PreAP of Water Lab Background The structure of the water molecule gives water unique properties. Water is a polar molecule, which means that it has a region with a slight negative charge (the oxygen atom), and a region with a slight positive charge (the hydrogen atoms). The oppositely charged regions of water molecules interact to form hydrogen bonds. A hydrogen bond is an attraction between a slightly positive hydrogen atom and a slightly negative atom. Hydrogen bonds are responsible for several important properties of water. High specific heat: Water resists changes in temperature; it must absorb a large amount of heat energy to increase in temperature. Cohesion: The attraction among molecules of a substance is called cohesion. Cohesion due to hydrogen bonds makes water molecules stick together. Adhesion: The attraction among molecules of different substances is called adhesion. Water molecules stick to many other materials because of hydrogen bonds. Many compounds that are important for life dissolve in water. Water is the largest component of cells interiors, and chemical reactions in the cell take place in this water. When one substance dissolves in another, a solution is formed. The substance present in the greatest amount is called the solvent. Substances that are present in lower amounts and dissolve in the solvent are called solutes. Polar solvents, such as water, dissolve polar molecules and ions. Pre-Lab Questions (15 pts) 1. Define polar molecule. 2. What is a hydrogen bond? 3. What types of particles dissolve in polar solvents, such as water? PART 1 - WATER, THE SUPER SOLVENT Universal Solvent two test tubes salt vegetable oil water 1. Fill a test tube ⅔ full with water. Add a pinch of salt. 2. Cover the open end of the test tube with your thumb, and invert test tube up and down for 30 seconds. Record observations. 3. The test tube with oil already has salt in it. Securely cover the Parafilm-wrapped end with your thumb and invert the test tube up and down for 30 seconds. Record observations. 4. Pour the test tube with salt and water down the sink. Rinse the test tube with clean water. Leave the test tube upside down on the test tube rack to dry. PART 2 - COMPARING WATER AND ALCOHOL Specific Heat beaker of water beaker of alcohol dropper 1. Place one drop of water onto the back of your partner s right hand. Place dropper back into correct container. 2. Place one drop of alcohol onto the back of your partner s left hand. Place dropper back into correct container. 3. Observe for thirty seconds and record observations.
PART 3 - PEPPER WATER Surface Tension Petri dish ground pepper water small beaker of soapy water Q-tip 1. Pour a small amount of water on a Petri dish, sprinkle the surface with some ground pepper. 2. Observe the pepper floating on the surface of the Petri dish. 3. Take the Q-tip from the soapy water and barely touch it to the center of the surface of the peppery water in the center of the dish, and record observations. 4. Replace the Q-tip in the soapy water, and dump the water and pepper in the Petri dish down the sink. Rinse Petri dish and leave it upside down on a paper towel to dry. PART 4 - THE FLOATING PAPERCLIP Cohesion, Surface Tension paper clip container with water 1. Using a steady hand, see if you can get the paper clip to rest on the surface of the water in such a way that it will not sink. You may bend the paperclip into a new shape, or keep it as it is. 2. After you succeed, remove the paper clip, and throw it away, and clean up any spilled water with a paper towel. 3. Record observations. PART 5 - DROP BEHAVIOR Cohesion, Surface Tension two dry pennies 10-mL graduated cylinder one dropper water detergent 1. Using a dropper, drop water into a 10-mL graduated cylinder, counting each drop, until there is 1 ml of water in the graduated cylinder. Record how many drops are in 1 ml. 2. Place a dry penny on a paper towel. Gently place drops of water on the penny until the water spills over. Record the results. 3. With your finger, spread one small drop of detergent on the surface of a dry penny. 4. Using the same dropper as before, add drops of water to the penny surface. Keep careful count of the number of drops, and record your results. 5. Rinse and dry the pennies, and wipe up any water spills with a paper towel. PART 6 - CAPILLARY ACTION Capillary Action capillary tube small beaker filled with water 1. Place the capillary tube in the small beaker of water. 2. Observe the water level in the capillary tube. Sketch your observations. 3. Remove the capillary tube from the water, and dry.
Data (30 pts) Your data table must be complete on your lab sheet before entering the lab. Part 1 Water, the Super Solvent Part 2 Comparing Water and Alcohol Part 3 Pepper Water Part 4 The Floating Paperclip Part 5 Drop Behavior Part 6 Capillary Action Drops of Water in 1 ml Drops of Water on Dry Penny Drops of Water on Soapy Penny
Post Lab Analysis Questions (30 pts) 1. Based on your observations from Part 1, is salt (an ionic compound) more soluble in polar solvents or nonpolar solvents? Use evidence from the lab to support your answer. 2. A. Which solvent evaporated more quickly in Part 2 water or alcohol? B. Liquid water has a specific heat of 4.2 J/g C and liquid isopropyl alcohol (rubbing alcohol) has a specific heat of 2.6 J/g C. Based on your observations, what can you conclude about specific heat and rate of vaporization? 3. Dish soaps works because the detergent molecules breaks the hydrogen bonds between water molecules, interrupting the surface tension of water. Explain why adding soap produced the observed effect. 4. A. What does it mean to float? (Hint: consider density) B. Was the paper clip floating on the water? Explain. C. What property of water allows the paperclip to rest on its surface? 5. Describe why water forms droplets on a surface. 6. Explain why water in a narrow tube placed in water will rise higher than the surface level of water.
Grading Rubric Heading (1 pt each): -name -date -class period -group members -title Purpose: -clearly states lab objective (4 pts) -written in a complete sentence (1 pt) Pre-Lab Questions (5 pts each): -written in complete sentences, if applicable (1 pt per question) Data Table: -straight lines (5 pts) -data is complete (35 pts) -data is clearly presented and easy to read (5 pts) Analysis Questions (5 pts each): -response correctly answers question (4 pts) -written in a complete sentence (1 pt) / 5 pts / 5 pts / 15 pts / 45 pts / 30 pts