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Kajal Laroia

on 21 February 2015

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Transcript of Water

Properties of Water Lab
Properties of Water
1 - Cohesion
2 - Adhesion
Properties of Water
Kajal Laroia
1) Solid (ice)
2) Liquid
3) Gas (vapor)
What is Water?
Molecule made up of two hydrogen atoms and one oxygen atom (H2O)
Forms a bent-shaped triangular molecule
Oxygen atom holds a small negative charge (dipole) and two hydrogen atoms hold small positive charge (dipole-dipole force)
Attracted to each other creating hydrogen bonds
Intermolecular bonds determine many physical and chemical properties of water
States of Water
About 70% of our total body weight is water
The physical and chemical properties of water have permitted living things to appear, survive, and evolve on this planet
Water is the universal solvent
The medium and the participant in most of the chemical reactions occurring in our environment
Water can dissolve more substances than any other liquid
Ex. sodium chloride (NaCl) dissolves in water very easily when placed in water; molecules fall apart
Positively charged sodium ion (Na+) binds to oxygen, while the negatively charged ion (Cl-) attaches to hydrogen
This property of water allows transportation of nutrients vital to life in animals and plants (a drop of rainwater falling dissolves atmospheric gases)
1) Surface Tension - H2O molecules held closely together
2) Cohesion - H2O molecules stick together
3) Adhesion - H2O molecules stick to other substances
4) Solvency - capability to dissolve
5) Thermal Properties - absorbs or releases more heat
6) Specific Heat - amount of energy required to raise temperature of water by one degree Celsius
7) Heat of Vaporization - H2O has highheat of vaporization
8) Boiling and Freezing - pure water boils at 100C, freezes OC
9) Water Density - H2O most dense at 4C
10) Solid Expansion - H2O less dense as a solid (ice floats)
11) pH - H2O molecules ionize - dissociates into +ve and -ve ions
Water molecules are sticky! Due to the hydrogen bonds among the molecules
H2O molecules at the surface have a greater attraction for each other than for molecules in the air
Cohesiveness creates a high surface tension at the surface of the water
H2O molecules at surface crowd together; create strong layer as they are pulled downwards by the attraction of other H2O molecules beneath them
H2O molecules stick to other substances!
Ex. when water creeps up the the inside of a graduated cylinder; a paper towel used to "soak up" spilled water, this is how water makes things wet
Water clings to living things
Most plants have adapted to take advantage of water's adhesion that helps move water from the roots to the leaves; capillary action
- Quarter
- Eye Dropper
- Water
- Graduated Cylinder
Part 1: Cohesion
Procedure & Observations
18 # drops = 1 cc
1 drop = 0.05556
1. Using a dropper count how many drops it takes to get 1 mL in the graduated cylinder.
2. Record the number of drops from your dropper it takes to produce 1cc (cubic centimeters) of water, and the amount of water per drop.
3. Predict how many cc's fit on the surface of a quarter. Each person should guess.
Number of Drops
Person 1 (Kajal)
Person 2 (Moe E.)
Person 3 (Hafsa)
Person 4 (Meghan)
4. Carefully squeeze droplets of water onto the quarter. Count carefully up until the point the quarter overflows. Record the number of drops.
Results: Number of drops to fit on a penny : 49
A quarter holds 2.72 cc's of water.
5. If the number of drops was different than your prediction, explain your results using the properties of water.
My entire lab groups' predictions were different than the actual result (including mine). We assumed that one single drop would spread among a very small surface of the quarter and eventually overflowing after a large number of drops, which is what we observed, however, our predictions were a little emphasized. Due to water's property "Cohesion", the water molecules have an attraction of sticking to other water molecules when they come in contact. They stick together because it is a polar molecule, this means that there is a positive charge (hydrogen) and a negative charge (oxygen) of the molecule. The negative charge on the oxygen attracts the positive charge on the hydrogen's of neighboring molecules. This causes the molecules to "stick" together forming the "dome" of droplets that allows the consequent drops to stick to the quarter with an excess amount and without flowing over. The increasing number of drops occurs due to water's strong intermolecular forces, that attract each other through the opposite dipoles between the molecules. Additionally, the actual number of drops (49) was different than my prediction (75), the actual number is a lot less since their was not enough surface area as I thought to hold the water; the water droplets in my prediction are excess then the actual surface area. Their are less cohesive forces on the edges of the coin because of the "film" the water forms within the first few droplets. This film allows following droplets to build on top until it reaches a certain point where the coin only has so much area for water to pile upon.
6. Dry the quarter using a paper towel. Put a small dab of detergent on your finger and rub it on the surface of the quarter. Repeat steps #4-#6. Explain your results.
Result: Number of drops to fit on a quarter: 11
A quarter holds 2.324 cc's of water.
Also, dish detergent limits water's cohesive properties. The cohesiveness is weaker because detergent is amphipathic. Meaning it is both hydrophobic & hydrophilic.The quarter is incapable to form the dome of water that sits on the surface of the quarter without soap. Additionally, the dome is incapable of forming the film that is normally formed because of the lack of surface tension. The soap creates bubbles (air pockets); droplets depend on a stable base to stand on or stick to, which is why the droplets collapsed a lot sooner with the dish detergent than without.
-Eye dropper
-Wax paper
-Glass Slide
Procedure & Results
1. Predict the shape of water droplets placed on each media below by drawing the droplets.
