Acids, bases and litmus paper Litmus paper is an ‘indicator’ that tells us whether a substance is acidic, basic or neutral. Blue litmus paper turns red.

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Presentation transcript:

Acids, bases and litmus paper Litmus paper is an ‘indicator’ that tells us whether a substance is acidic, basic or neutral. Blue litmus paper turns red in when put in an acidic substance. Red litmus paper turns blue when put in a basic substance. If you put both red and blue litmus paper in a substance and there is no change in colour, it is said to be ‘neutral’. neutral

Litmus paper

Instructions In your newly assigned groups get 1 test-tube rack, 3 test-tubes, a small beaker, and a bottle of HCl and NH 4 OH and make a station around the room. When one of the natural indicators are free, one of you come to the front with your small beaker and get some of the natural indicator. Go back to your station and pour 1/3 of the natural indicator in each test-tube. Add acid to one test-tube, Base to another and water to the last. View and compare the change in colours (if any!). Create your own table to record the data! (however you like).

Copy and complete the table Natural Indicator Acid (HCl)Water (H 2 O)Base (NH 4 OH) Ammonium Hydroxide Green Tea Black Tea Onion Skins Boysenberry Juice Red Cabbage Juice

Copy notes pH is the measure of the concentration H+ ions in an aqueous (containing water) solution. pH is a numerical measure from 0-14, where is acidic, 7 is neutral and is basic or alkali. If a substance forms H+ ions in a solution it is acidic, and if it forms hydroxide OH- ions it is basic. Extension: To calculate the pH we use the formula “– log(H+)” E.g. for dilute HCl the concentration of H+ ions is 1X10 -1 so using your calculator… pH = -log (1 exp -1) Therefore, pH = 1

Questions to consider What is the range of pH for the shampoos? Are there any shampoos outside the range recommended by the textbook on page 246? Why wouldn’t you use soap as a shampoo? What is the pH of some of the indigesion powders? Why? (what would it do in our body)? Extension: ENO is also an indigestion powder, however it has a far different pH then other indigestion powders. Why could this be?

Copy an complete the table NameFormulaIons Sulfuric acid Hydrochloric acid Sodium Hydroxide Ammonium Hydroxide Ammonium Nitrate Acetic acidCH 3 COOH

Answers NameFormulaIons Sulfuric acidH 2 SO 4 H + SO 4 2- Hydrochloric acidHClH + Cl - Sodium HydroxideNaOHNa + OH - Ammonium HydroxideNH 4 OHNH 4 + OH - Ammonium NitrateNH 4 NO 3 NH 4 + NO3 - Acetic acidCH 3 COOHH + CH3COO - Ammonia and WaterNH 3 H 2 ONH 4 + OH -