8.3 Precipitation reactions

It is important to realise that our hydrosphere exists in a delicate balance with other systems and that disturbing this balance can have serious consequences for life on this planet.

Group project : school action project

There is a lot that can be done within a school to save water. As a class, discuss what actions could be taken by your class to make people more aware of how important it is to conserve water.

Summary

• The hydrosphere includes all the water that is on Earth. Sources of water include freshwater (e.g. rivers, lakes), saltwater (e.g. oceans), groundwater (e.g. boreholes) and water vapour. Ice (e.g. glaciers) is also part of the hydrosphere.
• The hydrosphere interacts with other global systems , including the atmosphere, lithosphere and biosphere.
• The hydrosphere has a number of important functions . Water is a part of all living cells, it provides a habitat for many living organisms, it helps to regulate climate and it is used by humans for domestic, industrial and other use.
• The polar nature of water means that ionic compounds dissociate easily in aqueous solution into their component ions.
• Ions in solution play a number of roles. In the human body for example, ions help to regulate the internal environment (e.g. controlling muscle function, regulating blood pH). Ions in solution also determine water hardness and pH.
• Water hardness is a measure of the mineral content of water. Hard water has a high mineral concentration and generally also a high concentration of metal ions e.g. calcium and magnesium. The opposite is true for soft water.
• pH is a measure of the concentration of hydrogen ions in solution. The formula used to calculate pH is as follows: pH = -log[H ${}_3$ O ${}^{+}$ ] or pH = -log[H ${}^{+}$ ]. A solution with a pH less than 7 is considered acidic and more than 7 is considered basic (or alkaline). A neutral solution has a pH of 7.
• Gases such as CO ${}_2$ , NO ${}_2$ and SO ₄ ^$2-$ dissolve in water to form weak acid solutions. Rain is naturally acidic because of the high concentrations of carbon dioxide in the atmosphere. Human activities such as burning fossil fuels, increase the concentration of these gases in the atmosphere, resulting in acid rain .
• Conductivity is a measure of a solution's ability to conduct an electric current.
• An electrolyte is a substance that contains free ions and is therefore able to conduct an electric current. Electrolytes can be divided into strong and weak electrolytes, based on the extent to which the substance ionises in solution.
• A non-electrolyte cannot conduct an electric current because it dooes not contain free ions.
• The type of substance , the concentration of ions and the temperature of the solution affect its conductivity.
• A precipitate is formed when ions in solution react with each other to form an insoluble product. Solubility 'rules' help to identify the precipitate that has been formed.
• A number of tests can be used to identify whether certain anions are present in a solution.
• Despite the importance of the hydrosphere, a number of factors threaten it. These include overuse of water, and pollution .

Summary exercise

1. Give one word for each of the following descriptions:
   1. the change in phase of water from a gas to a liquid
   2. a charged atom
   3. a term used to describe the mineral content of water
   4. a gas that forms sulphuric acid when it reacts with water
2. Match the information in column A with the information in column B by writing only the letter (A to I) next to the question number (1 to 7)

   Column A	Column B
   1. A polar molecule	A. H ${}_2$ SO ${}_4$
   2. molecular solution	B. CaCO ${}_3$
   3. Mineral that increases water hardness	C. NaOH
   4. Substance that increases the hydrogen ion concentration	D. salt water
   5. A strong electrolyte	E. calcium
   6. A white precipitate	F. carbon dioxide
   7. A non-conductor of electricity	G. potassium nitrate
   	H. sugar water
   	I. O ${}_2$

3. For each of the following questions, choose the one correct answer from the list provided.
   1. Which one of the following substances does not conduct electricity in the solid phase but is an electrical conductor when molten?
      1. Cu
      2. PbBr ${}_2$
      3. H ${}_2$ O
      4. I ${}_2$
      (IEB Paper 2, 2003)
   2. The following substances are dissolved in water. Which one of the solutions is basic?
      1. sodium nitrate
      2. calcium sulphate
      3. ammonium chloride
      4. potassium carbonate
      (IEB Paper 2, 2005)
4. The concentration of hydronium and hydroxyl ions in a typical sample of seawater are 10 ${}^{-8}$ and 10 ${}^{-6}$ respectively.
   1. Is the seawater acidic or basic?
   2. Calculate the pH of this seawater.
5. Three test tubes (X, Y and Z) each contain a solution of an unknown potassium salt. The following observations were made during a practical investigation to identify the solutions in the test tubes: A: A white precipitate formed when silver nitrate (AgNO ${}_3$ ) was added to test tube Z. B: A white precipitate formed in test tubes X and Y when barium chloride (BaCl ${}_2$ ) was added. C: The precipitate in test tube X dissolved in hydrochloric acid (HCl) and a gas was released.D: The precipitate in test tube Y was insoluble in hydrochloric acid.
   1. Use the above information to identify the solutions in each of the test tubes X, Y and Z.
   2. Write a chemical equation for the reaction that took place in test tube X before hydrochloric acid was added.
   (DoE Exemplar Paper 2 2007)