonding.|
Students should observe the oil and water separating, although the process may occur slowly. Again, this is a case of polar molecules seeking a polar environment and avoiding a non-polar environment. Any substance that can dissolve in water is polar.
|Predict||1.||Take a clean beaker of water. Predict what will happen if you add
one small drop of oil to the water using a medicine dropper.|
Exercise 3d [Sheen] illustrates that a small amount of oil will form a monolayer of molecules on the surface of the water, and that the monolayer can be seen as a sheen. This is what often occurs in oil spills, making a slick on the surface of the water that is difficult to clean up. The heavier oil in spills forms a coating on any animals that pass through it, often killing them because the coating restricts their movements and oxygen exchange processes, and its toxins are absorbed through their skins.
|To Do||2.||Do this experiment. Can you see the oil? Was your prediction
correct? Add more drops of oil if necessary to see it clearly. Describe. Your
description should focus on the separation of polar and non-polar layers and why
Students should see a sheen of oil sitting on top of the water.
|Predict||3.||What will happen if you add a drop of detergent to the beaker.|
The students may anticipate the disruptive effect of detergent, predicting that the surface layer of oil may be broken up into individual patches of oil. or disappear altogether.
|To Do||4.||Now add a drop of detergent to the beaker of water with oil on top. Record your results.|
|Interpret||5.||Compare your results with your prediction, and explain how the
detergent works in molecular terms. Your explanation should focus on the ways in
which amphipathic molecules disrupt cohesion.|
The disruptive effect of detergent may break up the sheet of oil into individual patches. This is likely because of the amphipathic detergent molecules interacting with both oil and water, diminishing their separation. The cohesion of water and the cohesion of oil are both reduced by the detergent.
|Interpret||6.||Explain some of the consequences of oil spills in the sea. What
effects do they have on sea life and bird life, and what methods are used to
'clean up' oil spills?|
A small amount of oil will form a monolayer of molecules on the surface of the water, and the monolayer can be seen as a sheen. This is what often occurs in oil spills, making a slick on the surface of the water that is difficult to clean up. The heavier oil in spills forms a coating on any animals that pass through it, often killing them because the coating restricts their movements and oxygen exchange processes, and its toxins are absorbed through their skins.
It is useful to provide time in
class for students to work in groups to answer all embedded questions. They
learn a lot from their discussions with one another. It is also important that
you discuss their findings and interpretations with the class as a whole,
eliciting their ideas and bringing the class to consensus on the scientific
For example, all
groups should obtain a similar pattern of color bands in the water-climbing
experiment with Vis-a-Vis dye. If any group has contrary results, it is worth
trying to figure out why. Sometimes students will use their own black pens, for
example, which may have different properties.
For example, all groups should obtain a similar pattern of color bands in the water-climbing experiment with Vis-a-Vis dye. If any group has contrary results, it is worth trying to figure out why. Sometimes students will use their own black pens, for example, which may have different properties.
|To Do||1.||Summarize class results with respect to drops on a penny in the
table below. |
When you compare drop data across groups, you will likely discover similar patterns (for example, many more drops without detergent than with) but a fairly wide range in the numbers of drops obtained. This is due in part to differences in drop size produced by different droppers. Variation may also be due to whether the penny is clean or oily and how the students handle the dropper.
|Interpret||1.||Explain the variation from group to group.|
|2.||What general conclusions can you draw from the class data?|
Whether molecules are polar or non-polar has a powerful effect on where they may be found, what kinds of molecules they interact with, and on the nature of those interactions.
|3.||Summarize the most powerful ideas (1 to 5) you learned in this
Elicit the main ideas from your students. Their list should look something like ours below.
Organizing Your Knowledge
|1.||Describe at least one observation you have made outside the laboratory that
illustrates each phenomenon below.|
Many different examples are possible. Here are our choices.
in all interactions involving water|
bonds - in all interactions involving water
|c.||Cohesion - the way water balls up on a polished car|
tension - when we can float cream on the surface of
when water saturates our clothes|
action - when a milk shake climbs up a paper straw|
|g.||Amphipathic - when detergent
and water cleans oil off our frying pan|
|h.||Dissolving - when coffee or tea or sugar
dissolve in water|
ice cubes floating in water (because ice cubes are less dense than
|2.||The table below summarizes nine phenomena associated with water across the top
and list the exercises we have performed down the side. For each exercise,
indicate which phenomena are illustrated.|
You can think about these things in different ways, so the list generated by your students may not be exactly the same as ours, but we would identify the following ideas as being illustrated by each exercise
|The following books support the California Department of
Education's Science Frameworks.|
Watson, Lyall. (1988). The Water Planet. Beautifully illustrated, this book discusses the physics and chemistry. New York: Crown Publishers.
Dorsey, N. Ernest. (1968). Properties of ordinary water substance in all its phases: water vapor, water & all the ices. New York: Hafner Publishing.
Wetlist (http://www.uwin.siu.edu/WaterSites/index.html) - Comprehensive Water Topics
|Chapter 5: THE LIVING ENVIRONMENT|
Section C: Cells
Grade K-2 (Benchmark 2 of 2)
Grade 3-5 (Benchmark 1 of 2)
Grade 6-8 (Benchmark 4 of 4) About two thirds of the weight of cells is accounted for by water, which gives cells many of their properties.
Chapter 8: THE DESIGNED WORLD
Grade K-2 (Benchmark 1 of 4)