Click the following link for a student learning guide to the Chemistry and Properties of Water

1. Introduction

Living things, by weight, are mostly made of water. In other tutorials, we’ve looked at how water’s polarity and the hydrogen bonding that results affects water’s properties. In this tutorial, we’re going to look at another feature of water’s chemistry: acids, bases, and the pH scale.

03_Spoon_Sugar_Solution_with_Glass2. Solutions, Acids, and Bases

When you dissolve sugar, salt, or lemonade in water, you’ve made a solution. In a solution, one substance, the solute, is dissolved in another substance, the solvent. In the solution shown in the image to your left, the solute is sugar, and the solvent is water.

In this tutorial, we’re going to focus on two special types of solutions. Acidic solutions have a sour taste. If you have a cut on your skin, and you put an acidic solution on it, it will sting. Strong acids can cause burns, and even dissolve metals. Almost all fruit juices and sodas are acidic. Sulfuric acid and hydrochloric acid are two strong acids that you might have heard of.

Basic solutions taste bitter. Strong bases have a soapy feel when you touch them. Unlike acids, bases don’t dissolve metals, so they can be used to unclog pipes by dissolving the hair that might be clogging them, without dissolving the pipe. Many bleaches and cleaning agents are bases. The word “alkaline” means the same as “basic.”

To see if you’ve mastered the concepts above, take the quiz below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Solutions, Acids, Bases Quiz (M4)”]

[h]Solutions, Acids, and Bases

[i]The following questions include labeling, multiple choice and fill-in the blanks.

[!]Question 1 [/!!]

[q]A liquid mixture in which one thing is dissolved in another

[hangman]

[c*]solution

[f]Excellent. A liquid mixture in which one thing is dissolved in another is a solution.

[!]Question 2 [/!!]

[q]The thing that does the dissolving in a solution is the

[hangman]

[c*]solvent

[f]Nice.The thing that does the dissolving in a solution is the solvent

[!]Question 3 [/!!]

[q]The thing that gets dissolved in a solution is the

[hangman]

[c*]solute

[f]Nice.The thing that gets dissolved in a solution is the solute.

[!]Question 4 [/!!]

[q]A solution with a bitter taste is a(n)

[hangman]

[c*]base

[f]Nice. A solution with a bitter taste is a  base.

[!]Question 5 [/!!]

[q]A solution with a sour taste is a(n)

[hangman]

[c*]acid

[f]Nice. A solution with a sour taste is an acid.

[!]Question 5 [/!!]

[q]_____ can dissolve metals

[hangman]

[c*]acids

[f]Nice. Acids can dissolve metals.

[!]Question 6 [/!!]

[q]_____ sting when you get them on a cut

[hangman]

[c*]acids

[f]Nice. Acids sting when you get them on a cut.

[!]Question 7 [/!!]

[q]Soda drinks are _________

[c*]acidic

[c]basic

[f]Nice. Soda drinks are acidic.

[f]No. Soda drinks have a sour taste. Sour liquids are ….

[!]Question 8 [/!!]

[q]____ have a bitter taste.

[hangman]

[c*]bases

[f]Excellent. Bases have a bitter taste.

[!]Question 9 [/!!]

[q]____ have a slippery or soapy feel

[hangman]

[c*]bases

[f]Correct. Bases have a slippery or soapy feel.

[q labels = “right”]

[l]solvent

[fx] No. Please try again.

[f*] Great!

[l]solute

[fx] No. Please try again.

[f*] Great!

[!]Question 12 [/!!]

[q]A word that means the same as basic is _______

[hangman]

[c*]alkaline

[f]Correct.A word that means the same as basic is alkaline

[x][restart]

 

[/qwiz]

3. The Chemistry of Acids and Bases: What Biology Students Need to Know

What causes a solution to be acidic or basic? A water molecule, or H20, can dissociate, or fall apart, to form two charged particles, or ions. One of these ions is called a “hydrogen ion,” written as H+ (and pronounced “H-plus”). An H+ is simply a proton, floating around in a solution. The other ion is written as OH (which is pronounced as “O” “H” minus). The OH ion is also called “hydroxide.” Here’s the equation for the dissociation of water.

