Liquids have a number of interesting physical properties, such as viscosity and surface tension. Keep in mind that differences in these properties are largely due to differences in the intermolecular forces explained in Lesson 15, but length of molecules may also be a factor. For example, long molecules may change viscosity due to molecular entanglement.
Experimental Title: Lab 15: Physical Properties of Liquids
Date of laboratory: September 9, 2014
Purpose: The purpose of this laboratory is to investigate three physical properties of liquids: surface tension, viscosity (including the special case of thixotropy), and evaporation.
Surface tension is the tendency of the molecules in liquids to interact with each other in such a way to pull together to minimize surface area. This inward pulling is due to cohesive forces.
Viscosity is how much the molecules in a liquid resist flow. Thixotropy is when certain gels or fluids are viscous (resistant to flow) under static conditions and then become less viscous when shaken, stirred or agitated.
Viscosity has a number of real world applications:
- Food scientists study the viscosity of foods, such as the how viscous a given batch of strawberry jam is.
- Health professionals check the viscosity of a patient’s blood.
- Volcanologists monitor the viscosity of molten rock or magma
to determine how easily a volcano will erupt.
- Auto mechanics calculate the viscosity of oil needed for different engines and climates.
- Artists pick paints for different projects based on their viscosity.
Evaporation or vaporization is a liquid changing into a gaseous state. The rate of evaporation is influenced by the strength of intermolecular forces, as well as temperature and surface area. A drop in temperature may also be an indication that a liquid is evaporating rapidly.
Special safety concerns for Lab 15:
- If anything spills, please clean it up immediately with a paper towel and let your instructor know.
- If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
- Alcohol is highly flammable, so keep it away from heat sources.
- Be sure to wash your hands when you are finished with this lab
- Plastic bins
- Graduated cylinders
- Metal paper clips
- Plexiglass sheet
- Plastic chopstick for stirring
- Plastic cylinders
- Glass marbles
- Large bottle of light-colored shampoo
- Sharpie marking pen
- Hot water (hot water from a faucet is fine)
- Ice cubes
- Paper towels
- Temperature probe
- Rubbing alcohol
- Laundry detergent
- Corn syrup
- Vegetable oil
Note: Today you can do the parts in any order, so go ahead to another part and come back to finish if you need to do so. No need to wait for materials.
Part 1. Determine the surface tension of water and other liquids
- Obtain a bowl and partially fill it with tap water.
- Take a metal paper clip and devise a way to float it on the surface of the water via surface tension.
- Answer the following questions: How many paperclips can the surface tension hold? Does the shape of the paperclip affect its floating ability?
- Obtain another liquid. Using the floating metal paper clip test, compare the surface tension of that liquid to water.
Part 2. A. Determine the viscosity of Various Liquids
1. Obtain four plastic cylinders. If there are no lines, draw a line as a starting point about 3 cm below the top of each cylinder and and a second line as a stopping point at least 3 cm above the blue part (to allow marbles to accumulate in the bottom for each run) with a Sharpie marking pen. You don’t want the ending line to be right at the bottom of the cylinder because the marble will slow down as it approaches the bottom. With the ruler, measure and record the distance between the lines in cm.
Length between marks (fall distance):
2. If the tubes provided are not already filled, fill one each with:
- Tap water
- Corn syrup
- Vegetable oil
- Laundry detergent
3. You or the helper should hold a marble at the surface of one of the liquids. The other person should zero out the stopwatch.
4. The person holding the stopwatch should say “Go!” and have the other person drop the marble. As the marble passes the starting point, which was marked in the previous section, the person holding the stopwatch should start the stopwatch. As the marble passes the ending point, which was marked in the previous section, the person holding the stopwatch should stop it.
5. Record the time elapsed in a data table. Leave the marble in the tube until the end of the trial.
6. Repeat steps 3-5 of this section, with the same liquid and cylinder four more times with four other marbles. When you have recorded the results, tip the fluid back into the original container and retrieve the marbles to wash before using with the next liquid.
7. Repeat the process for all the liquids.
Note: Technically in this experiment you are calculating velocity = distance/time rather than viscosity. Hawaii Space Grant has a more in depth viscosity experiment, which includes calculating viscosity using this equation:
- delta p = difference in density between the sphere and the liquid
- g = acceleration of gravity
- a = radius of sphere
- v = velocity
Part 2. B. Investigate the effect of Temperature on Viscosity
1. Draw two lines as start and stop points all the way around the side of the shampoo bottle (about 3 cm from each end) with a Sharpie marking pen. (You don’t want the ending line to be at the bottom of the shampoo bottle because the marble will slow down as it approaches the bottom.) With the ruler, measure and record the distance between the lines in cm.
Length between marks (fall distance):
2. Uncap the bottle and insert a marble. Fill the bottle to the top with shampoo from another bottle. Take the temperature of the shampoo with the thermometer. Close the cap tightly.
Temperature of room temperature shampoo:
3. Turn the bottle upside down and observe the marble as it sinks downward. The marble should come to rest in the cap. This ensures that the marble will drop down the center of the bottle when it is inverted once more.
4. Invert the bottle once more and use the stopwatch to measure the time it takes for the marble to sink down the center of the bottle from the top line to the bottom line. Record the time.
5. Repeat step 4 four more times, for a total of five time measurements. Then calculate the average time it takes the marble to sink through the shampoo.
6. Next, investigate the viscosity of shampoo at a warmer temperature. Fill a bin with hot water from the faucet. Tighten the cap on the shampoo-filled bottle so it cannot leak. Then lay the bottle in the bin so it is completely covered. The hot water bath will heat up the shampoo. Leave the bottle in the hot water for about 1o minutes. Carefully rotate the bottle every five minutes with tongs to heat the shampoo evenly. Open the cap and take the final temperature of the shampoo with the thermometer.
Temperature of heated shampoo:
7. Repeat step 4 five times and record the data. Then calculate the average time it takes the marble to sink in warm shampoo.
8. Now cool the shampoo. Fill the bin provided with cold water and ice cubes and lay the bottle of shampoo on its side in the bin. Leave the bottle in the cold water for about 1o minutes. Carefully rotate the bottle every five minutes or so to cool the shampoo evenly. Open the cap and take the temperature with the thermometer.
Temperature of cold shampoo:
9. Repeat step 4 five times and record the data. Then calculate the average time it takes the marble to sink in cold shampoo.
Part 2. C. Investigate the thixotropic Behavior of Ketchup
Remember all those commercials showing people struggling to get ketchup to come out of to bottle? Ever wonder why that is the case?
We will be investigating the question: which is more viscous (thicker), undisturbed ketchup in the container or vigorously-stirred ketchup?
1. Look for two plastic cups labelled “undisturbed ketchup” and “stirred ketchup.” Take the plastic chopstick and stir the ketchup labelled “stirred ketchup” vigorously for at least one minute.
2. Using a dropper, gently suck up a sample from each type of ketchup and place approximately 0.5 mL dots on the piece of plexiglass as indicated. Tip the sheet of plexiglass upright against the wall in the tray provided. Record the distance each sample has moved after one minute and after five minutes.
Part 3. Evaporation
1. Obtain the temperature probe. Wrap the probe in a small amount of tissue and then dip it into a sample of tap water. Record the initial temperature and the temperature at one minute, two minutes and three minutes.
2. Remove the wet tissue. Dry the probe and then wrap in a small amount of tissue as before. Now dip the wrapped probe in a sample of rubbing alcohol. Record the initial temperature and the temperature at one minute, two minutes and three minutes.
Compare your results.
Once you have completed the five parts, sit down and write a sentence or two to explain the results of each part.
Record any thoughts you have about the experiments, including:
- Possible improvements to the procedures or how to tweak techniques
- How the results differed from your expectations
- Suggestions for other experiments
- What key concepts you learned about the physical properties of liquids
We’ll go over the key concepts together at the end of lab.
Please leave a comment or send an e-mail if you have any questions before our meeting.