Biology Junction Team - BIOLOGY JUNCTION

Water Properties Prelab

Properties of Water

Pre-Lab Questions:

1. Explain why water is referred to as the universal solvent.

2. What is the overall charge on a molecule of water?

3. Water is a polar molecule (appears to have a charge). Explain why this is so.

4. Which end of a water molecule “acts negative”? Which “acts positive”?

5. Is water the only molecule that is polar?

6. Explain what occurs whenever several water molecules are near each other in a droplet. Include a sketch of this.

7. The property of water molecules being attracted to other water molecules is called ________________.

8. Explain what causes water to have surface tension.

9. Surface tension causes causes water to _____________ on surfaces such as glass.

10. In order to clean a surface, what must happen to surface tension? What type of chemicals can do this? Give an example

11. Besides reducing surface tension, what 4 other things can surfactants perform?

Water Properties Handout

Physical Properties of Water

All of water’s unique physical properties are caused by water’s polarity.

Cohesion: water molecules stick to each other. This is caused by hydrogen bonds that form between the slightly positive and negative ends of neighboring molecules. This is the reason why water is found in drops; perfect spheres. It’s hard to imagine water behaving any other way.

Adhesion: water molecules stick to other surfaces. This causes water to move upward against gravity in plant stems and to be absorbed by paper towels. It also causes water to adhere to spider webs.

Surface Tension: water has the ability to support small objects. The hydrogen bonds between neighboring molecules cause a “film” to develop at the surface.

Organisms like the water strider can be seen taking advantage of this property.

4. Water has a high boiling point. Water is one of the few substances that remain a liquid at such a large range of temperatures (O-100 °C). A large amount of energy must be invested to overcome the hydrogen bonds in liquid water to change it to the gas phase.

Liquid Water Molecules with hydrogen bonds Water Vapor Molecules

5. Capillary Action: water has the ability to “climb” structures. Think about what happens when you stick the tip of a straw in a glass of water.

6. Ability to Dissolve: water is consider to be the universal solvent. More substances will dissolve in water than any other liquid. This includes other polar substances (such as sugar) and ionic compounds (such as salt).

When a sugar crystal is placed in water, the slightly positive and negative ends of the water molecule attract the sugar molecules in the crystal (they are also polar) and pull them into solution.

When an salt crystals are placed in water, the slightly positive and negative ends of the water molecules attract the ions in the crystal. The ionic bonds holding the sodium and chlorine ions together are broken and the ions are pulled into solution.

7. High Heat of Vaporization:

Water Properties

Water Properties
States of Water
Adhesion and Cohesion
Surface Tension
Capillary Action

The States of Water

Water has three states. Below freezing water is a solid (ice or snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a gas. There are words scientists use to describe water changing from one state to another. Water changing from solid to liquid is said to be melting. When it changes from liquid to gas it is evaporating. Water changing from gas to liquid is called condensation (An example is the ‘dew’ that forms on the outside of a glass of cold soda). Frost formation is when water changes from gas directly to solid form. When water changes directly from solid to gas the process is called sublimation.


Gas	Liquid	Solid

Most liquids contract (get smaller) when they get colder. Water is different. Water contracts until it reaches 4 C then it expands until it is solid. Solid water is less dense that liquid water because of this. If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the bottom up!

Water is found on Earth in all three forms. This is because Earth is a very special planet with just the right range of temperatures and air pressures.

Adhesion and Cohesion

Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion.

The oxygen end of water has a negative charge and the hydrogen end has a positive charge. The hydrogens of one water molecule are attracted to the oxygen from other water molecules. This attractive force is what gives water its cohesive and adhesive properties.

Surface Tension

Surface tension is the name we give to the cohesion of water molecules at the surface of a body of water. Try this at home: place a drop of water onto a piece of wax paper. Look closely at the drop. What shape is it? Why do you think it is this shape?

What is happening? Water is not attracted to wax paper (there is no adhesion between the drop and the wax paper). Each molecule in the water drop is attracted to the other water molecules in the drop. This causes the water to pull itself into a shape with the smallest amount of surface area, a bead (sphere). All the water molecules on the surface of the bead are ‘holding’ each other together or creating surface tension.

Surface tension allows water striders to ‘skate’ across the top of a pond. You can experiment with surface tension. Try floating a pin or a paperclip on the top if a glass of water. A metal pin or paper clip is heavier than water, but because of the surface tension the water is able to hold up the metal.

Surface tension is not the force that keeps boats floating. If you want to know why a boat floats look here: Why do boats float?

stability1.jpg (8249 bytes)
The key to floating is that the object must displace an amount of water which is equal to its own weight.

