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Acids and Bases

Directions: Please read the following passage and answer all of the accompanying questions.


1 The planet earth is covered with water. In fact, 71% of the earth’s surface is covered by water. Almost all of that liquid is either an acid or base. In fact, the only exception is pure, distilled water. The rest is loaded with the H+ and OH- particles that give water acidic and basic properties.
2 It is probably no surprise to learn that pure water is a neutral substance. It might surprise you to learn that any sample of water, even if perfectly pure, contains tiny amounts of charged particles. These particles, called ions, are created in equal amounts when two water molecules react. Thus, even in the purest water, there is a small amount of positive hydrogen ions (H+) and an equal amount of negative ions (OH-). These ions can collide and reform into a neutral molecule of H2O. Who would have guessed that common water is such a dynamic substance, in an ongoing state of breaking into ions and reforming back into neutral molecules, or that the abundance of these ions determines whether a substance is classified as an acid, base, or neutral?
3 Why are some substances acids while others are bases? Some chemicals, like lemon juice, have the ability to react with water to create H+ ions. Any substance which has more H+ ions than OH- ions is an acid. Other chemicals, like soaps, react with water to create OH- ions. When these ions outnumber H+, the solution is classified as a base. The relationship between the amounts of H+ and OH- is well-established mathematically. It will suffice to understand that both acids and bases contain both types of ions and that the concentration of one ion is inversely proportional to the other.
4 Everywhere you go you encounter acids and bases. Even your body carries acids and bases from the hydrochloric acid in your stomach to amino acids that build every protein in your body. Even DNA (DeoxyriboNucleic Acid) the blue print for your body, is an acid. In your kitchen, you probably have fruit juices with citric acid, or you might keep your refridgerator fresh with baking soda, a base. Our household cleaners are often bases, too; bleach, ammonia and soap are all bases.
5 Clearly acids and bases permeate our lives, so understanding their properties gives us a perspective on the chemistry that we encounter everyday. As you would probably guess, acids and bases look, taste, and react differently. Bases feel slippery and will actually react with oils in your skin to form a soapy film. (Bases are used to create all soaps. That is why some people with sensitive skin use non-soap based products.) Furthermore, the name acid comes from the Latin word acere which means sour. Not surprisingly, many acids, like lemon juice, have a sour taste. (However, we don’t recommend that you taste substances in your chemistry class to find out.) Bases, unlike acids, taste bitter.
6 Not all acids and bases are equal, however. Some are extremely corrosive, while others will not harm you at all. To tell the difference scientists use something called the pH scale. It runs from 0 to 14. Neutral substances, like pure water, are rated 7. Anything less than 7 is an acid, and those things with a pH greater than 7 are bases. The farther away the number is from 7 the stronger the acid or the base. The closer a chemical's pH value is to 0 and 14, the more reactive the chemical. These chemicals must be handled extremely carefully as they can cause severe burns. The strength of an acid or base depends on how much it dissociates (breaks up) in water. Hydrochloric acid is an example of a strong acid. It is strong because in water every molecule of hydrochloric acid will release a hydrogen ion.


images courtesy of Chem4Kids.com


7 So what happens chemically to make something an acid or a base? Well, acids are substances that release an excess of H+ ions when placed in an aqueous solution (liquid). An H atom consists of a proton with a positive charge and an electron with a negative charge. When the atom loses an electron, it becomes a hydrogen ion. Bases, on the other hand, release OH- ions.
8 An acid may also be defined as a substance that serves as a proton donor, a molecule that readily gives up a proton, like the H+ ion. A broader definition might be that an acid is an electron pair acceptor. When this happens the electron pair acceptor and the base electron pair donor combine by sharing electrons. Even in the able to accept or doneat electrons, acids and bases are chemical opposites.
9 Acids can also be divided between inorganic and organic acids. An organic acid is any acid that contains carbon atoms. These include citric acid (C6H8O7); asorbic acid (C6H8O6) otherwise know as Vitamin C; and acetylsalicylic acid (C9H8O4), asprin. Other acids, those that do not contain carbon, are called inorganic acids. They are just as common and as important as their organic counterparts. Two good examples are sulfuric acid (H2SO4), commonly used as the fluid in automobile batteries, and muratic acid (HCl), which is used to maintain swiming pools.
10 Another distinctive property of acids is that they will turn litmus, a blue vegetable dye, red. In chemistry class many students use litmus paper to test if a substance is an acid. Bases can restore the blue color of litmus after it has been turned red. This is because acids and bases neutralize each other, forming water: H+ + OH- = H2O.