H2O–>H+ + OH–  

In a chemistry class, you’ll learn that the H+ immediately combines with a water molecule to form H3O+, also known as the hydronium ion. For biology students, it’s very useful to continue to think of water as dissociating into H+ and OH

If dissolving something in water causes it to have more H+ than OH, then the solution is acidic. For example, if the compound HCl (hydrochloric acid) is dissolved in water, the hydrogen ions dissociate from the chlorine as shown below:

HCl–>H+ + Cl–  

The added H+ ions make the resulting solution an acid.

Conversely, if dissolving something in water causes it to have more OH than H+, then the resulting solution is a base.For example, when Sodium Hydroxide (lye) is dissolved in water,the sodium dissociates from the hydroxide ion:

NaOH–>Na+ + OH–  

If the amount H+ is equal to OH, then the solution is neutral. Pure water has a pH of 7, but what comes out of the tap is usually a weak acid.

Acidic and basic solutions are measured on the pH (pronounced “P” “H”) scale. In the pH scale, the acids with most hydrogen ions are at 0 (or rarely, below 0). The most concentrated bases are at pH 14 (or above). A neutral liquid is 7.

The scale works by powers of 10, so a solution with pH 5 is 10 times more acidic than a solution that is pH 6. A solution that is pH 12 is 10 times more basic than a solution that is pH 11.

Here’s a graphical version of the pH scale.

06_ph scale
The pH scale. Note that the pH values listed for the solutions above assume that the solutions are of a certain concentration (1 molar).

4. Acids, Bases and pH: Checking Understanding

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Acids, Bases, pH Quiz (M4)”]

[h]Quiz: Acids, Bases and pH

[i]

[!]Question 1 [/!!!]

[q]The change of a water molecule into an H+ and an OH is called

[hangman]

[c]dissociation

[f]Excellent. The change of a water molecule into an H+ and an OH is called dissociation.

[!]Question 2 [/!!!]

[q]Another word for the H+ ion is ____________ ion.

[hangman]

[c]hydrogen

[f]Nice job.Another word for the H+ ion is ____________ ion.

[!]Question 3 [/!!!]

[q]A solution with more H+ than OH is a(n) ____________

[hangman]

[c]acid

[f]Good work. A solution with more H+ than OH is an acid.

[!]Question 4 [/!!!]

[q]A solution with more than OH than His a(n) ____________

[hangman]

[c]base

[f]Good work.A solution with more than OH than His a base.

[!]Question 5 [/!!!]

[q]A solution with pH 9 is a(n) ____________

[hangman]

[c]base

[f]Good work.A solution pH 9 is a base.

[!]Question 6 [/!!!]

[q]A solution with pH 4 is a(n) ____________

[hangman]

[c]acid

[f]Correct. A solution with pH of 4 is an acid.

[!]Question 7 [/!!!]

[q]A solution with pH 7 is

[c]acidic

[c]basic

[c*]neutral

[f]No. Take a look at this diagram, and notice where 7 lies.

[f]No. Take a look at this diagram, and notice where 7 lies.

[f]Correct .A solution pH 7 is neutral.

[!]Question 8 [/!!!]

[q labels = “top”]Label the following.

ph 6 pH 8
____________ ____________

[l]more acidic

[fx] No. Please try again.

[f*] Great!

[l]more basic

[fx] No, that’s not correct. Please try again.

[f*] Great!

 

[!]Question 9 [/!!!]

[q labels = “top”]Compare the following:

ph 3 pH 5
____________ ____________

[l]more H+

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]more OH

[fx] No, that’s not correct. Please try again.

[f*] Great!

[!]Question 10 [/!!!]

[q labels = “top”]Label the following.

ph 3 pH 9
____________ ____________

[l]less basic

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]more basic

[fx] No, that’s not correct. Please try again.

[f*] Good!

[!]Question 11 [/!!!]

[q]Compared to a solution that’s pH 3, a solution that’s pH 1 is ___ times more acid.

