Lab 03 - Chem & Water

Biology 101 PCC - Cascade Lab 3: Chemistry & Water

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Lab Section: ____________________ Name: ______________________________________

Pre-lab Homework Lab 3: Chemistry & Water After reading over the lab and Chapter 2 in your textbook, answer these questions to be turned in at the

beginning of the lab!

1. Refer to the periodic table on the last page, and then provide the atomic number and the atomic mass

for each of the following elements. You may round off the atomic mass to one decimal place.

atomic number atomic mass

Calcium (Ca): ________ ________

Phosphorus (P): ________ ________

2. Give the total number of electrons and the number of electrons in the outermost shell (valence shell)

of each of the following elements.

Element

(symbol)

Total electons Valence

electrons

Element

(symbol)

Total electons Valence

electrons

Carbon (C)

Hydrogen (H):

Oxygen (O):

Nitrogen (N

3. The element fluorine (F) has the atomic number of 9,

However, it has an approximate atomic weight of 19!

Give the total number of each of the following particles fluorine in its neutral state. (recall the atomic

weight is due primarily to the weight of the protons plus the neutrons, each of which weigh 1 )

# protons = ____ # electrons= ____ # neutrons = ____

4. Match each description ion the left with an item in the right-hand column by placing a letter in each

blank.

a. A weak attraction between the positive region of one molecule and

the negative region of another molecule.

(Ex. between two water molecules.)

____ covalent bond

b. An attraction between a particle that has a full positive charge with

another particle that has a full negative charge.

____ polar

c. An attraction where electrons are shared .

____ ionic bond

d. The uneven distribution of electrons in a single molecule, producing

regions of partial negative and regions of partial positive charge.

____ hydrogen bond


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Intentionally left blank


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Name: _______________________________________ Date/Lab time: ___________________

Lab 3: Chemistry and Water

LAB SYNOPSIS:

We will review characteristics of atoms, elements and chemical bonding.

We will be exploring several unique properties of water resulting from its polar nature and

H-bonding including:

o Specific heat, its polar nature, cohesion, surface tension, adhesion, capillarity &

density.

OBJECTIVES: After successfully completing this lab a student will be able to:

Know the location and electrical charge on protons, neutrons and electrons

Explain how atomic number and atomic weight are determined

Use the periodic table to identify atomic number and atomic weight of several elements

Define ionic bond, covalent bond and hydrogen bond

Define specific heat

Relate the high specific heat of water to its property of hydrogen bonding

Define water cohesion and relate it to the surface tension of water

Define adhesion and cohesion and relate them to capillarity

Explain why water is more dense at 4oC than at 0oC

Overview:

92 elements exist naturally. Only about 25 are found in living organisms. Why are certain elements

consistently associated with life processes while others are not? The answer can be found in the

structure of atoms. This atomic structure determines the relationships between atoms (the interactions

necessary for life).

Atomic Structure

Matter- Anything that has mass and takes up space.

All matter is composed of atoms..

Note the form of the atom helium (figure 1), the atom’s nucleus with orbiting electrons.

Table 1. Characteristics of subatomic particles

Particle and Symbol Charge Mass (Daltons) Location

Electrons e- - 0* Orbits nucleus

Neutrons n None 1 Nucleus

Protons p + 1 Nucleus

*electrons have a small mass = 1/1,837 that of a proton

To aid in understanding atoms and elements, refer to the periodic table (last page)


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Atomic number- is the number of protons in an atom.

For example, helium has 2 protons thus its atomic number is 2. The number of protons is usually the

same as the number of neutrons and the number of electrons. The number of protons determines the

element. For example boron has 5 protons, sulfur has 16 protons etc. (see periodic table)

Elements- A pure chemical substance composed of atoms

Atoms come in 92 natural forms based on their number of protons (see periodic table). These include

many familiar elements and their chemical symbol, iron Fe, carbon C, gold Au, potassium K, etc.

Isotopes- Atoms having the same number of protons, but differing numbers of neutrons.

Although all helium has 2 protons, the number of neutrons can vary. Helium has eight isotopes, 2He, 3He, 4He, 5He, 6He, 7He, 8He, 9He and 10He. The superscript indicates the number of protons plus

neutrons. Ex. Helium always has 2 protons. So, 5He has 2 protons and 3 neutrons.

1. How many protons and neutrons does isotope 8He have? ________ protons and _______ neutrons.

Atomic mass (weight) - is the mass of all the subatomic particles in an element.

The atomic mass is determined by averaging the various ratios of the isotopes for the element. The

periodic table indicates He has an atomic mass of 4.0026 (see last page). This number is due to

averaging all the relative weights of the eight He isotopes.

Electron orbital shells.

