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activity: Investigating temporary and permanent hard water

What you will need:

  • 6 boiling tubes and bungs
  • 5 beakers
  • distilled water or rain water
  • tap water
  • solid calcium hydrogencarbonate
  • solid calcium sulphate
  • detergent (soap solution) in a dropping bottle (or with a pipette)

  • spatula
  • glass stirring rod
  • test tube clips
  • test tube rack
  • safety glasses

What you will do:

  1. Fill 3 beakers half full of distilled water or rain water.
  2. Add one spatula of calcium hydrogencarbonate to one beaker and one spatula of calcium sulphate to the second beaker. Stir the contents of both beakers well.
  3. Filter the contents of both beakers and collect the filtrate (the liquid that comes through the funnel) in clean beakers. Label both beakers clearly so you know which contains which solid.
  4. Pour about 3 cm of water from your labelled beakers and straight from the tap into 3 separate boiling tubes. Add 5 drops of detergent to each tube and shake them twenty times each.
  5. Record your results in a table like the one below. You could also draw what you see in the table.

What do you observe?

Water sample Before heating After heating

 

calcium hydrogencarbonate

 

    

 

calcium sulphate

 

    

 

tap water

 

    
  1. Keep your boiling tubes with the water samples so you can compare your results.
  2. Pour about 3 cm more of water from your labelled beakers and water straight from the tap into 3 separate boiling tubes. Using test tube clips to hold onto the tubes, carefully heat the water inside the tubes. Make sure you are wearing safety glasses and the end of the boiling tube points away from anyone's face. Heat the water until it boils, letting it boil for 3 minutes.
  3. Let each boiling tube cool down.
  4. Add 5 drops of detergent to each cool tube and shake them twenty times each.
  5. Record your observations in a table and compare your results to those you made before heating.

Questions

  1. Which compound makes water temporarily hard?
  2. Which compound makes water permanently hard?
  3. In this experiment you found that boiling temporary hard water can make it soft again.You used heat to break down the compound in temporary hard water. What is this kind of reaction called?
  4. Was your sample of tap water permanently or temporarily hard? Explain your answer.

Click here to check your answers.

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