Capillary Action

Surface tension is related to the cohesive properties of water. Capillary action however, is related to the adhesive properties of water. You can see capillary action ‘in action’ by placing a straw into a glass of water. The water ‘climbs’ up the straw. What is happening is that the water molecules are attracted to the straw molecules. When one water molecule moves closer to a the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw. Capillary action is limited by gravity and the size of the straw. The thinner the straw or tube the higher up capillary action will pull the water (Can you make up an experiment to test this?).

Plants take advantage of capillary action to pull water from the into themselves. From the roots water is drawn through the plant by another force, transpiration.

Water Movement in Celery Stems

Water Transport in Plant Stems

INTRODUCTION:

As water evaporates from the leaves of a plant, more water is drawn up by osmosis from the tissues below to replace it. The replacement of water lost through transpiration is possible because water molecules have polar covalent bonds. This causes one end of the molecule to have a slightly positive charge and the other end to have a negative charge. Because of this, the water molecules act like “small magnets”. The positive end of one water molecule sticks to the negative end of another in a long chain that is pulled upward against the force of gravity.
When enclosed in a narrow tube, such as the transport vessels of a plant, water molecules can withstand a large force without being pulled apart.

MATERIALS NEEDED:

Celery stalk with leaves intact	Metric ruler
400-mL beaker	Distilled water
Glass bowl	Red food coloring
Razor blade	Stirring rod

PROCEDURE:

1. Fill the beaker with 100 mL of distilled water. Add drops of red food coloring, stirring with the stirring rod, until the water is a dark red color. Set this aside.

2. Put some distilled water in the glass bowl. While holding the bottom end of the celery stalk under water, cut off the bottom two centimeters of the celery stalk.

3. Quickly place the freshly cut celery stalk upright in the beaker of colored water. Record the beginning time on your DATA TABLE

4. Allow the celery to remain in the food coloring until the color is visible in the upper stem and leaves. Record the ending time on your DATA TABLE, and remove from the beaker of food coloring.

5. Measure the length the red color traveled up the celery stalk in centimeters. Record on your DATA TABLE

DATA TABLE:

Beginning time: ___________

Ending time: ___________

Length food color traveled up stalk. ______ cm

CALCULATIONS:

6. Calculate the number of minutes it took for the coloring to reach the top.

Time for color to reach the top of stalk. = _________ minutes

7. Calculate the rate of travel of the food coloring up the celery stalk in centimeters per minute.

Rate of Travel	=	length of celery stalk (cm) time for color to reach top of stalk (min)

Rate of travel = _________ cm / min

QUESTIONS:

1. What type of tissue moves water upward in a plant stem?

2. Name and explain 2 properties of water that enable it to move upward against gravity in a stem.

3. What is transpiration and where does it occur in plants?

4. How does transpiration help the upward movement of water?

Water in Carrot Lab

How Much Water is in a carrot?

Introduction:

Life exists on Earth because of the abundance of liquid water. Water makes up anywhere from 70 to 90% of the body weight of living things. Living things are composed of atoms and molecules within aqueous solutions (solutions that have materials dissolved in water). At most temperatures on the surface of the earth, water is a liquid. In this state, water is an excellent solvent, and because there is so much of it available on the earth’s surface, water is home (oceans, lakes and rivers) to much of life. Water has been referred to as the universal solvent. Water is also involved in many metabolic processes within organisms.

Water is a polar molecule and can bond both to itself and to other water molecules by weak attractions called hydrogen bonds. Hydrogen bonding is responsible for the unusual thermal properties of water including a high specific heat capacity and a high heat of vaporization.

Specific heat is defined as the amount of heat energy needed to raise the temperature of one gram of a substance 1°C. Since it takes much more energy that normal to break all the hydrogen bonds in liquid water, water resists rapid temperature fluctuations, adding stability to earth’s environments where liquid water is plentiful.

The heat of vaporization is defined as the energy needed to change the phase of a liquid to a gas. Again, because of the number and relative strength of water’s hydrogen bonds, it takes a great deal of energy to break a molecule free of its liquid partners. Heat of vaporization causes a cooling effect because as the warmer molecules evaporate from your skin they take the heat energy with them, leaving you cooler.

Objective:

Students will design and conduct an experiment to determine the amount of water present in a carrot.

Materials:

Some materials that will be available for you to use are plates, vegetable peelers, knives, graters, knee-hi stockings, foil, microwave, blow dryers, plastic bags, and paper towels. Any other materials you use must be approved by the teacher first (No dehydrators!).

Procedure:

Begin by weighing and recording the mass of the carrot.
Estimate the water content present in your carrot.
Develop a hypothesis for the amount of water in a carrot.
Write the materials needed and procedure you will be using to extract the water.
After having your hypothesis and procedure approved by the teacher, conduct the experiment.
Be sure to include an introduction, procedure, data, data analysis, and a conclusion in your lab report.