Electrons orbit their nucleus in defined orbitals called orbital shells. Rules apply to where these

electrons can be. Figure 2 shows this rule. The first shell holds a maximum of 2 electrons, the 2nd shell

can hold up to 8, the 3rd shell also can hold up to 8 electrons. This is the 2,8,8 rule

Figure 2. The first 18 elements and how electrons appear in orbital shells.

The Noble Gases

Note from above that helium, neon and argon all have full outer shells of e-. Having a full outer shell

means, these elements are stable; they do not form chemical bonds. All the other elements do not have

full outer shells, thus they will form chemical bonds (they are reactive).

Noble gasses


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Example of a reactive element: Carbon has 6 protons, thus it has an atomic number of 6. It has 6

electrons; 2 in the 1st shell, 4 e- in the second shell (table 2)

Table 2. Electron orbital shells for carbon

Shell Maximum # electrons Ex. carbon (atomic # 6) Ex. carbon (atomic # 6)

1st shell 2 2 Full

2nd shell 8 4 Not full

3rd shell 8 na na

Carbon only has 4 electrons in its 2nd shell. It “wants” 4 more electrons to get a full outer shell. Therefore,

carbon is reactive. It will undergo chemical reactions to get the 4 more electrons it “wants”.

There are 2 ways elements can get the full outer shells they “want”.

1. Ionic bonding

2. Covalent bonding.

1. Ionic bonds- form between oppositely charged atoms (or molecules).

For example in the formation of NaCl (table salt) (Fig. 3)

Initially Sodium (Na) has only one e- in its outer shell. It “wants” to have a full outer shell of e-.

Initially Chlorine (Cl) has 7 e- in its outer shell, one more e- would give it the 8 e- it “wants”.

Chloride can easily take Sodium’s 1 outer shell electron (Fig. 3).

Note: following e- transfer both sodium and chloride have full outer shells of electrons.

Figure 3. Formation of Na+ and Cl- ions following e- transfer.

Following electron transfer, these atoms become charged, they become ions.

Following electron transfer

Element (symbol) Atomic # # + protons # - electrons Net charge

Sodium (Na) 11 11 10 1 +

Element (symbol) Atomic # # + protons # - electrons Net charge

Chloride (Cl) 17 17 18 1 -

Ions- An atom (or molecule) that has lost or gained one or more e-, giving the atom an electrical charge. Ex.

Positive charged ion of sodium (Na+) and negatively charged ion of chloride (Cl-).

Ionic bonds- Form between the oppositely charged Na+ and Cl-

Salts- Molecules held together by ionic bonds ex. NaCl


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2. Covalent bonds- Form when elements share pairs of electrons.

Ex. Covalent bonds in a methane molecule, CH4

Carbon has 6 e-. 2 e- in the first shell, 4 e- in the second shell. Carbon “wants” 4 more e-

Hydrogen has 1 e-. Hydrogen “wants” 1 more e- to have a full outer shell.

Figure 4. Formation of CH4 (Methane)

Note following the chemical reaction C + 4H → CH4

The carbon has 8 e- in its outer shell (full)

Each hydrogen has 2 e- in their outer shell (full)

The general shape of methane is tetrahedral.

Methane is not charged.

Methane is not polar. See below for what a polar molecule is.

________________________________________________________________________________

Ex. Covalent bonds in a water molecule, H2O Oxygen has 8 e-. 2 e- in the first shell, 6 e- in the second shell. Oxygen “wants” 2 more e-

Hydrogen has 1 e-. Hydrogen “wants” 1 more e- to have a full outer shell.

Figure 6. Formation of H2O (Water)

Note following the chemical reaction O + 2H → H2O

The oxygen has 8 e- in its outer shell (full)

Each hydrogen has 2 e- in their outer shell (full)

The general shape of water is tetrahedral.

Water is not charged.

Water is polar.


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Figure 8. H-Bonding B/T Water Molecules

Why is water polar?

1. Tetrahedral shape. Due to the shape of electron orbitals, the 2 hydrogen atoms

cannot be on opposite sides of each other.

2. Oxygen has a vary strong pull on electrons.

Hydrogen side is mostly + charged protons.

Oxygen side is mostly – charged electrons.

Hydrogen bonds-

Due to water’s polar nature, it can form opposite charge attraction forces

with other water molecules. These are weak compared to ionic bonds. Since

these attractions always involve a hydrogen, they are termed hydrogen bonds.

A single drop of water has 2.7 x1019 water molecules. Each water molecule can

form 4 H-bonds. That is 108,000,000,000,000,000,000 H-bonds per drop.

Water is one of the most abundant molecules on earth. All life as we know it

require water. Water is a solvent for hydrophilic molecules thus most

metabolism occurs in water. Water also has unique properties resulting from its

polar nature, and its ability to form H-bonds. We will explore some of these in

the following exercises.

