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Title: Mystery Powders: Identifying substances
Principle(s) Investigated: Physical and Chemical properties
8.5.a Students know reactant atoms and molecules interact to form products with different chemical properties.
8.7.c Students know substances can be classified by their properties, including their melting temperature, density, hardness, and thermal and electrical conductivity.
Materials:
- Baking soda
- Cornstarch
- Water, vinegar, iodine solution
- Test mat with wells or plastic-foam egg carton (1 for each group)
- Spatulas or popsicle sticks (2 for each group)
- Eyedroppers (3 for each group)
- Stirring rod or coffee stirrers
Procedure:
1. Start the lesson by telling students that they are going to help the local police unit to solve a crime. (you can present a scenario) Police found two white powdery substances spilled on the kitchen floor. They found similar substances on the shoes of the suspect. Now, the police needs your help in finding out what the substances are.
The teacher’s demonstration may be necessary for the procedure of testing and recording observations for Substance 1.
Important: Substances should be tested one at a time. Samples should not get mixed together.
Step 1: Use a spatula (popsicle stick) to place a small amount of Substance 1 into three micro-wells of your testing mat (or egg carton).
Step 2: Record your observations about the substance sample’s appearance, such as color and texture into the data table in the column labeled “Description”.
Step 3: Use an eyedropper to add 5-6 drops of water to the substance sample in the first well. Stir with the stirring rod. Record your observations in the column labeled “Mixed with water”.
Step 4: Use a clean dropper to add 5-6 drops of vinegar to the second well of the substance sample. Use a clean stirring rod, stir and record your observations in the data table column “Mixed with vinegar”.
Step 5: Use a clean dropper to add 4-5 drops of iodine solution to the third well and stir using a clean stirring rod. Record your observations in the column “Mixed with iodine solution”. Caution: Be careful when using iodine, because it will stain your skin and clothes.
(Extension: A heat test can also be added: place a small amount of the substance on a piece of aluminum foil, bend the edges and hold the sample over the candle flame for few seconds using a pair of tongs or tweezers.)
Step 6: Repeat steps 1-5 for the Substance 2. Use a clean spatula (teaspoon).
Students work independently within their groups.
Student prior knowledge: Students should have knowledge about physical and chemical properties of a matter. Students also should be able to identify physical and chemical changes of a matter.
Explanation:
A physical property of matter is a characteristic that can be observed or measured without changing the matter’s identity. For example, in this experiment students needed to describe the color and texture of the substances, which are physical properties of those substances. Other examples of physical properties such as thermal conductivity, ductility, state, malleability, solubility, and density help to identify
In this experiment we observed the physical property of solubility, which is the ability of a substance to dissolve in another substance. For example, baking soda dissolved in water.
Chemical Properties describe matter based on its abilities to change into a new matter. Chemical properties include flammability, oxidation, digestion, decomposition etc.
in this experiment we observed the chemical reaction of baking soda with vinegar. Vinegar is acetic acid and baking soda is a sodium bicarbonate. Mixing the two creates an acid base reaction. The CO2 leaves the reaction in the form of fizzing.
NaHCO3 + HC2H3O2 ===> Na+ + C2H3O2- + CO2(g) + H2O
Questions & Answers:
1. What physical change did observe during your testing? Baking soda dissolved in water.
Name the physical property of that change. Solubility
2. What chemical changes did you observe during you testing? Fizzing, color change
Name the chemical property of that change. Reactivity
3. Why is important to use different methods to determine what substance it is? Some substances may have similar physical properties and react similarly with vinegar for example but differently with iodine.
Applications to Everyday Life:
- The Statue of Liberty is made of copper, which is orange-brown. It is green now because the copper due to the interactions with moist air undergone through a chemical change and formed copper compounds. Over time, the compounds turned the statue green.
- Rust- The iron has the chemical property of reactivity with oxygen. When iron is exposed to oxygen, the iron rusts.
- Melting ice cream is an example of the physical change. It changes from a solid to a liquid form.
Photographs:
Physical Science, Teacher’s Edition; Holt, Rinehart and Winston, A Harcourt Education Comp, 2007
LaRosa, Liz, “Mystery Powder Lab”. http://www.middleschoolscience.com
“The Case of Similar Substances”, Discovery Online Curriculum, 2002
http://www.discoveryschool.com
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