image courtesy of 4to40.com


You can also use liquids, such as the universal indicator bromothymol blue shown below, to determine the pH of a solution. These indicators usually have pH color scales against which you can compare your results.

pH1

pH2

pH3

pH4

pH5

pH6

pH7

pH8

pH9

pH10

pH11

pH12

pH13

 pH14

color scale courtesy The Open Door Website


11 Acids do not play a small role in our life either. In addition to being in our bodies, drinks, batteries, and medicines, they can appear in our rain. Cars, factories, and some power plants release acid-forming compounds like sulfur dioxide and nitrogen oxides into the atmosphere. When these compounds come in contact with moisture in the air, they form acid. Eventually this acid makes it back to the Earth as acid rain, harming lakes, rivers, and streams and causing damage to monuments, bridges, crops, and forests. In some areas, lime has been added to lakes to neutralize the acid, but it is only effective temporarily and may have side effects. Thankfully, new innovations, like safer fuels and filters in some factory exhaust stacks have helped to alleviate the problem.
12 As we have seen acids and bases have distinct chemical and physical properties. They are composed of different ions, have different tastes, react to produce different colors and gases, and even feel different. Yet they are very similar; they both conduct electricity and can both be highly corrosive. They can be strong or weak, and can have varying pH values. Finally, acids can be organic or inorganic and are often released from burning certain fuels, creating acid rain. Acids and bases clearly play a direct and dramatic role in our lives.

  General Questions
Although acids and bases are chemical opposites, they both can conduct electrical current. Which explanation offers the most reasonable explanation for this similarity?
1. 



 

A liquid is tested with litmus paper. A drop of the unknown liquid is placed on red paper. The red litmus paper stays red. A drop of the liquid being tested is then placed on blue paper. The blue litmus paper stays blue. The test indicates that
2. 



 

Use this information to answer the next two questions.

The pH scale
image courtesy of PHYSCHEM.co.za

This diagram shows the relationship between concentrations and pH values. Note that when the concentration of H3O+ ions in a solution is 1 x 10-14, the pH is 14. It is important to notice that the pH scale is logarithmic; that is, a difference of one pH unit represents a factor of 10. For example, if a sample has a concentration of H3O+ ions of 1 x 10-10 and another sample has a concentration of H3O+ ions of 1 x 10-8, the pH values are 10 and 8 respectively. The first sample's acidity is 1010-8, or 102 times greater.


As we have learned, acids have lots of extra H+ ions so that when they are mixed with water (H2O) as indicated in paragraph 6, we would get molecules that are H3O+. When there is a higher concentration of H3O+ than OH- then the substance is considered an acid.

Which of the following combinations of values would represent the strongest acid?
3. 



 

Which of the following combinations of values would represent the strongest base?
4. 



 

The pH scale is from 1-14. Which substance would be the most dangerous to ingest?
5. 



 

A lowercase “p” is used by chemists to mean ‘concentration.’ Which of the following statements is a correct definition of pH?
6. 



 

What will happen when an extremely alkaline (basic) substance is mixed with water?
7. 



 

What is the cause of acid rain?
8. 



 

Which one of the following numbers would indicate the most harmful acid based on the pH scale?
9. 



 

Which of the following is NOT a property of acids?
10. 



 

All of the following are contrasts of bases and acids except:
11. 



 




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