Introduction: (Initial Observation)
I have seen leaking faucets and leaking roofs many times. It is still amusing and interesting to see how the drops of water form, slowly grow, and finally fall.
Sometimes, I look at them for a long time and try to predict the exact moment that each drop will fall. Also sometimes, water drops remain hanging there for a long time and make me wonder what is holding them? Why doesn’t the drop fall?
Obviously, there is some sort of stickiness between the water molecules and the leaking object that keeps the water drops hanging there.
Later in a camping trip I saw mosquitoes that land on the water. I could see that their weight has actually pushed the water surface inward. I wondered why they didn’t get wet and why their legs didn’t go under the water.
My final observation that I think has something to do with the tension between molecules is when I touched some paint and I had to wash my hands with some solvents. I immediately noticed that the layer of solvent on my hand is much thinner than the layer of water.
This project is an opportunity to research on the tension between molecules of various liquids.
Information Gathering:
Find out about the tension between molecules of different liquids. Read books, magazines or ask professionals who might know in order to learn about the surface tension, its causes, properties, hazards, and benefits.
Keep track of where you got your information from.
Following links offer you valuable information about surface tension.
http://www.exploratorium.edu/ronh/bubbles/sticky_water.html
http://www.brantacan.co.uk/surface_tension.htm
http://antoine.frostburg.edu/chem/senese/101/liquids/faq/tates-law.shtml
Structure, Intermolecular Forces and Solubility (addendum)
Polarity:
Several experimental measures of solvent polarity exist; these include the dielectric constant (the ability of a solvent to screen charges on the solutes from interacting with each other) and the chromatographic elution strength, ε (Table 5.2). In practice the polarity of a liquid or solid substance is dependent of at least four factors:
1. The polarity of individual chemical bonds, arising from the differences in electronegativities.
2. The geometry of the molecule
3. The presence of hydrogen bond donating or accepting atoms
4. The relative proportion of polar versus nonpolar bonds in the substance.
To illustrate this point, let us examine butanoic acid, a carboxylic acid:
The carboxylic acid functional group on the right-hand side of the molecule is polar. There is a slarge electronegativity difference between carbon and oxygen, making the carbon-oxygen bonds polar. In addition, a carboxylic acid functional group is both a hydrogen bond donor and a hydrogen bond acceptor. However, the propyl group on the left-hand side of the molecule is made of C-C and C-H bonds, which are not polar. If we added more -CH2- groups to butanoic acid, it would become less polar, but if we subtracted -CH2- groups from butanoic acid, the molecule would become more polar.
Miscibility and Solubility:
If two liquids are mixed and two layers form, the liquids are immiscible in each other. If two liquids are mixed and only a single layer forms, the two liquids are miscible in each other. If a turbid solution forms, the two liquids are not miscible, but they are not as immiscible as when two layers form. The maximum amount of a solute that dissolves in a solvent is the solubility of a solute. This can be thought of as how much solute can fit into the solvent until it becomes saturated. The solute may be a liquid or a solid, whereas the solvent is usually a liquid. Even when two liquids A and B are immiscible, there is always a very small amount of liquid B in the liquid A layer, and vice versa. In other words, two immiscible liquids A and B are slightly soluble in each other.
What determines how soluble a solute is in a solvent? There are intermolecular forces (IMFs) between solute molecules, IMFs between solvent molecules, and IMFs between solute and solvent molecules. The IMFs between the solute and solvent must be as favorable as the other IMFs between molecules of solute and between molecules of solvent in order for the solute to be highly soluble. Likewise, when two liquids are miscible, the forces between the two liquids must be as favorable as each solvent’s IMFs are with itself. For example, hexane and pentane are miscible because any given hexane molecule experiences London dispersion forces with a pentane molecule almost as easily as with another hexane molecule. Water and methanol are miscible because any given water molecule can form hydrogen bonds with methanol very effectively. But water and pentane are not miscible because water interacts more strongly with itself, via hydrogen bonds, than it interacts with pentane.
Ion-dipole Forces:
One other intermolecular force that is important in the solubility of many solids and liquids are ion-dipole forces. Ion-dipole forces are the attraction between an ion (an electrically charged species) and a polar solvent. The more polar a solvent is, the stronger its attraction to ions. Ion-dipole attraction explains why sodium chloride (NaCl) is soluble in water, but is insoluble in hexane. The oxygen atoms of water bear a partial negative charge that is attracted to the positively charged sodium atom (Na+) in solution, and the hydrogen atoms of water are similarly attracted to the chlorine atom (Cl-). These attractions compensate for the loss of attractive ionic forces when sodium and chloride ions in a crystal are separated by forming a solution. Hexane does not have a partial positive or a partial negative charge, and therefore sodium chloride remains a solid when mixed with hexane.
Many organic solids are insoluble in water, but soluble in organic liquids (solvents). However, sometimes the conjugate acid or the conjugate base forms of the organic solids are soluble in water, but insoluble in organic liquids. The explanation lies within the ion-dipole forces. When an acid is neutral, the conjugate base must be negatively charged. For example, benzoic acid (below left) is soluble in organic solvents but is only slightly soluble in water. The Benzoate ion (below right), on the other hand, is soluble in water, because of strong ion-dipole attractions.
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Likewise, when a base is electrically neutral, its conjugate acid is positively charged. Aniline is only slightly soluble in water. However, the anilium ion, its conjugate acid, is much more water soluble because of ion-dipole attractions.
Question/ Purpose:
The purpose of this project is to compare the surface tension of various liquids.
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
The independent variable is the type (chemical formula) of liquid.