[c]twice

[c]three times

[c]ten times

[c*]100 times

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). For example, a pH of 5 is 10 times for acidic than a pH of 6. Use this to figure out the difference in acidity between pH 1 and ph 3.

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). For example, a pH of 5 is 10 times for acidic than a pH of 6. Use this to figure out the difference in acidity between pH 1 and ph 3.

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). For example, a pH of 5 is 10 times for acidic than a pH of 6. Use this to figure out the difference in acidity between pH 1 and ph 3.

[f]Yes. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude) So, a pH of 1 is 100X more acidic than a pH of 3.

[!]Question 12 [/!!!]

[q]Compared to a solution that’s pH 7, a solution that’s pH 8 is ___ times more basic.

[c]twice

[c]three times

[c*]ten times

[c]100 times

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). Use that to figure out how much more basic 8 is than 7.

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). Use that to figure out how much more basic 8 is than 7.

[f]Yes. The pH scale works by powers of 10. So a solution that’s pH 8 is 10 times more basic than ones than one that’s pH 7.

[f]No. The pH scale works by powers of 10 (much like the Richter scale for earthquake magnitude). Use that to figure out how much more basic 8 is than 7.

[!]Question 13 [/!!!]

[q]A solution that is 1000 times less basic than a solution that is pH 12 would have a pH of ___

[textentry single_char=”true”]

[c*]9

[f]Excellent. A solution that is 1000 times less basic than a solution that is pH 12 would have a pH of 9.

[c]*

[f]No. Each decrease in pH number represents a 10 times decrease in acidity. So, ph 11 is 10 times less basic than 12, and pH 10 is 100 times less basic than 12. What pH is 1000 times less basic than 12?

[!]Question 14 [/!!!]

[q]A solution that is 100 times less acidic than a solution that is pH 2 would have a pH of ___

[textentry single_char=”true”]

[c*]4

[f]Excellent. A solution that is 100 times less acidic than a solution that is pH 2 would have a pH of 4.

[c]*

[f]No. Each increase in pH number represents a 10 times decrease in acidity. So, ph 3 is 10 times less acidic than pH 2. What pH is 100 times less acidic than pH 2?

[/qwiz]

5. Buffers

A buffer is a substance which when added to a solution, enables that solution to resist changes in pH when an acid or a base is added to it (note: this definition is a slightly modified version of one in the New Oxford American Dictionary). Blood, for example, resists changes in its pH much more than an equivalent quantity of water. That’s because blood contains buffers which are capable of soaking up hydrogen ions (H+) when acids enter the blood, and which release hydrogen ions when blood’s pH starts to rise.

As described in Campbell, Biology, 9th Edition, one of the buffers which enables blood to do this is carbonic acid. The formula for carbonic acid is H2CO3. Carbonic acid can dissociate by losing a hydrogen ion, making it into the bicarbonate ion (HCO3), which is a base. You can see this in the chemical reaction below.

H2CO3 → HCO3+ H+

Carbonic acid itself is a weak acid. When added to water, a small amount of carbonic acid will dissociate and release H+ into the solution (which is why it’s an acid). But if another acid is added to the same solution, then the H+ released by the second acid will combine with the bicarbonate ion, creating more carbonic acid. Because carbonic acid is a buffer, that actually decreases the pH of the solution (making it less acidic) by absorbing the H+. By contrast, when a base is added to the solution (causing the pH to rise), more carbonic acid will dissociate, releasing H+ into the solution, causing the pH to fall.

6. Conclusion: Acids, Bases, pH, and Life

Acids and bases are incredibly important to understanding how living things work. One of the most important aspects of homeostasis in any organism is keeping the pH in a cell’s cytoplasm, outside the cell, and or within a body compartment (the stomach, or the inside of a blood vessel) within a very specific range. Exceeding that range, even by a little bit, can be harmful or fatal. We’ve seen above how buffers help maintain the pH of the blood. We’ll continue to see, throughout this course, the variety of adaptations that organisms have for controlling pH.

Links

This is the last tutorial in this series about the chemistry and properties of water. Proceed to Carbon and Functional Groups