This lesson is based on California's Middle School Integrated Model of NGSS.
NGSS Performance Expectation (PE): (MS-PS1-2) Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
Science and Engineering Practice 3: Planning and carrying out investigations
Disciplinary Core Ideas (DCI): PS1.B: Chemical Reactions - In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.
Crosscutting Concepts (CCC): Energy and Matter - The transfer of energy can be tracked as energy flows through a designated or natural system.
Common Core ELA Literacy/Writing Literacy in History/Social Studies, Science, & Technical Subjects: (WHST.6-8.3) Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.
This lab uses ordinary hydrogen peroxide 3% (H2O2) and active yeast - both easily found in grocery stores - to produce an exothermic decomposition reaction using a catalyst. An exothermic reaction releases heat and the decomposition reaction tears a complex molecule (H2O2) into simpler molecules of oxygen (O2) and water (H2O). This reaction occurs naturally over several months and can be accelerated by sunlight (hence hydrogen peroxide is sold in brown bottles). A catalyst (active yeast) is used to accelerate this reaction, but is not part of the reaction.
Your students should observe the properties of the reactant (H2O2) and the properties of the products (O2) and (H2O) to determine if a chemical reaction has occurred (MS-PS1-2) (see Evidence of a Chemical Reaction).
The reaction is described below:
2H2O22H2O + O2
The higher percentage of hydrogen peroxide (H2O2) used, the more energetic the reaction will be. I use 3% because it is cheap, available, slow, and safe.
CAUTION: Higher concentrations of hydrogen peroxide 30% (H2O2) are caustic and will produce a nasty burning itch along with a mark. Hydrogen peroxide is a base and is slippery (properties of a base). It is used as a disinfectant because it burns the infection away.
I use hydrogen peroxide 30% (H2O2) as a demonstration in class to get a bigger wow-factor (see Elephant Toothpaste Demonstration). Hydrogen peroxide 3% (H2O2) is safe to touch, Hydrogen peroxide 30% (H2O2) is not. The percentage refers to the amount of hydrogen peroxide in distilled water. Hydrogen peroxide 100% (H2O2) is used as rocket fuel in rocket packs.
TIP: This lab requires students to measure a liquid in a graduated cylinder, make sure you have explained that a liquid in a graduated cylinder forms a meniscus (curve) and the measurement should be taken at the bottom of the meniscus.
This reaction will occur naturally over several months, it needs a catalyst (active yeast) to accelerate it to something that can be observed in a few minutes.
TIP: When weighing a granulated solid, I have my kids fold a full sheet of paper in half and place it on the scale. If the scale is set to 4g then the scale should be balanced. Since this lab needs five grams of active yeast, set the scale to 9g with the paper on it. Use the handle of a spoon to carefully add active yeast until the scale is again balanced. Pick up the paper and place the edge of the fold at the opening of the water bottle and dump the contents into the bottle.
Once the reaction has begun, I have my students start a timer, and record their observations at regular intervals.
NOTE: If the students hold the bottle they will be able to feel a temperature difference, as this reaction gives off heat (exothermic). I try to have them feel the bottle without giving away the fact that it is exothermic. I also use a lighter and touch the flame to the oxygenated soap. The soap should fizzle and pop, proving the presence of oxygen (O2) (see Evidence of a Chemical Reaction).
I was once showing this reaction to a group of Cub Scouts and thought is would be beneficial to show them an example of a base. So I had them rub hydrogen peroxide 30% (H2O2) between their fingers. In a few minutes the boys were crying and washing their hands with soap didn't help. Needless to say, I wasn't popular with the moms that night.
Each student receives a lab sheet with directions and a data chart to record their observations.
Materials needed: hydrogen peroxide H2O2 – 3%, clean/empty water bottle, graduated cylinder, triple-beam balance scale, liquid soap, food coloring, active yeast, eye protection
CAUTION: This lab may spit hydrogen peroxide (H2O2) into the air. Always insist upon student eye protection.
TIP: You can perform this experiment without the graduated cylinder and triple-beam balance scale, just eyeball the amounts. I collect used clean water bottle for this activity.
CAUTION: I purposely do not include a cap with these water bottles, we talk about how this reaction, if sealed, is a potential bomb. Bomb making is NOT chemistry and is a potential expellable offense.
This sort of lab requires the students to follow precise steps in order to produce the desired results (SP3: Planning and carrying out investigations/ CCLA: 6.SP.B.5).
As part of their post-lab, I have my students prove that they observed a chemical reaction (color/light, temperature change, solid if formed, odor is generated, or gas is produced (MS-PS1-2). Through this investigation they should understand that the atoms that made up hydrogen peroxide (reactant) were rearranged into oxygen and water (products) and these new products have different properties from the original reactants (DCI: PS1.B). You may also want to explain that the reactant energy was transferred to the product energy in a predictable fashion (CCC: Energy and Matter).
TIP: I teach my students a mnemonic sentence for remembering the evidence of a chemical reaction.
Cats tell stories of ghosts.
C - color/light
T - temperature change
S - solid if formed (precipitate)
O - odor is generated
G - gas is produced
They must also identify the type of reaction they observed (synthesis, decomposition, combustion, replacement). They must present evidence in order for their observations to be confirmed.
I also created a mnemonic for remembering the types of chemical reactions.
Squids don't carry roses.
S - synthesis
C - combustion
R - replacement
I have my students draw two pictures, one for evidence of a chemical reaction and the other for the type of reaction in their interactive notebooks. As part of their training, I explain the difference between drawing for fun and creating accurate scientific drawing. Scientific drawings require a title, minimum of three colors (pencil doesn't count), label with arrows, and a brief text explaination. I often require students to redo their work if it is not scientific enough.