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Topic: Physical and Chemical Changes
Demonstration: Distinguishing between chemical and physical change Concepts:
A physical change does not involve any bond breaking or making and no new chemical is formed, only the state of the chemical is changed; eg. From solid to liquid
A chemical change involves the breaking and making of bonds and the formation of a new substance, identified by, e.g;, a change in colour Safety:
Safety: glasses must be worn, handle lead nitrate and potassium iodide with care minimise contact with skin by wearing gloves/ using spatula if possible when handing these chemicals
Physical Change: Ice
Chemical Change: 2 x large test tubes, rubber stopper to fit test tube, 10 g lead nitrate 10 g potassium iodide black background
Preparation: Make sure that both lead nitrate and potassium iodide are in fine powder form, if not; grind-up separately each solid using mortar and pestle, prior to the demonstration.
Physical change: Using tongs, transfer some of the ice into a conical flask. Shake the flask to increase the rate of change from solid to liquid.
1. Place about 15g of lead nitrate into one test tube and about 15g of potassium iodide into another test tube then transfer one solid into another.
2. Stopper the test tube and shake vigorously until there is a colour change.
Physical Change: Ice changes into liquid i.e; water.
Chemical Change: Both lead nitrate and potassium iodide are white solids. Upon mixing, a yellow solid (lead iodide) is produced. A change in colour indicates chemical change rather than physical change. Mixing of two solids to produce chemical reaction is unusual, as normally this reaction is done by mixing lead nitrate solution and potassium iodide solution, however, using solids is more effective as a demonstration. Students will observe that in the first instance there was only physical change since solid H2O turned into liquid, whereas in the second example, a new substance with different colour was formed with no white powder remaining, which means that both potassium iodide and lead nitrate have converted to a new substance.
Students will be able to distinguish between physical and chemical changes.
Students will be able to write balanced equations from observations and include physical states for all reactants and products.
Solid waste should be disposed of into a special toxic waste bin.
Pb(NO3)2 + 2KI → PbI2 + K2(NO3)2
Topic: Chemical Reactions
Demonstration: Burning magnesium
Characteristics of a chemical change Identification of products of a reaction Balancing chemical equations
Safety: Safety glasses MUST be worn at all times DO NOT look directly at the light produced by the magnesium Work on a heat-proof mat when using open flames Turn off gas/spirit lamp when not in use.
Requirements: safety glasses, heat-proof mat, metal tongs, gas burner/spirit lamp, matches, magnesium ribbon strips, phenolphthalein. Preparation: take small strips of magnesium and clean off (use sand paper) any oxide layer formed on the magnesium prior to burning.
1. Put spirit lamp on top of the heat-proof mat. Turn on the spirit lamp.
2. Using metal tongs pick up a piece of magnesium ribbon and hold it inside the hottest part of the flame. Wait until the magnesium starts to burn then take it out of the flame. Please make sure you tell students NOT TO look directly at the light produced by the magnesium.
3. Drop the burning magnesium ribbon onto the heat-proof mat.
4. Transfer the burnt magnesium into water, shake the content, heat the content.
5. Add few drops of phenolphthalein in the content.
i. The heated magnesium strip produces a bright white light while burning.
ii. The magnesium changes from a silver metallic strip to a white powder.
iii. The burnt magnesium is magnesium oxide.
iv. Magnesium oxide is sparingly insoluble in cold water but fairly soluble on hot water.
v. Aqueous solution of magnesium oxide is basic that is why it changes the clour of phenolphthalein from colourless to pink.
Mg(s) + O2(g) → 2MgO(s)
MgO(s) + H2O(l) → Mg(OH)2(aq)
Students should be able to convert observations into a chemical equation.
Students should be able to balance chemical equation.
Students should identify states of matter and include those when writing equation.
Disposal of chemicals:
All solids may be placed into a laboratory bin.
Demonstration: Reaction between sodium and water
Characteristics of a chemical change
Converting observations into chemical equation
Identifying products of a reaction
Balancing chemical equations
Safety glasses MUST be worn at all times MUST use tweezers when handling sodium Take only one piece of sodium at a time out of the paraffin oil
Observe from a distance. Do not stand too close while the reaction is taking place Only use small pieces of sodium (no bigger than pea size)
Requirements: safety glasses, overhead projector, large glass bowl, water, tweezers, scalpel, ceramic plate, sodium metal in a container with paraffin oil, phenolphthalein indicator
Cut three pea-size pieces of sodium metal and place them in paraffin oil in an enclosed
container. Half-fill the large glass bowl with water.
1. The bowl of water should be placed on top of the overhead projector.
2. Place about five drops of phenolphthalein indicator into the water.
3. Drop a small piece of sodium metal into water
Figure: Using tweezers drop small piece of sodium metal into water.
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