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Title: Light Relight
Principle(s) Investigated: Observations, inferences, chemical reactions, the role of oxygen, the combustion triangle
Grade 8
5. Chemical reactions are processes in which atoms are rearranged into different combinations of molecules. As a basis for understanding this concept:
a. Students know reactant atoms and molecules interact to form products with different chemical properties.
Grade 9-12
a. Students know the observable properties of acids, bases, and salt solutions.
c. Students know strong acids and bases fully dissociate and weak acids and bases partially dissociate.
d. Formulate explanations by using logic and evidence.
Materials: White vinegar, hydrogen peroxide, popsicle stick, 2 graduated cylinders, lighter, 1 tsp measuring spoon, yeast (mystery powder #1), baking soda (mystery powder #2)
Procedure:
1. Add 1 teaspoon of baking soda (mystery powder 1) to both graduated cylinders.
2. Add 1 teaspoon of yeast (mystery powder 2) to both of graduated cylinders.
3. Hold the graduated cylinders and swirl it in order to mix the two powders.
4. Add a generous amount of white vinegar one of the graduated cylinders.
5. In the other graduate cylinder add a generous amount of hydrogen peroxide.
Note: the mixtures will create a reaction with bubbling and fizzing in both cylinders.
6. Use the lighter to light the popsicle stick. Make sure you have strong consistent flame before continuing.
7. Stick the popsicle stick with the flame down the graduated cylinder that contains the white vinegar. Do not touch the bubbles, liquid, or cylinder. The flame will extinguish.
8. Take the smoking popsicle stick over to the graduated cylinder that contains hydrogen peroxide. Do not touch the bubbles or the cylinder itself. The ember will begin to glow more intensely until the flame relights.
9. Let your partner try. Again place the popsicle stick in the vinegar cylinder and watch it extinguish. And again place popsicle stick in hydrogen peroxide cylinder and it will reignite once again.
Student prior knowledge: Students should know about chemical reactions, acids, bases.
Explanation:
The fire triangle or combustion triangle is composed of three components: oxygen, fuel, and heat to ignite and stay lit. Removal of any of the three will cause the flame to extinguish. When we first light the popsicle, the three necessary components are present. The heat generates from the separate from which came from the lighter, the wood of the popsicle stick is the fuel. The oxygen in the atmosphere is enough to keep the flame lit.
When we first stick the flaming popsicle stick, the flame goes out meaning one of the three components is not present. This is where students really observe and infer what they believe is happening rather than simply reading about combustion and chemical reactions.
The baking soda (aka sodium bicarbonate) is a base. The vinegar, or acetic acid CH3COOH is a weak acid. When the baking soda and vinegar are combined, the acid-base reaction creates carbonic acid. Carbonic acid decomposes into carbon dioxide and water. The bubbling seen inside the first cylinder is the CO2 being produced. When you dip the flaming popsicle into the cylinder filled with CO2, the lack of oxygen extinguishes the flame.
Next, we put the extinguished popsicle stick into the second graduated cylinder, where we see the ember glow more until the flame reignites. The high concentration of O2 in this cylinder makes the heat source more intense until the flame ignites again. Hydrogen peroxide (H202) is fairly stable and is always trying to decompose into water (H20) and oxygen (O2). When yeast is mixed in with it, it acts as a catalyst to the decomposition. This creates oxygen at a much faster rate as seen with the bubbling and fizzing. By placing the popsicle stick back into this cylinder, the oxygen completes the triangle and reignites the flame.
The Fire Triangle
1. There must be Fuel to burn.
2. There must be Air to supply oxygen.
3. There must be Heat to start and continue the combustion process.
Questions & Answers:
1. How do firefighters use the concept of the fire triangle to suppress forest fires?
A: Fuel separation in a wildfire separation. Fire fighters focus on the fuel aspect of the fire triangle. Because trees and other shrubs acts as fuel to continue fires, firefighters create control lines that contain to combustible material. They may be man made or natural.
2. What clues did you have that a chemical reaction occurred in this activity? What is the catalyst in this activity?
A: The bubbling. The cylinder may even feel a bit warmer, that is because a change in temperature is also sign that a chemical reaction may be occurring. The catalyst is the yeast.
3. Why did the acid-base reaction (vinegar and baking soda) not react more violently? What if we used hydrochloric acid instead of vinegar.
A: Vinegar, an acetic acid is a weak acid and has much lower concentration compared to hydrochloric acid. The base, or sodium bicarbonate is a weak base with a pH of 9. We are still able to observe the vinegar baking soda reaction as seen with the fizzing and bubbling. Hydrochloric acid is a strong acid, with a pH of -1 and dissociates completely in water.
Applications to Everyday Life:
1. Suppressing a small fire around the house, such as a kitchen fire. Water typically reduces the heat and therefore is one way to extinguish a fire. Sand and baking soda are can also accomplish this. Removing the fuel reduces the heat. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. The oxidizer is the reactant of the chemical reaction, therefore to remove oxygen, one can remove oxygen by smother the fire with a heavy blanket, for a example a fire blanket. The most common dry chemical foam or powder is typically made from sodium bicarbonate (baking soda) or potassium bicarbonate. When baking soda starts to decompose, it releases carbon dioxide. The CO2 along with the insulation of the foam smothers the fire.
2. Oxygen is essential for life and takes part in combustion, its biological function in respiration make it important.
3. It is used for welding and cutting metals. It is used with hydrogen or coal gas in blowpipes and with acetylene in the oxy-acetylene torch.
4. In medicine, oxygen is used in the treatment of pneumonia and gas poisoning, and it is used as an aesthetic when mixed with nitrous oxide. Carbon dioxide can mixed with oxygen to stimulate breathing and this mixture is used in cases of when someone is poisoned and needs to restore respiration.
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.
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