Demonstrations Demonstration 1: Comparison of the Electrical Conductivity of Distilled Water with Tap Water and Seawater Purpose The electrical conductivity of distilled (or deionized) water is compared with tap water and seawater as one indication of the amount of salt present. (The Bonding module describes the pertinent apparatus.) Materials 9-volt Battery conductivity apparatus Distilled water Seawater Safety Use normal safety precautions. Procedure Test electrical conductivity of distilled water and seawater with the apparatus. Extensions Discuss why it is inadvisable to go swimming during a lightning storm. Demonstration 2: Buoyancy and Density Purpose Part A simply demonstrates what buoyancy is and how we recognize it. Part B shows the relationship between density and buoyancy. Materials Part A Beaker, 2-L or larger (or clear plastic pail) Ice Water, 2 L Canned soft drinks, sweetened and sugar-free Part B 2Graduated cylinders, 1-L Water (tap water or deionized water), 2 L 400g Sodium chloride, NaCl, saturated solution, per L solution 2 Eggs (raw) Safety Use normal safety precautions. Procedure Part A Fill a very large (2 L or larger) beaker or clear plastic pail with water and a little ice. Drop in one can of Pepsi and one can of Diet Pepsi (or Coke and Diet Coke). The sugared solution will sink to the bottom while the artificially sweetened solution, which is much less dense, will float at or near the top of the container. Part B Fill one graduated cylinder with tap water or deionized water; in the other place 250 mL saturated NaCl solution. Now, carefully pour more water down the side of the cylinder without stirring until the cylinder is nearly full. Now drop an egg into each container. In pure water, the egg falls. In the salt solution, the egg will remain suspended somewhere in the bottom half of the cylinder where the density of the solution equals that of the egg. Remarks One way to introduce this demonstration is to ask if any students have ever swum in the Great Salt Lake or the Dead Sea. If so, they can attest to the difference in buoyancy between those very salty oceans and fresh water. Maybe students will have noticed the phenomenon illustrated in Part A at picnics where large trash cans or barrels are often filled with ice and canned drinks. If there is freedom of movement, the sugared drinks will be on the bottom and diet drinks on top. The greater the sugar content, the deeper the can sinks. Demonstration 3: Osmotic Pressure Purpose In this activity, students will observe increased osmotic pressure as a saturated solution of sodium chloride is diluted by distilled water passing through a membrane causing the water level of a thistle tube to rise. Materials Ring stand and clamp Beaker, 400-mL and 150-mL Semipermeable membrane (dialysis tubing or cellophane) Saturated salt water (40 NaCl per 100 mL solution) Thistle tube or similar tubing Procedure Fill 400-mL beaker approximately 3/4 full of distilled water. Put saturated NaCl solution inside of thistle tube (which has membrane over the large open end) until saturated NaCl solution is just above the distilled water level in the beaker (see Figure 15). Mark initial level on Day 1 and mark final level the next day. [NOTE: To fill the thistle tube, place piece of cellophane in a 150-mL beaker and depress it to the bottom with thistle tube. Remove thistle tube and fill depression with NaCl (sat.). Replace thistle tube and allow to fill with solution. Secure cellophane around bottom of thistle tube with thread or rubber band.] The distilled water passed through the membrane and diluted the saturated sodium chloride solution. As this happened, pressure increased, the fluid level inside of the thistle tube increased, and the fluid level in the thistle tube rose.Extensions 1. Evaporate some seawater and look at the remaining solids under a microscope. 2. Again, using a microscope, look at plant cells and what happens when they are immersed in distilled water vs. salt water. Remarks Osmosis is used to desalinate water. In light of the severe water shortage in certain parts of the world, there is great interest in using osmosis to purify water. GROUP AND DISCUSSION ACTIVITIES Counterintuitive Examples 1. Water can be purified by passing through a filter. [Some impurities or substances found in water can be filtered. Suspended solids can be removed with filter paper; some organic compounds can be absorbed on pulverized charcoal. Soluble salts are not so easily removed.] 2. All salts dissolve more in warm water than in cold water. [While this fact is true for many salts, it is not universal. For example, when NH 4 NO 3 dissolves in water, the reaction is endothermic, meaning that heat is absorbed during the reaction. The surrounding air and solution get colder as they supply this heat.
According to LeChatelier’s Principle raising the temperature of the system, which is adding more of the reactant heat, will cause more NH 4 NO 3 to dissolve. In contrast, when MgSO 4 dissolves in water, the opposite effect is noted; the surroundings get warmer because heat is produced.
In this case, raising the temperature shifts the equilibrium to the left so that less MgSO 4 dissolves in hot water than in cold. As a side note, mention to students that NH 4 NO 3 is the salt used in many first-aid cold packs, while MgSO 4 is used in first-aid hot packs.] 3. Gases dissolve better in warm water than cold. [Open a cold can of soda and one at room temperature. Which one loses its carbonation more quickly? (The warmer one because CO 2 is less soluble at higher temperatures.)] Pictures in the Mind 1. Distribution of the world’s water supply (see Appendix). 2. Seawater contains an average of 35,000 parts per million of dissolved solids. In a cubic mile of seawater, weighing 4.7 billion tons, there are about 165 million tons of dissolved matter, mostly chlorine and sodium (see Appendix).
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