Exploring the Chemical World, PGCC, 2003 135 NAME________________________________ SECTION_________________ PARTNERS________________________ DATE___________________ PERFORMANCE TASK # 3 You must work in teams of three or four (ask instructor) and will turn in ONE report. Answer all questions. Write in complete sentences. You must hand this activity in before leaving today. This activity is worth 50 points. Part I. Qualitative Relationships (20 pts) In a previous lab, you considered the relationship between the appearance of a tea solution and the concentration of the solute - the tea. You were able to tell by the intensity of the color of the tea which was the weakest and which was the strongest sample. Could you tell the difference in a dark room? No. Light passes through the solutions and it is the light that affects your eyes and stimulates your brain and your brain then makes the decision about the strength or concentration of the tea based on the color. Your eyes and your brain may be able to distinguish among weak, strong, and medium strengths of colored solutions, but you many have difficulty distinguishing solutions that are either 10% or 15% solute as the intensity of color may be hard or impossible to distinguish. Today you will substitute a technological device (for your eyes)- a light probe and probe meter to measure the light going through various materials. Obtain a light probe, CBL, and light source. Connect the probe to the CBL according to your instructor's directions. Turn the light source on and shine it onto the probe. Align the light and the probe so that the light shines directly into the probe in a straight line. Hit the ON/HALT button and then the MODE button. Adjust the position of the probe so that the meter reading is close to 0.80. The meter reading is the intensity of the light beam. MicroscopeLight probeCBLPlace lenses herelightadjust distanceso that meter on CBLreads 0.8Exploring the Chemical World, PGCC, 2003 136 DO NOT CHANGE the space between the light source and probe for the remaining activity, as it will influence the intensity of light hitting the probe and hence, the meter reading. It may be convenient to mark the probe and light source positions with masking tape. Record the initial meter reading. meter reading ____________________ Block the light beam with your hand to verify that the CBL responds rapidly by dropping radically in value. If it does not, check with your instructor before proceeding. 1. Place a sunglass lens directly in front of the light source. What happens? Why? meter reading ___________ Is there any light going through the lens? How do you know? Add another lens, so that you have two in place. What happens? Why? meter reading ___________ Add a third lens and note what happens. meter reading ___________ State a relationship between the amount of light absorbed by the lens material and thickness in words and as a mathematical equation. As more light is absorbed, what happens to the amount of light transmitted (light that goes through the material)? State a relationship between absorption and transmission in words and as a mathematical equation.Exploring the Chemical World, PGCC, 2003 137 Now let's explore how light behaves when passing through a solution. 2. Place a bottle with only distilled water in it between the light source and the probe. What happens? meter reading________________________ Remove the distilled water bottle and place a bottle of green solution (a green solid dissolved in distilled water) between the light source and the probe. What happens? Why? meter reading_______________________ If you were adjusting or standardizing a machine (to make sure the settings were all correct) before taking a light measurement through a colored solution, why might you begin by placing a sample of distilled water between the light and the probe? Place a second bottle of green-colored solution in the light beam, in line with the first bottle. What happened? Why? meter reading_______________________ State the relationship between pathlength, which is the distance light traveled through the solution, and the amount of light absorbed in words and mathematically. Obtain the three bottles of blue solution of varying concentrations. See if you can line them up in order from least concentrated to most concentrated just by looking at them. Place them one at a time at the same spot between the light source and the probe. Was your estimation correct?_______ How do you know if it was correct or incorrect? State a relationship between concentration of a solution and the amount of light absorbed by that solution in words and as a mathematical equation.Exploring the Chemical World, PGCC, 2003 138 Part II. Quantitative Relationships: Using a Spectrophotometer (25 pts) The spectrophotometer is an electronic device that can take the place of your eye and the light source with probes and the light detector. A simplified diagram of what is inside the Spec-20 looks like this: Io is the intensity of the light hitting the sample. An added feature of the spectrophotometer is that it can select a particular wavelength of light to shine on the sample. When we used the light probes, we were using white light which contains all the wavelengths in the visible spectrum. I is the intensity of the light being transmitted through the sample, the amount of light that does not get absorbed by the sample. The amount of light absorbed by the sample will be different depending on the wavelength of light hitting the sample. The spectrometer displays absorbance, which is -log I/Io. Absorbance is directly proportional to the concentration of the sample. The samples are inserted into the machine through a covered slot on the top. The glass used to hold the sample is special optical glass (the light must go straight through the sample). It is important that these tubes be clean and free of finger prints as any marks on the tube can block some of the light and change your results. There is lens paper available for cleaning the tubes. Only hold the tubes near the top. Overall Task: There are five tubes of a