Exercise 1: Atomic Structure

Use the attached periodic table (Fig. 2 & last page) and what you know about atomic structure to

determine the atomic symbol, atomic mass, number of electrons in the outer shell and whether or not the

element would be reactive.

Atomic

number

(# of

protons)

Element

(symbol)

Atomic

mass

Number

of e- in

outer

shell

Reactive?

Yes or

No

Atomic

number

(# of

protons)

Element

(symbol)

Atomic

mass

Number

of e- in

outer

shell

Reactive?

Yes or

No

1 6

2 Helium

(He)

No 7 Nitrogen

(N)

3 8

4 2 Yes 9 7

5 10


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Exercise 2: The Polar Nature of Water (Demo)

Your instructor will demonstrate the polar nature of water by rubbing a plastic rod with fur or wool

and then place this rod next to a thin stream of water. By rubbing the rod with fur or wool, your

instructor is creating a negative charge on the rod.

1. What did you observe?

2. Based on what you have learned about water molecules, describe what is happening.

Exercise 3: Specific Heat of Water Vs. Oil You should start this activity first! While you are recording cooling data, you can move on to the next

exercises in the lab.

Specific Heat- The amount of heat energy to raise the temperature of a substance. Also, relates to the

amount of heat energy stored.

The specific heat of water and oil means these fluids are able to “store heat”. This can be

demonstrated by measuring the rate of cooling. For water, the same hydrogen bonds that oppose

molecular motion upon heating resist the removal of energy upon cooling. In this experiment, the rate

of cooling of water and oil will be compared.

READ THROUGH ALL INSTRUCTIONS BEFORE BEGINNING THIS PROCEDURE.

After reading through the setup, what do you predict will happen? Which, water or oil will cool more

slowly (i.e. which has a higher specific heat)?

1. From the 75°C water bath, obtain one Erlenmeyer flask of hot water and one of hot oil.

(75° is hot so handle with care).

2. Return to your work station. Work quickly and record cooling

temperature in the table below. You will record temperature every 2

minutes for a total of 30 minutes.

3. When done, return flasks to the75°C water bath.


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Time (min.) Water Temp (oC) Oil Temp (oC)

Initial temp

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

Construct a line graph of your results below: (for clarity: use two different color lines).

1. Explain your results. Did results support your prediction? Why? Why not? Explain using the concept

of water’s hydrogen bonding.


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Figure 9. H-Bonding Forming Surface Tension

Water’s Cohesive Properties

Cohesion- water sticks to itself

Due to its extensive H-bonding, water is cohesive. The strength of cohesion allows water to be

pulled all the way up tall trees without breaking apart. Cohesion also allows water to form droplets.

Surface Tension- The extensive H-bonding at the surface of

water producing a surface that resists breaking.

Due to cohesion, water form an air/water barrier. Force is

required to break the H-bonds in order to penetrate into water.

Surface tension is surprisingly strong-it can support water

striders (medium-sized insects that stand on the surface of

relatively still waters). Surface tension would cause human

lungs to collapse if it weren't for the presence of a soap-like

material (surfactants), which disrupts the surface tension.

Exercise 4: Surface Tension

To demonstrate surface tension you will sit a paperclip onto the surface of water by performing the

following:

1. Get a Petri dish and rinse well with water. Fill ~3/4 full with tap water.

2. Float a piece of paper towel or lens paper (a little bigger then your paper clip) on the water and then

carefully place a well rinsed paper clip on the paper towel.

3. Submerge the paper using two toothpicks. What did you observe happen to the paperclip?

4. Add a drop of diluted detergent as far away from the paper clip as possible. What did you observe?

Keep adding drops till the paper clip sinks. Why might this have happened? Relate this to the figure.

Hint: Detergents are long molecules. Part of the detergent molecule is polar, while the other part is non-

polar (see figure below).


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Tube # Diameter (narrowest

to widest)

Height of water

(mm)

1 0.5 mm

2 1.0 mm

3 1.5 mm

4 2.0 mm

5 18.0 mm

Water’s Adhesive Properties

Adhesion- Water sticks to other materials

Water’s H-bonding makes it sticky to other types of molecules. You observed this when you

measured water in the metric system lab. Water tended to stick to the sides of graduated cylinders

forming a meniscus. In nature, this allows for water droplets to cling to things; leaves, spider webs,

etc.

Exercise 5: Adhesion

To demonstrate the property of adhesion, perform the following:

1. Obtain two new microscope slides or wash two microscope slides until they are “squeaky-clean”

and dry them.

2. Place a single drop of water on one slide and sandwich it with the other slide.

3. Now, attempt to pull the two slides straight apart without sliding them apart.

4. The two slides are held together by the hydrogen bonding of water to glass!

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Water’s Adhesive and Cohesive Properties

Capillarity- wicking of water due to both water’s adhesion and cohesion

properties.

