How does ocean acidification change ocean chemistry? Lesson 2 of 5 Developed by Brian Erickson (Erickson.bd@gmail.com) ©2019

Refresher What do you remember from yesterday’s lesson? (Use this question as an informal probe to see what registered and what needs to be reinforce from yesterday.) Discuss student answers as full class. If needed, the following questions may prompt student recall: What happened at Whiskey Creek? Where is Whiskey Creek Shellfish Hatchery? What is ocean acidification? What did we see in the visualization we watched?

A review of lesson 1 Ocean acidification = the change in ocean chemistry due to increasing amounts of anthropogenic CO2 in the atmosphere Humans release CO2 into the atmosphere The ocean absorbs ~25% of anthropogenic CO2 Ocean chemistry changes This makes it harder for some organisms And could have impacts on ecosystem and humans yesterday today & tomorrow Yesterday, we learned about the general process of ocean acidification. We looked at a visualization of how CO2 cycles throughout the year: down in the summer when plants are actively carrying out photosynthesis, up in the winter, when many plants go dormant or die. Today we are going to focus on what chemistry changes occur due to additions of CO2 from the atmosphere. We’ll also use real data from Whiskey Creek Hatchery to learn about how these changes in chemistry impact oysters. future

What do you know about pH? Gauge student knowledge with responses.

What is pH? Students often say: A scale ranging from 0-14 Describes whether a solution is acidic (0-7), neutral (7), or basic (7-14) Today we’ll learn: What happens when CO2 is added to water Relationship between pH and [H+] Different definitions of acids and bases pH is often described as measuring the “acidity” of a solution. This is a useful starting point, but it is not completely correct. We often describe solutions as acids, bases, or neutral. We typically define pH as ranging from 0 to 14. Anything below pH 7 is an acid, anything above 7 is a base, and anything at pH 7 is neutral. As you move down the scale, solutions become more acidic, as you move up the scale they become more basic. (Teacher note: this pH scale has 14 at top and 0 at bottom to match the way we think about numbers (bigger above smaller). Most pH scales are drawn with 0 or 1 at top, which could be confusing for students) Today we are going to learn

Demo: What will happen to the pH of water when CO2 is added? At this point we’re ready for a quick demonstration. I have a beaker with some indicator, which means it turns colors depending on the pH of the solution. I have a beaker of water here with universal indicator in it. Universal indicator, turns colors depending on the pH of the solution. It is blue around pH 8, which it is now. If it were more basic, it would turn darker blue, then purple, and if it becomes more acidic it will become green, yellow, orange, and then red. I also have a Soda Stream here. Does anyone know what gas makes soda and carbonated water bubbly? (CO2) What we’re going to do is add CO2 to water and see what happens to the pH. What do you think will happen when we add CO2 to water? (get 1 or 2 student predictions: direction of change and why). Then do it. Student volunteer presses the soda stream button for a few seconds, until the solution turns green/yellow. Discuss as class: What happened? (It turned green/yellow, which means the solution became more acidic) Do you think we could reverse this effect? How? (Add aquatic plants to remove the CO2 from the water, add a base) Add soda ash (sodium carbonate) to beaker and stir. Add soda ash until color changes back toward blue. Say: Hatcheries use soda ash to buffer the seawater and make it more favorable for larvae. Say: Let’s connect this demo with what we learned yesterday. Image: http://www.foodbev.com/wp-content/uploads/2015/06/sodastreampowerxrgbx1220.jpg

Why does distilled H2O have pH = 7? A small number of water molecules naturally dissociate. Dissociate = break apart into ions (charged molecules) H2O ↔ H+ + OH- When water dissociates, it forms an equal number of H+ and OH- ions [H+] = 1.0 x 10-7 M [OH-] = 1.0 x 10-7 M [H+] = 0.0000001 M [OH-] = 0.0000001 M

A general definition of acids and bases Acidic: [H+] > [OH-] Basic: [H+] < [OH-] Neutral: [H+] = [OH-] Neutral: [H+] = [OH-] (concentration of hydrogen ions equals the concentration of hydroxide ions) Acidic: [H+] > [OH-] Acids have more hydrogen ions than hydroxide ions Basic: [H+] < [OH-] Bases have less hydrogen ions than hydroxide ions

Not all solutions form H+ and OH- ions HCl hydrochloric acid HCl + H2O → H3O+ + Cl- Because not all HCl → H+ + Cl- Acids: chemicals that produce hydrogen ions/protons (H+) in aqueous solutions

Not all solutions form H+ and OH- ions NaOH sodium hydroxide NaOH + H2O → Na+ + OH- + H2O Let’s take another example: NaOH NaOH → Na+ + OH- Bases: chemicals that produce hydroxide ions (OH-) in aqueous solutions

Is vinegar an acid/base? CH3COOH ↔ ______________ + ____ CH3COOH ↔ CH3COO- + H+ Image: https://theoptimisticvoice.files.wordpress.com/2013/10/white_vinegar.jpeg

CO2 enters the ocean Carbon dioxide CO2 Mention that students should write out a description of the steps on the right side of their notes. CO2 in the atmosphere is absorbed by the ocean.

CO2 & water react to form carbonic acid (H2CO3) CO2 enters the ocean CO2 & water react to form carbonic acid (H2CO3) Carbon dioxide CO2 Water H2O Carbonic acid H2CO3

CO2 & water react; form carbonic acid CO2 enters the ocean CO2 & water react; form carbonic acid Carbonic acid dissociates; forming hydrogen (H+) and bicarbonate (HCO3-) ions Carbon dioxide CO2 Bicarbonate ion HCO3- Carbonic acid H2CO3 Hydrogen ion H+

In what ways does this graph connect to the equations we discussed earlier? How does the SodaStream demo connect with this graph and the equations? Look at the pH values on the graph. Is seawater currently acidic, neutral, or basic? Future ocean forecasts predict an average ocean pH of 7.8-7.9 by 2100. Will the ocean be acidic then? Considering your answers to #3-4, why do you think we call it ocean acidification?