Properties of Water Lab: AP Biology

For this lab, you should record your answers to the questions on a separate document and share them with Ms. Poole.

Cohesion: Water sticks to itself

  1. Predict how many drops of water you can you place on the surface of a penny before it overflows. How many drops do you predict?
  2. Drop water from the dropper onto a penny, keeping count of each drop.
  3. How many drops were you able to place on the penny before it overflowed?
  4. If the number of drops is different from your prediction, explain what accounts for the difference. Explain your results in terms of water sticking to itself.
  5. Draw how water can stick to itself through cohesion. Make a brief sketch of your model.
  6. Repeat the above procedure with rubbing alcohol. How many drops were you able to place on the surface of the penny before it overflowed? Is rubbing alcohol more or less cohesive than water? Explain.

Cohesion and Surface Tension: Water sticks to itself and forms surfaces: Part 2

  1. With your finger, spread one small drop of soap on the surface of a dry penny.
  2. How many drops do you think this penny will hold after being smeared with soap: more, less, or the same as before? Why?
  3. Using the same dropper as before, add drops of water to the penny surface. Keep careful count of the number of drops.
  4. How many drops were you able to place on the penny before it overflowed this time?
  5. Did the soap make a difference? Describe the effect of the soap.
  6. What does the soap do to have this effect on water? Explain your results in terms of water sticking to itself.

Adhesion: Water sticks to other polar things.

  1. Observe water in a graduated cylinder. Note how the surface of the water curves and sticks to the sides of the cylinder. (You may have to crouch down to do this.) This is called a meniscus. Explain this in terms of water sticking to other polar things.
  2. Measure and cut a length of string 15 cm long. Soak the string in a beaker containing water for about 15 seconds. Put one end of the string into the beaker so that the end of the string is submerged in the water. Have one team member hold the string so it stays submerged in water. Put the other end of the string into another beaker. Have one team member hold the string so that it stays in the beaker. Lift the first beaker up so that it’s above the second beaker. Position the beaker so that the string is taut. SLOWLY pour the water down the string and into the other beaker.
  3. Repeat the procedure with a string 30 cm long.
  4. Describe what happens with the first string. Relate this to water sticking to other things. Do the results change for the longer string? Why do you think your results were similar/different?
  5. If you have time, coat the string with soap and see what effect this has on water’s ability to stick to the string.
  6. Draw how water can stick to other polar things through adhesion. Make a brief sketch of your model.
  7. Plastic, like many substances made from petroleum products, is nonpolar. Predict what would happen if you repeated the above procedure with a plastic graduated cylinder or plastic string.

Adhesion and capillary action:Water sticks to and climbs polar things.

  1. Cut out two strips of filter paper or chromatography paper 2.5 cm wide and 10 cm long.
  2. Fill two beakers with water. Put several drops of dish soap in one of the beakers. Stir gently. Too much foam will interfere with this experiment.
  3. Use a marker and draw a horizontal line 2 cm from the bottom of the paper.
  4. Time the following step using a watch with a second hand or the classroom clock.
  5. Put the edge (about 1 cm) of the bottom of paper into the water without soap. Time the rise of the water for 2 minutes. Measure how far up the paper the water went.
  6. Repeat previous with the remaining piece of paper and the soapy water.
  7. Compare your results. What happened to the marker ink? What accounts for the difference between the two pieces? Relate your answer to water sticking to particular materials.
  8. Draw how water can stick to other polar things and climb them through capillary action. Make a brief sketch of your model.

Ice Floating/ High Specific Heat/Evaporative Cooling

  1. Cold water is denser than warm water, and tends to sink. However, ice is less dense than water and tends to float. Explain, why cold water sinks but ice floats.
  2. “A watched pot never boils.” Explain why water takes more time and heat to boil than oil or other nonpolar liquids.
  3. Compare a drop of water to a drop of ethanol. Which will take longer to evaporate? Evaporative cooling – the evaporation of water off skin in the form of sweat – is a component of homeotherms (endotherms) ability to cool themselves. Why does it take more energy for water to evaporate than for less polar liquids (like ethanol)?

Solvent: Water dissolves polar substances. We will do this as a class!

  1. Fill a large beaker about halfway up with water. Add an inch or two of oil. Does the oil dissolve in water? From this, is oil more hydrophilic (liking water) or hydrophobic (fearing water)? Polar or nonpolar?
  2. We will dissolve several substances in the water and oil: salt, sugar, starch, pepper, etc. If a substance remains in the oil layer – and is dispersed in the oil layer – it is hydrophobic and nonpolar. If a substance remains in the water layer – and is dispersed in the water layer – it is hydrophilic and polar.
  3. Substances may be both hydrophobic and hydrophilic – called being amphipathic or amphiphilic. Soap is one such molecule. We will add soap to the oil/water mixture. Note the effect the soap has on the oil’s ability to dissolve in water.