The dependent variable is the relative surface tension.
Constant is the amount of each liquid used for test.
Controlled variable is the temperature. Try to perform all experiments at room temperature.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1:
The purpose of this experiment is to see if surface tension can cause heavy objects to remain on the surface of water. Density of paperclips is about 9 times more than that of water.
Procedure:
Before you try this you should know that it helps if the paper clip is a little greasy so the water doesn’t stick to it (rub it on your nose or forehead.) Place the paper clip on a fork and lower it slowly into the water. The paper clip is supported by the surface-tension skin of the water.
Experiment 2:
The purpose of this experiment is to compare the surface tension of various liquids. The method that we use in this experiment is to measure the area and layer thickness of certain amounts of different liquids poured on a flat glass surface. Lower tension between molecules will allow the liquid to cover a wider area and have less thickness.
Procedure:
Use a pipette that can measure exactly one milliliter of subject liquids. If you want to test multiple liquids at the same time, you should have the same number of pipettes (to avoid errors caused by contamination). Use the pipette and pour 1 ml of each liquid on a flat glass surface. Place the glass where you can look at it horizontally and read the height of each liquid. Use a ruler to measure the height and diameter of the circle created by each liquid. Record the results in a table like this.
| 1 ml of these liquids | liquid thickness | liquid diameter |
| Water | ||
| Acetone | ||
| Isopropyl alcohol | ||
| Olive oil | ||
| mineral oil |
You may try this experiment with 0.1 ml of each liquid instead of 1 ml.
One way to measure the diameter of a drop of liquid is using a compass. Open the compass equal to the diameter of the drop and then transfer this measurement to a ruler.
Height of liquid has a relation with the diameter. If you think of a layer of liquid as a cylinder (like a coin), you can use its diameter and its volume to measure its thickness.
You can also place a ruler behind the drop and look at the drop horizontally. In this way you can read the line on the ruler that aligns with the top of the liquid.
You may also use a micrometer to measure the diameter of the liquid.
Measurement of the liquid thickness can also be achieved using some optical devices that can magnify the drop size about 10 to 40 times. You may use your own creativity to come up with new methods as well.
If you cannot measure the thickness, complete your experiments just by measuring the diameter of the drop circle.
Experiment 3:
In this experiment, we will use the weight of drops to compare the surface tension. When a drop of liquid is still hanging there, it simply means that the the weight of the drop is less than the tension between molecules that are holding it. So the weight of each drop can be used to measure the surface tension. (Note: Although the tension between molecules is obvious on the surface, it actually exists everywhere in the liquid.)
Procedure:
Get samples of different liquids with known density. If you don’t know the density of the liquid that you are testing, calculate it yourself. To calculate the density you need to know the weight and and the volume. By dividing weight by volume, you will calculate the density. For example if one liter of a liquid is 1.3 kilograms, its density is 1.3. Or if 7 milliliter of a liquid weight 6 grams, it’s density is 6 divided by 7 or 0.85.
Use a pipette and slowly drop 1 milliliter of each liquid and count how many drops comes out.
Record the results in a table like this:
| 1 ml of these liquids | Number of drops | Density | Weight of each drop |
| Water | 23 | 1 | 0.043 grams |
| Acetone | |||
| Isopropyl alcohol | |||
| Olive oil | |||
| mineral oil |
Note:
- Since we are using one milliliter of each sample, the weight of our sample is the same as the density.
- By dividing the weight by the number of drops, we can calculate the weight of each drop.
- Instead of 0.043 grams, you could write 43 milligrams
- You may add some liquid detergent to water and use it as one of your test samples to see how liquid detergent affects the surface tension.
Experiment 4: (Just for Fun)
Surface tension driven boat.
This experiment shows how a floating object (foil boat) may be driven across a surface of water by an expanding front of soap molecules.
Equipment:
Piece of foil, scissors, large (preferably flat) dish or cooking pan filled with water, drop of any dishwashing/laundry detergent or soap solution.
How to do the experiment:
Fill a large cooking pan with water, in order to create a reasonably large water surface. Get a piece of cooking foil, and smooth it by “ironing” it with a fingernail. Press the foil against some flat surface to ensure that the surface is flat. Cut a small “boat” out of foil.
The back of the boat has a channel cut through it with a small cavity at the end of the channel
Smooth the boat gently, in order to eliminate all the irregularities created by cutting.
Place the boat (!)gently(!) on the surface of water, nose pointing towards a reasonably large area of free water surface. If everything is done with care, it will hang, suspended by surface tension forces. Dip a match in detergent or soap solution, and gently deposit a (!!)small(!!) drop of the solution on the boat, so that it touches the cavity. The boat should accelerate rapidly, like a “rocket” and skid a reasonable distance before stopping.
Explanation:
When you place soap solution in contact with water, soap molecules try to spread over the surface of the water, at first, since they are confined in the cavity of the boat with only one way out they jet from the rear end of the boat creating a reaction force strong enough to drive the boat across the water. As soon as all the water surface is covered with a monolayer of soap molecules, the motion stops. To facilitate the experiment, one may place a small crystal of camphor in the cavity, instead of soap. The boat motion would last much longer.
Simple boat:
Cut the wood to a length of about one inch. Place a small piece of soap in the back of the wood, or boat. Place the boat in the water and watch to see what happens. Does the boat sink? Does it move?
Materials and Equipment:
List of material can be extracted from the experiment section.
Isopropyl Alcohol is also known as rubbing alcohol.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
You may need to calculate the density. You will also need to calculate the weight of each drop in experiment 3. Write your calculations in your report.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
