Chemistry_Grade_10-12 (1).pdf

A light blue precipitate forms when sodium carbonate

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1. A light blue precipitate forms when sodium carbonate reacts with copper(II) chloride 2. No precipitate forms when sodium sulphate reacts with copper(II) chloride It is important to understand what happened in the previous demonstration. We will look at what happens in each reaction, step by step. 1. Reaction 1: Sodium carbonate reacts with copper(II) chloride When these compounds react, a number of ions are present in solution: Cu 2+ , Cl , Na + and CO 2 3 . Because there are lots of ions in solution, they will collide with each other and may recom- bine in different ways. The product that forms may be insoluble, in which case a precipitate will form, or the product will be soluble, in which case the ions will go back into solution. Let’s see how the ions in this example could have combined with each other: Cu 2+ + CO 2 3 CuCO 3 Cu 2+ + 2Cl CuCl 2 Na + + Cl NaCl Na + + CO 2 3 Na 2 CO 3 You can automatically exclude the reactions where sodium carbonate and copper(II) chlo- ride are the products because these were the initial reactants. You also know that sodium chloride (NaCl) is soluble in water, so the remaining product (copper carbonate) must be the one that is insoluble. It is also possible to look up which salts are soluble and which are insoluble. If you do this, you will find that most carbonates are insoluble, therefore the precipitate that forms in this reaction must be CuCO 3 . The reaction that has taken place between the ions in solution is as follows: 2 Na + + CO 2 3 + Cu 2+ + 2 Cl CuCO 3 + 2 Na + + 2 Cl 2. Reaction 2: Sodium sulphate reacts with copper(II) chloride The ions that are present in solution are Cu 2+ , Cl , Na + and SO 2 4 . The ions collide with each other and may recombine in different ways. The possible com- binations of the ions are as follows: Cu 2+ + SO 2 4 CuSO 4 Cu 2+ + 2Cl CuCl 2 Na + + Cl NaCl Na + + SO 2 4 Na 2 SO 4 If we look up which of these salts are soluble and which are insoluble, we see that most chlorides and most sulphates are soluble. This is why no precipitate forms in this second reaction. Even when the ions recombine, they immediately separate and go back into solution. The reaction that has taken place between the ions in solution is as follows: 2 Na + + SO 2 4 + Cu 2+ + 2 Cl 2 Na + + SO 2 4 + Cu 2+ + 2 Cl Table 20.1 shows some of the general rules about the solubility of different salts based on a number of investigations: 390
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CHAPTER 20. THE HYDROSPHERE - GRADE 10 20.8 Table 20.1: General rules for the solubility of salts Salt Solubility Nitrates All are soluble Potassium, sodium and ammo- nium salts All are soluble Chlorides All are soluble except silver chlo- ride, lead(II)chloride and mer- cury(II)chloride Sulphates All are soluble except lead(II)sulphate, barium sul- phate and calcium sulphate Carbonates All are insoluble except those of potassium, sodium and ammo- nium 20.8 Testing for common anions in solution It is also possible to carry out tests to determine which ions are present in a solution.
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