2. Carefully drop an equal number of droplets onto each surface medium, and draw the results below.
3. Explain your results.
The concept of water being adhesive is when water molecules stick to other substances, this attraction also relies on the polarity between substances. Our predictions were quite accurate to the final results due to the different materials that water interacts with. During part 1 of the lab, we discovered that a drop of water on the quarter did not spread much and stayed in it's place due to cohesion. When we conducted this experiment we realized that the water drop in fact did not spread among the entire coin.
Therefore, water has stronger adhesion forces towards the quarter and lesser to glass due to their surface tension that results water to make a flatter drop sticking to the surface. At this point, bonding and potential energy occurs, their is too much contact between the glass and water; area between glass and water is so maximized that it causes the water to spread out. Whereas wax paper results water to being cohesive to it's self forming a round drop (greater surface tension). Also, wax has an oil layer; lipids, which are in insoluble in water and do not interact (hydrophobic) leaving the droplet in its spheric shape.
- Water
- Graduated Cylinder
- Stop Watch
- Pencil
- Chromatography paper
- Overhead Marker
- Tape
- Scissors
Procedure & Results
1. Predict the amount of time it will take to the water to creep up a strip of filter paper 2.5 cm wide?
- I, Kajal predicted that it would take about 15 minutes to creep up the strip of filter paper
2. Trim one end of your filter paper so that it ends in a triangular point.
3. Obtain a 25 mL graduated cylinder, and using the pencil and tape attach the squared end of the paper to the pen so that the paper "hangs" down with the tip just touching the bottom of the graduated cylinder.
4. Remove the paper and carefully make a solid dot with the marker at the point shown below.
5. Place 5 mL of water into the graduated cylinder. Place the paper carefully, but quickly, into the cylinder. Start the timer the moment the paper tip touches the water.
6. Note and record the Time.
Time (min)
41 ml
0 ml
45 ml
53 ml
60 ml
47 ml
7. When the water climbs to the top of paper. Stop the time, remove the paper and carefully lay it on your table to dry.
8. Explain what happens.
At 0 minutes, the water did not move. At 5 minutes the water traveled a distance of 41 ml. After another 5 minutes, by the time it reached 10 minutes, the water traveled a total distance of 45 ml. At 15 minutes, the water reached a total distance of 47 ml and at 20 minutes it reached 53 ml. Finally, by the time 25 minutes passed, the water traveled to a total distance of 60 ml. Therefore, we realized that the water rised the fastest up until the first 5 minutes and then decreased it's speed as each 5 minutes passed, with a 3-7 ml difference.
Discussion Questions
1. What would the effect on the properties of water molecules be if oxygen and hydrogen had equal electronegativity?
2. Explain how detergents act as cleaning agents.
3. A water strider's legs are coated with a hydrophobic substance.
a) What might be the benefit?
b) What would happen if the substance were hydrophilic?
4. Use the properties of water to discuss the problem of plants, photosynthesis and gravity.
As for the glass slide, we predicted that the water would spread among a larger surface than the coin due to it's flat surface and minimal surface tension. This prediction was correct, the water spread considerably even and laid evenly on the slide. Finally, for the wax paper, our prediction was that the entire molecule would spread (disperse) and not stick to the wax material. However, in the end we realized that although the water did not stick to the wax paper, it's shape was smaller and shaped as glob of water sticking to it's own molecules rather than the wax. In no way interacted with wax, completely stayed in its own shape.
When we tilted the materials to see how water attracted to the materials, we realized that the water's molecules spread slowly together but left a trace behind on the quarter and glass slide. However, since wax paper and water have different polarity, water being polar and wax paper being non-polar, the adhesion forces between them does not exist because wax is once again, insoluble in water.
My group and I predicted that the quarter would be able to hold less droplets of water, due to the space the detergent is taking up. Moreover, due to the detergent's types of molecules being mostly non-polar, this causes a lower resistance for a formation of water to occur. Considering that soap is hydrophobic, this would cause each water molecule to repel and spread apart, resulting in the water to have weak intermolecular forces. As a result of having less surface tension, the water was not able to pull the top water molecules downward as strongly, causing the molecules to spread further apart and overflow instead of keeping its dome shape.
If oxygen and hydrogen had an equal electronegativity difference between the hydrogen and oxygen bonds being zero, this would make the bonds a "non-polar covalent bond." One of the main properties of water is cohesion, the water molecules stick to each other due to intermolecular bonds. These intermolecular bonds occur between the partial negative oxygen atoms and the partially positive hydrogen atoms. These bonds are also known as hydrogen bonds. The water molecules stick to each other due to the intermolecular bonds that occur. With oxygen having an electronegativity of 3.5 and hydrogen having an electronegativity of 2.1, the oxygen atom has the strength to pull in electrons from the Hydrogen. However, when having the same electronegativity, both the oxygen and hydrogen would not have a partial charge and there would be no attractions. Without these bonds that occur as a result of the partial charges on the Hydrogen and Oxygen atoms, then water will no longer be able to stick to each other losing it's cohesive property. Also, water wouldn't be able to dissolve polar solutes as well.

