Unit 4: Chemical Reactions

We observe different bath bombs and what they do when added to water and then develop individual models and explanations to show what is happening at a scale smaller than we can see. We develop an initial class consensus model, brainstorm related phenomena, develop a DQB and ideas for investigations to pursue. We figure out that:

• We had competing ideas for whether the matter in the solid that we started with is still there after it is added to water. Some thought that it was all still there, while others thought not all of it was still there.

• We had competing ideas for where the gas came from that was in the bubbles that appeared. One was that it was there to start with (trapped inside the solid). The other was that it formed from some of the stuff we started with (e.g., in the solid and/or water).

We investigate bath bombs, measuring their mass in a closed and open system before and after crushing them and before and after we add the bath bomb to water. We argue from evidence about where the gas came from. We figure out that:

• The gas we observed from the bath bomb does not come from any gas that was originally trapped in the bath bomb itself.

• Instead, the gas we observed when the bath bomb was placed in water comes from some change to the matter that is already there.

In this lesson, we make observations and collect data on each of the main ingredients in a bath bomb, recording the properties of each. We also investigate each ingredient as it mixes with water and record our observations. However, we see that the ingredients interact with water in different ways. We figure out that:

• Substances in the bath bomb have properties that can help us identify them (e.g., solubility, odor, state of matter at room temperature, melting point, density, and color).

• Mixing only one substance from a bath bomb with water does not cause gas bubbles to appear.

We will discuss and record what we’ve figured out so far in the unit. We will plan and carry out an investigation to test different combinations of substances from a bath bomb, and we will use the results to argue that the gas produced must be a new substance. We figure out that:

• Citric acid and baking soda combined are the only substances from the bath bomb that, when combined with water, cause gas bubbles to form.

• The gas(es) in the bubbles are substance(s) that are different from any of the substances we started with.

We brainstorm phenomena related to gases and identify some different properties. We analyze the data by taking into account common gases and their known densities and flammabilities. We test the flammability of air from the room, gas from the bath bomb, and helium gas. We carry out an investigation to see if gas from the bath bomb rises or sinks. We argue from evidence (density and flammability data) that the gas from the bath bomb can be narrowed down to three candidate substances. We figure out that:

• Density and flammability are properties.

• In high concentrations, gases that are non-flammable will extinguish a flame.

• Materials that are less dense float upward when surrounded by matter that is more dense; materials that are more dense sink downward when surrounded by matter that is less dense.

• The gas from the bath bomb could be nitrogen, argon, or carbon dioxide.

We apply what we have figured out about properties to explain a related phenomena (elephant’s toothpaste). We revisit our DQB and reflect on what other related phenomena we might explain using the same key model ideas. We figure out that:

• The mass of a partially open system where potassium iodide and hydrogen peroxide are combined decreases because a gas is formed and some of it escapes the system.

• The gas makes a glowing ember burst into flame and an already burning flame glow brighter.

• Flammability data can help identify the types of gases that aren’t being produced.

• Testing the melting/freezing point, density, and/or comparing the results of the flammability test to results from controls could help identify additional gases that aren’t being produced in this process.

We work as a class to summarize and review all of the science ideas we have figured out through the investigations we have done so far in order to put all the pieces together. We develop a new way to represent what we figured out, using an input/output table. We identify an unanswered question about where the particles that make up the substance(s) of the gas came from and individually develop a model to try to explain this. We figure out that:

• The same substance is made of the same type of particles throughout.

• Different substances are made of different materials throughout.

• The particles that make up the substances in the gas bubbles from a bath bomb must be a different type of particle than any of those in the substances that were combined together to make it (water, baking soda, and citric acid).

We develop alternate models for how new particles might be made from old particles using manipulatives (printed colored circles). We formulate questions we have about how we could figure out what happens when new substances are made from old. We read about what Dalton and other scientists did to see if adding energy to water could form new particles. We figure out that:

• When new substances form from old substances, the particles of the old substances might break apart and/or stick together to form new combinations of particles.

• We have a new line of investigations to pursue to see if new substances are formed when energy is added to a single substance (water).