Grade 9/10 Alex Foo – Alexfoo138@gmail.com Chemistry Demos Grade 9/10 Alex Foo – Alexfoo138@gmail.com

Overview Introduction to chemical changes Colour change reactions Blue bottle Cobalt chloride equilibrium Glow in the dark Slime/polymers

Blue Bottle - Protocols Methylene blue NaOH Glucose Dissolve 1.5g NaOH, 1.7g glucose in 250mL water Add 5-7 drops of 5% methylene blue solution Wait until blue colour disappears Shake flask vigorously to produce colour change Methylene blue is commonly used as a biological stain - Also available from pet stores – antiparasitic drug for fish

Cobalt Chloride - Protocols Cobalt chloride (CoCl4 · 6H2O) Concentrated HCl Ice/hot water Test tube Dissolve 4g of CoCl4 · 6H2O in 40 mL water to form a red solution Add HCl until solution becomes slightly purple Add more water if you overshoot Add the CoCl4 solution to a test tube. Immerse the test tube in hot water to produce a bright blue colour. Immerse in cold water to reverse the reaction

Teaching Notes Blue bottle: Oxidation of methylene blue by atmospheric O2 produces blue colour Glucose reduces methylene blue back to colourless form Cobalt chloride: chemical equilibrium between two coloured species altered by change in temperature Very visual examples of colour changes as a result of chemical reactions Higher grades – le chataliers principle Leading questions – observe what happened. Is this a chemical change or a physical change? How do you know? Highlight the fact that chemical changes are reversible Highlight the fact that sometimes it is not easy to differentiate chemical changes from physical changes based on macroscopic observations. However, IF we know the chemistry behind a given compound, we can predict whether a given change it undergoes is a physical or chemical change Red Blue

Glow in the dark - Protocols Luminol (ebay) Fluorescein (optional – highlighter juice) Bleach Sodium hydroxide Water

Glow in the dark - Protocols Luminol (ebay) Fluorescein (optional – highlighter juice) Bleach Sodium hydroxide Solution A: 0.046 g luminol and 0.4g NaOH in 100 mL water. Add a pinch of fluorescein if desired Solution B: 1% bleach Chill, mix and watch! Note: reaction works best when both solutions are cold

Glow in the dark – Teaching notes Luminol is oxidized by the bleach to form a diacid Product initially contains an electron in the excited state Electron quickly relaxed, releasing light Example of the production of light in a chemical reaction Release of chemical energy

Slimes and Polymers - Ingredients Polyvinyl alcohol Borax Water Slime 2: Sodium alginate Calcium lactate or any other calcium salt ebay - Amazon.ca Fine food stores Spherification

Slimes and Polymers - Protocol Prepare 6% PVA solution in water Need to boil water to ensure complete dissolution Prepare saturated borax (4%) solution Combine 30 mL of PVA solution with 8 mL of borax. Add food colouring to taste Mix well to form a slime

Slimes and Polymers - Protocol Prepare a 0.75g/100mL solution of sodium alginate in distilled water Add food colouring to taste Prepare a 2g/100mL solution of calcium lactate in distilled water Pour 200mL of calcium solution into a clear beaker Add the sodium alginate solution slowly The alginate solution will polymerize instantly upon contact with the calcium solution, producing long “snakes”

Teaching notes: Polymerization involves the formation of chemical cross-links between individual molecules Resulting solid can signify a physical change Are all reactions which produce a solid the result of a chemical change?

Hard Water - Protocol Calcium acetate (ebay) Isopropanol Prepare saturated (~35g/50mL) solution in water Isopropanol Pour calcium acetate solution into a cup Add equal amounts of isopropanol Calcium acetate should precipitate out to form a gel

Hard Water – Teaching notes Calcium acetate highly soluble in water Insoluble in isopropanol Precipitates out to form flammable gel Segue into solubility, physical vs chemical properties Is this a chemical change or a physical change? Again, chance to discuss the difference between chemical change and physical change, show that sometimes it can be hard to tell However, if we understand the chemistry behind it, we can make an educated guess, and propose a hypothesis or mechanism through which this change occurs.