a) The benefit of the water strider's legs being coated with a hydrophobic substance is that this creature doesn't not drown and has the ability to walk on water. Considering that the substance on the insect's legs is non-polar means that it repels water. Due to it's solvency and not being polar, the opposite substances allows the water's surface tension to not provide an adhesive property to the water strider, which allows it to walk on the water without sinking.
b) If the legs were covered with a hydrophillic substance (water loving), then the water strider would not be able to walk on the water and would drown. Due to being a polar substance, the water has the adhesive property of dissolving the polar substance because of it's concept "like dissolves like."
The properties of water are related to the problem of plants, photosynthesis and gravity. They are affected by water through cohesion and adhesion. Water is known as the "universal solvent", meaning that it dissolves most substances based on it's solvency and properties. So the nutrients in the plant are dissolved in water. The strong cohesive and adhesive properties of water allow the nutrients to move up through the root to the plant, nurturing it's leaves and allowing it to grow while acting against the force of gravity. Water's cohesive property allows the molecules to stick to each other. It's adhesive property allows the water molecules to stick to other substances. Due to these properties, water clings to living things, allowing the water in plants to go against gravity and move up from the roots to the leaves during the process of capillary action (he ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity).

In the photosynthesis case, adhesion and cohesion are contributors because they move water throughout the plant. However, carbon dioxide is also needed in order to allow this process to continue, along with lighter energy and water to produce the oxygen gas and glucose. If adhesion and cohesion inside the water molecules was not there, then it would not be able to allow the water to transport nutrients through the water to reach the cells that need it for functioning.
Overview of Topics
Capability of dissolving another substance
Solute- Substance being dissolved
Solvent- Substance is which solute is dissolved
Ex. Water (solvent) dissolving sugar (solute)
Drop results vary on how steady the person's hand is, or how large the dropper is, as well as the force at which the drop hits the dome.
Additionally, tension is created as the water molecules at the surface are pulled into the body of the water since the most surface tension, cohesive forces at present in the centre which allows the dome to form. The tension at the surface produces strong layers as molecules are pulled downward by the attraction of the other molecules underneath; molecules that are closer together have a stronger force of attraction. Also, drops on top stick to like molecules which forms "film" on the surface. Thus, forming the dome shape. The reason for the dome shape is because the water droplets adhere to the like molecules present in the center since their are less like molecules on the edges; there is more cohesive forces and surface tension located in the middle between the like molecules causing the dome to be the highest in the middle. This tension causes water to bead up on surfaces (glass, fabric), which slows wetting of the surface and inhibits the cleaning process. You can see surface tension at work by placing a drop of water onto a counter top. The drop will hold its shape and will not spread.

Further, we noticed that as the water traveled up the filter paper (adhesion; water sticks to paper and rises), the coloured dot that the water interacts with raises up along with the water's direction (upwards). Due to the overhead marker's solvency and being a polar solvent, the water molecules dissolved the substance as it travels up. It is evident through the paper experiment that the marker is a polar substance due to dissolving, this lies under that concept that water being polar dissolves other polar substances, following "like dissolves like."
Additionally, water rises up the coffee filter because of a phenomenon called capillary action. It is the same action which a tree uses to soak water up its trunk and its branches.
Works Cited
The separation of the different dyes along the paper increases if the carrying liquid is allowed to carry the ink up a longer distance. At the beginning of the experiment, the colors are still bundled together. As the carrying liquid moves up the paper, the separation becomes more and more complete. Note, however, that the order of the colors does not change with the distance traveled. Also, the most soluble dye will move up the paper the most and the least soluble dye will move up the least.
Water is a polar molecule and they are attracted to each other (cohesion). Oils are nonpolar, so they don't mix with water, so the water can't wash it away. Detergents have both properties--polar on one end, nonpolar on the other (amphipathic). The nonpolar end dissolves into the grease and the polar end dissolves into the water. When you agitate this mixture, the grease form smaller and smaller droplets and detergent molecules surrounds the grease droplets, preventing them from reforming into larger droplets (this is why it emulsifies). Now because the grease is surrounded by polar detergent molecules, they are easily washed away by water. Referring back to the concept of "like dissolves like", the non-polar oils and greases do not dissolve in the polar water. Since water is unable to dissolve grease/stains/oils, detergents take this role of being surfactants that lower the tension of water by making it more adhesive. Detergents consists of hydrocarbon chains that are polar on one end and non-polar on the other end. Through their structure, the polar ends mixes with water and the other end mixes with the oils; combing these substances. The oil molecules are then reduced by size when the detergent molecules surround them; leaving polar detergent molecules that are easily washed by water. Therefore, through detergent's complex structure, it is able to bind water in cleaning and interacting with oils/grease/stains.

Detergents also takes away the cohesive properties of water. If you put water into a glass until it bulges up at the top and then put detergent in, you'll lose the bulge and the water will spill. I'm not sure how to explain this molecularly nor how this relates to cleaning properties.
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