For this activity, students use a variety of phenomena including weathered and eroded tombstones, dissolving salt, and carbonated soda to explore the concept of dissolved substances. This is designed to lead them to an understanding of how gases can be dissolved in liquids, which will be important for understanding how air pollution can get into the Chesapeake Bay.

Approximate Time: 30-45 minutes​

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Objectives:

• Students will understand the concept of a gas dissolved in a liquid

• Students will understand that “polluted rain” can have a significant effect on objects

Materials:

• Salt

• Cups of water

• Stirrer

• Bottle of carbonated beverage (ex. soda)

• Word wall words: dissolve

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Handouts:

• Disappearing Salt

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SEP: Constructing Explanations & Designing Solutions

CCC: Cause & Effect

Show students a picture of monuments or tombstones affected by acid rain, such as the one below, and have them write what they think happened to the monument/tombstone:

Depending upon what they have already learned, students may discuss weathering and erosion, or them may say things like a piece broke off or the letters wore off. Use their responses as a way to gauge student background knowledge on the concept of acid rain and weathering/erosion, but do not give them a definitive answer.

Remind students that during the last activity, they investigated how pollution from the ground gets into the Chesapeake Bay. Show them the infographic from Activity 5 and remind them that 1/3 of the nitrogen pollution in the Bay comes from air pollution. How is that possible? How does air pollution get into the Bay? Today they are going to start answering that question.

Pass out the “Disappearing Salt” handout. Then give each pair of students a small cup of water, a tablespoon of salt on a paper towel, and something to stir with. Have students write observations about the salt and the water on their sheets. For example, the salt is a solid, it’s white, it’s hard, it’s opaque; the water is a liquid, it’s colorless, it’s clear.

Have students mix the salt into the water, and then make an observation of what they see. For example: the salt slowly disappears. The mixture is a clear, colorless liquid.

Ask students if they think the salt is still in the water. How can they tell? (If permitted, and students are interested, you can have one student from each group taste the water to verify that the salt is still in it).

Ask students if they know what happened to the salt – some may know that the salt “dissolved” in the water. Put the word “dissolve” up on the word wall, and ask students what this means. Help them to develop the definition that when something dissolves in water it mixes with the water and changes form but it is still there. Have students write an explanation of what happened to the salt using the word dissolve in the space on their handout.

Make sure students understand that the first example was a solid dissolved in a liquid. Ask them if they think gases can be dissolved in water. They should remember talking about dissolved oxygen earlier in the module, or they may have other examples.

Take out a bottle of soda or another carbonated beverage. Have students write down observations of the liquid in the bottle in the “before” section of their paper. For example: it is brown, it is a liquid, it is opaque.

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Ask students if they think anything is dissolved in the soda. They may say things like sugar or flavorings (which are correct). Ask students if they think there is a gas dissolved in the soda. They will likely know that there is.

Ask students how they could show that there is a gas dissolved in the liquid. There are many possible responses: when you open it, you can hear a sound, when you shake it or pour it, the gas will fizz, if you drink it you might have to burp, etc. After hearing student responses, have students prepare to make observations in the “after” section of their handout. Then shake the bottle and then open it so that the carbon dioxide is released quickly and becomes very visible (preferably over a sink or in a plastic bag). Have students write down their observations (ex. there are lots of bubbles coming out of the liquid, it changes shape, it changes color, etc.)

Ask students if they know what the gas is that was dissolved in the soda. Use questions to help them connect the idea of “carbonated” drinks with “carbon dioxide”.

Have students answer the question at the bottom of their handouts: “What does it mean for something to be dissolved in something else” Student explanations should include the idea that the first thing changes form and mixes in with the other thing. Often the thing that is dissolved can no longer be seen.

Make sure students agree that solids and gases can both be dissolved in water. Have students think back to Activity 2 when they measured dissolved oxygen in water in order to help them make the connection that oxygen can dissolve in water, and that living things like fish need the dissolved oxygen to survive.

Point at different parts of the map, and ask students “If it rains here, where will the water go?” Point to parts of the map that are in the watershed, and not in the watershed. You can be specific about where the water will go before it gets to the Bay (ex. it goes into the Susquehanna River and then into the Chesapeake Bay), but the key is that all the water in the Chesapeake Bay watershed goes to the Bay. Have students write the definition of the Chesapeake Bay watershed on their notes sheet: “All the land that drains its water into the Chesapeake Bay.”

Show students the picture of the tombstones again. Ask them again what they think may have happened to them. Use questions to focus their thinking around key ideas:

• The tombstones are solid, but they partially washed away. Part of the tombstones must have dissolved in rainwater and washed away

• Rainwater on its own is not strong enough to dissolve a tombstone

• Something must be in the rainwater that allows it to dissolve the stone

Ask students what they think may be in the water that allows it to dissolve the tombstone. Depending on their background knowledge, you may or may not need to guide them towards the idea of acid in the rain.

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Ask students where the acid in the rain could have come from. Use questions to help them narrow it down (it can’t be from the ground – it must be from the air). Key takeaways:

• Air pollution in the atmosphere can dissolve in water in the air and clouds

• The dissolved pollution makes the water acidic

• When it rains, the acid rain can affect things on the ground

Have students draw a simple picture showing how air pollution can get into the rain and end up affecting things on the ground. For example: