Lesson 8 of 10
Objective: SWBAT clearly describe observations in lab and describe and recognize combustion reactions.
The lab from the previous lesson left me unhappy with student observation skills, so I decided to slow down the unit and work on these skills for a day.
These skills were interfering with our ability to properly carry out investigations (SP3) and to analyze and interpret data (SP4). By not recording quality data, it was impeding student ability to process it.
I decided to revisit the previous day's experiments to improve our skills, so students could see the concrete improvement when they compared the two papers. Having the unfiltered waste products and some filtered products, helps us to make our observations and draw conclusions from them.
The final portion of the lesson is concerned with recognizing the chemical equations for combustion, and being able to predict the products of combustion of hydrocarbons, which aligns with HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Improving Observation Skills
When students enter, I have provided them a fresh copy of the Replacement Reactions Lab from the day before. This confuses them, and they ask if they are repeating the lab.
I explain that many of their observations weren't very good, so we are going to take some time to improve on that skill today.
On the board I have written all 8 chemical from the experiments, and have two markers, a black marker for old observations, and a red for improved observations.
I begin with asking how they described the silver nitrate on their lab paper. Students respond "Clear" "Looked like water" "liquid." I record these in black on the board, and then ask how they described the copper and they respond "reddish" "orange" "metal" "square" "Solid" "shiny".
I ask which set of observations is better, and students cite the copper, as it is more complete. So I focus on the silver nitrate and ask what clear means to the class. Students respond that it is see through. I hold up a test tube of copper (II) nitrate solution which is light blue, but is also see through and ask if it is clear. Students respond "Yes, but its blue also".
I follow up with asking "How would you describe the color of the silver nitrate?" Students are puzzled, and one of my seniors ventures "You wouldn't." I ask why and he states "It doesn't have a color, its colorless." I ask the students if they agree, and then we go through the experiment and label all the colorless liquids. (silver nitrate, potassium iodide, lead (II) nitrate, hydrochloric acid, ammonium hydroxide)
We then focus on the "like water" aspect, and I ask if water is always colorless. Students reply that it can be muddy, or blue-green like at the aquarium. I caution them against using comparisons, because these terms can mean different things to different people. I ask they be really precise in what they're seeing.
So we turn to the product of reaction 1, which many students labeled as "mossy" "moldy" "dirty". I pass around the filtered silver, and ask students to come up with better observations. Students volunteer "solid", "Grey", "sparkly" and "crystals". From this, I ask them to take an extra step: "Of the four elements in the reaction, copper, silver, nitrogen and oxygen; which one would fit those descriptions?" Students think about it and respond "Silver" and I explain how they can figure that out from good observations, but from the starting observations they could not. I then give students time to improve all their observations. Examples are below side by side.
I ask students what the first reaction we saw this semester was. Students have to think, and some dig into their binders to check for it. They find the burning of methane.
I explain this is our fifth type of chemical reaction and I ask them to get out their notes, or they can write on the reactions lab if they want.
I turn on the document camera and pull down the screen so the observations will still be on the whiteboard if students need them later. I write the word "Combustion" and ask what it means.
"Blowing up" "Burning"
I ask what the difference is between burning and an explosion, students aren't sure, and I ask which is faster. They reply the explosion, and I point out that explosions are things that burn very quickly, and release both energy and gases very rapidly.
I ask what three things we need to burn something, which is referred to as the "Fire Triangle"
I ask "Are the fuel and oxygen products or reactants?" "Reactants" so I begin to write a chemical equation.
Fuel + Oxygen -->
I ask "Does anyone remember how we write heat in a reaction?" In each class someone remembered, "the triangle over the arrow," so I add it to our generic example. I then ask, "What are the two products when we burn things?" Students freeze here, so I give a more concrete example, writing the formulas for methane and oxygen beneath fuel and oxygen. I ask "When these elements get rearranged, what will we make from the carbon, hydrogen and oxygen?" Students respond "Carbon dioxide" and "water?"
I add the formulas for carbon dioxide and water, and explain there are five things common to the combustion reactions we will see this year:
- A fuel that is made of hydrogen and carbon, called a hydrocarbon
- Oxygen gas
- The heat sign over the arrow
- Carbon dioxide as one product
- Water as the other product
Students ask "Why is water a gas?"
"Do you see any liquid from the bunsen burner flame? Why not?" (If students aren't sure, I light the burner)
"Its so warm the water is a vapor"
I explain that this is our last type of reaction, so now students will need to tell all five types from each other. I pass out a worksheet with various reactions for them to classify.
To put all their reaction work together, students have time to work on the Classification of Chemical Reactions worksheet from Instructional Fair's Chemistry worksheets. This book contains a lot of good, concise practice worksheets on a variety of topics. Some go more in depth than we need in our curriculum, so we also use the Physical Science book at times.
Students have ten chemical equations to classify as synthesis, decomposition, single or double replacement.
Students struggle if they haven't internalized the rules of classifying reactions.
- Single product = synthesis
- Single reactant = decomposition
- Multiple reactants and products, single element on each side = single replacement
- Multiple reactants and products, compounds on each side = double replacement
The quiz on the types of reactions will be in two days, so I encourage students to look back at their notes and practices. This could be a good place to have students create a dichotomous key to classify reactions, but I did not do so this year due to class time constraints following our missed days due to weather.