How is water held in soil? Water moves into fine fibrous materials

and packed soil particles due to capillary action. This is the force behind

the movement of water into a paper towel, plant roots and of water

soaking into seeds. The climbing and pulling movement of water stops when the weight of the water

balances the capillary force. Capillarity also allows many insects and tree frogs to use specialized pads

that employ a water film to stick to smooth surfaces.

Exercise 6: Capillarity

Capillarity may be demonstrated as follows:

1. Observe the capillary setup containing

water and food coloring.

2. Note the diameter of each of the 5 tubes

containing fluid. (i.e. narrowest to widest)

3. Measure the height of the colored water

column in millimeters for each capillary tube

and record in the table. Recall there are 10

mm per cm. Make sure you measure in mm!


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Now graph the results as a bar graph:

1. What are your conclusions? Which diameter of tube had the strongest capillary action?

Based on these results, which type of soil would hold water more strongly: soil with small particle size like

clay soils, or soil with larger particle sizes like sandy soils?

2.What are the two properties of water that result in the observation you made on capillary action? (refer to

capillary section above)

Note: capillary action pulls water up the tubes against gravity!

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Exercise 7: Temperature and Density

Ice floats- When water freezes all H-bonds become fixed, forming an ice crystalline lattice that is less dense

then liquid water. Thus ice floats. In most other types of materials, the solid form is denser than the liquid

portion (ex. Wax, butter, gold etc.). Thus the solid forms sink.

The densest form of water is at 4°C (39°F). At warmer or colder temperatures water is less dense. A

consequence of this is that warm and cold form layers in lakes and ponds.

We will demonstrate how temperature affects water density and layering below:

Procedure:

Supplies: 3 large test-tubes, 150ml beaker

1. Make an ice bath by placing ice into a 150 ml beaker.

2. Get cold blue food colored water (~4°C) from the ice chest, half fill two large test-tubes

and put into your ice bath.


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3. From the warm water bath, bring the whole bottle of yellow food colored warm water (~30-35°C).

4. Using a transfer pipette, GENTLY layer the warm yellow water onto the cold blue water in one of the

test tubes from your ice bath. ADD THIS GENTLY OR IT WILL NOT WORK. Leave your other

blue test-tube on ice.

5. Although you will need it again later, return the bottle of yellow food water to the warm water bath.

6. Remove that test-tube from your ice bath and let it come to room temperature. Observe occasionally and

note what happens.

1. Draw or describe what happened when you try to layer warm water onto cold. What color is the water

between the layers, and how broad is this region?

2. If you have ever gone swimming in natural bodies of water, like lakes, you know summer time warm

water floats on cold water. This layering is called thermos-stratification, separated by a thermocline, the

layer that separates the warm and cold water.

3. What happened to your test-tube as it all came to room temperature?

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Now you will attempt to layer cold water onto warm.

1. Half-fill another test-tube with the yellow food colored warm water (if this has cooled off you may need to

get a new batch).

2. From your other test-tube still on ice, draw a transfer pipette full of cold, blue water and GENTLY layer

the cold water onto the warm, yellow water. Try to do this gently.

1. Draw or describe what happened when you try to layer cold water onto warm.

How is this similar to what happens in lakes as weather changes. (i.e. what do you think happens during

autumn as warm water in lakes cools)?

1. Ice is less dense then liquid water and so floats. Why is ice less dense? (relate your answer to H-bonds)

2. What would happen to our planet if ice didn’t float? (what would happened during winter then summer?)


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QUESTIONS on water: (this can be done in lab or as homework)

1. Draw two water molecules interacting. Name each atom. Label areas of the molecules that have partial

charges with a plus (+) or minus (-) sign. Show covalent bonds with a single line between atoms, and

hydrogen bonds as a single dotted line between atoms.

2. Temperatures in coastal areas, like in Newport are more stable than temperatures

east of the Cascade Mountain, like in Bend because:

a. the climate is drier east of the Cascades

b. hydrogen bonds are present in the ocean

c. the coast is usually cloudy

d. the ocean absorbs and releases heat more slowly than the land

e. all of the above.

3. Two properties of water contribute to the formation of a meniscus. One of these is

that water molecules attract each other (cohesion). What is the other property? (Use the word for this

property OR describe it).

4. What property(s) of water causes it to “bead up” on a window pane or leaf? (Use the word for this property

OR describe it). See figure 9.

5. Water is more dense at 4 C than at 0 C. This is because: (check exercise 7)

a. at 4C water molecules slow down and can get close together as hydrogen bonds form and reform.

b. water molecules are farther apart at 4C than they are at 0C

c. at 4C, all four hydrogen bonds form simultaneously between each molecule and the next

d. b and c

e. none of the above


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Lab 03 - Chem & Water

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