Indicators

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Lower Secondary School Chemistry- Indicators

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  • How do you “recognise” the pH scale? What do you want the students to be able to do to show you that they “recognise” the pH scale? E.g. State that the pH scale ranges from 0 to 14 etc?
    Similarly, what do you expect students to be able to do when they understand what indicators are? E.g. state the colour changes when litmus is used to test for acid and alkali?
    Be more specific. The objectives need to be measureable.
  • Indicators

    1. 1. After this lesson, you should be able to: Recognise the pH scale Understand what indicators are
    2. 2. PH SCALE  To measure the acidity or alkalinity of a substance  It is a range of values between pH 0 to pH 14. Acids pH < 7 Neutral Alkalis pH > 7 0 High acidity High alkalinity
    3. 3. 0
    4. 4. HOW TO FIND OUT PH VALUES OF SOLUTIONS?  pH papers pH meter -Electronic device to measure the acidity or alkalinity of solutions accurately  Datalogger - Comes with a pH sensor which will measure the change in pH value of solutions over time. - Connected to the computer to transfer the data collected. 
    5. 5. RECAP…  Acids turn blue litmus paper red  Alkalis turn red litmus paper blue
    6. 6. WHAT IS AN INDICATOR?  A substance which changes colour depending on whether the solution tested is acidic or alkaline  They are usually solutions  Can come from natural sources or synthetic sources Red Cabbage Phenolphthalein
    7. 7. COMMON INDICATORS Litmus paper Universal Methyl Indicator orange Phenolphthalein
    8. 8. pH 0 to pH 3: red pH 4 to pH6: orange/yellow pH 7: green pH 8 to pH 10: dark green/blue pH 11 to pH 14: purple
    9. 9. COLOUR CHANGES OF OTHER INDICATORS Methyl Orange pH 0 to pH 3: Orange to red pH 4 to pH 14: Orange to Yellow  Phenolphthalein pH 1 to pH 9: Remain Colourless pH 10 to pH 14: Colourless to Pink 
    10. 10. QUESTIONS! What is the function of a litmus paper? To test if a solution is an acid or alkaline.  When would it be more suitable to use the Universal Indicator pH paper? When you need to find out the exact pH of a solution. 

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