When ionic substances are dissolved in water the cations and anions go into solution separately. We would write, for example Fe(NO 3 ) 2 (s) ! Fe 2+ (aq) + 2 NO 3 - (aq) Not all ionic substances are soluble in water. Solubility is a complex issue, which we will revisit in CHE 152 (and again in CHE 300), but qualitatively it is governed by a few factors such as ionic charge (highly charged ions attract one another strongly in the solid, and so are more
CHE151Lab11_Precipitates.doc Dr . Roderick M. Macrae CHE 151 difficult to dissolve) and size (large ions with diffusely distributed charge do not interact so strongly with water, and so are more difficult to dissolve). A general rule of thumb is that all sodium, potassium and ammonium salts, and all nitrates, acetates, and perchlorates are soluble, while most hydroxides, sulfides, phosphates and carbonates as well as most silver, lead, and mercury salts are insoluble. (Solubility data for common ionic compounds can be found in Appendix F on page A-32 in McQuarrie.) In this experiment you will use the technique of gravity filtrationto separate an insoluble solid (precipitate) from a solution (supernatant). In this method (to be contrasted with suction filtration, which we will use in a later laboratory) a piece of filter paper of the appropriate size and grade (fine precipitates require finer-grade paper) is folded into quarters, then one side is opened into a cone and the filter paper is placed into a filter funnel which has been carefully distilled-water rinsed. You may need to dampen the paper slightly with distilled water to make it sit in the funnel. Then gently pour the solution to be filtered into the funnel, taking care not to overflow the top of the paper. To remove the last residue of the precipitate from your beaker, the best approach is to pour some of your supernatant back into the beaker and use it to rinse through the remainder of the precipitate. (The supernatant is a saturated solution, and none of your precipitate will dissolve in it, while a small amount of your precipitate may dissolve if distilled water is used. This will be important if quantitative recovery of your precipitate is required.) A. Data Collection – Qualitative (Wear your safety glasses for all laboratory experiments. Dispose of all waste in the container provided. Cobalt salts are toxic and must not be disposed of in the sink.) 1. Dissolve about 1 cm3of cobalt (II) nitrate in about 20 mL of distilled water in a beaker. Dissolve a similar amount of sodium phosphate in a second 20 mL of water. Describe the appearance of each solution. 2. Pour half of each solution into a third beaker and mix thoroughly with a glass rod. (Save all three beakers for use in part 4 below.) (a) Describe the appearance of the mixture.
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