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296 Chapter 8 8.3 8.3 Concentrated acids and bases are very hazardous. For example, concentrated hydrochloric acid is so corrosive that it will burn through clothing. A concentrated base is also highly corrosive. However, hydrochloric acid is also found in your stomach, and baking soda is safe enough to put in the food that you eat. A dilute solution of hydrochloric acid is less acidic and less likely to react than a concentrated solution. How can we determine how acidic or basic a substance is? Chemists use the pH scale to represent how acidic or basic a solution is. Most acids and bases can be ranked on this scale. A very acidic solution has a very low pH value. A neutral solution, like pure water, has a pH of 7. A very basic (or alkaline) solution has a very high pH value. The pH values for a variety of common substances are shown in Figure 1. The pH Scale Figure 1 The pH scale was developed in 1909 to allow scientists to compare the concentration of hydrogen ions in various solutions. The pH scale was so-named because it represents the power of Hydrogen”— that is, the concentration of hydrogen ions — in a solution. Did You Know? battery acid (0.5) stomach acid (1.5) lemon juice (2.0) tomatoes (4.0) black coffee (5.0) normal rain (5.6) milk (6.5) blood (7.3) baking soda (8.2) borax (9.1) milk of magnesia (10.0) ammonia cleaner (11.0) bleach (12.4) Bleach drain cleaner (13.5) sea water (7.9) water (7.0) wine (3.8) apples (3.0)

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Page 1: 8.3 Textbook

296 Chapter 8

8.38.3

Concentrated acids and bases are very hazardous. For example,concentrated hydrochloric acid is so corrosive that it will burn throughclothing. A concentrated base is also highly corrosive. However,hydrochloric acid is also found in your stomach, and baking soda is safeenough to put in the food that you eat. A dilute solution of hydrochloricacid is less acidic and less likely to react than a concentrated solution. Howcan we determine how acidic or basic a substance is?

Chemists use the pH scale to represent how acidic or basic a solutionis. Most acids and bases can be ranked on this scale. A very acidic solutionhas a very low pH value. A neutral solution, like pure water, has a pH of 7.A very basic (or alkaline) solution has a very high pH value. The pH valuesfor a variety of common substances are shown in Figure 1.

The pH Scale

Figure 1

The pH scale was developed in 1909 to allow scientists to compare the concentration of hydrogenions in various solutions.

The pH scale was so-namedbecause it represents the“power of Hydrogen”— that is,the concentration of hydrogenions — in a solution.

Did You Know ?

battery acid

(0.5)

stomach acid

(1.5)

lemon juice (2.0)

tomatoes (4.0)

black coffee (5.0)

normal rain (5.6)

milk (6.5)

blood (7.3)

baking so

da (8.2)

borax (9.1)

milk of m

agnesia (10.0)

ammonia cleaner (1

1.0)

bleach (12.4)

Bleach

drain cleaner (1

3.5)

sea water (7.9)

water (7.0)

wine (3.8)

apples (3.0)

Page 2: 8.3 Textbook

Dilution and pHTry This

ActivityIf your school lab is equipped with a pH meterthat gives digital readouts of pH, you could use itto investigate the effects of dilution on pH.

• Put on safety goggles and an apron.• Obtain 10 mL of a solution with a pH of 1.

(A 1% hydrochloric acid solution would beappropriate.) Measure the exact pH of thesolution using the meter.

• Dilute the solution by a factor of 10. Forexample, place 10 mL of the acid in agraduated cylinder and add 90 mL of waterto make 100 mL of solution. Measure the pHof this dilute solution.

• Repeat the process with successive dilutionsof acid. Record and analyze your resultsusing a table and graph.

(a) Explain your results.

• Repeat the process using a solution with apH of 13. (A 0.5% solution of sodiumhydroxide would be appropriate.)

(b) Explain your results.

Acids and Bases 297

pH: A Logarithmic Scale

The pH scale is a logarithmic scale (Figure 2),which means that every change of one unit onthe scale represents a tenfold effect on theconcentration of the solution. For example, anincrease of one on the scale represents dividingthe concentration by 10 (or 101), and a decreaseof two represents multiplying the concentrationby 100 (or 102).

As the pH decreases, the solution becomesmore acidic. A solution with a pH of 3 is 10 timesmore acidic than a solution with a pH of 4 and100 times more acidic than a solution with a pHof 5. As the pH increases, the solution becomesmore basic. Thus, a solution with a pH of 13 is 10times more basic than a solution with a pH of 12and 100 times more basic than a solution with apH of 11.

The pH is defined according to the followingformula:

pH = −log10 [H+]

where the square brackets around the H+ ionrepresent the “concentration of hydrogen ions insolution.” The units of concentration are mol/L(read as “moles per litre”).

Figure 2

The Richter scale is a logarithmic scale. An increase of three units on the scalerepresents an earthquake that is 1000 times worse. The 1989 earthquake thatcaused serious damage in parts of California measured 7.1 on the Richter scale.

Sodium hydroxide is corrosive. Anyspills on the skin, in the eyes, or onclothing should be washed immediatelywith cold water. Report any spills toyour teacher.

Hydrochloric acid is corrosive. Anyspills on the skin, in the eyes, or onclothing should be washed immediatelywith cold water. Report any spills toyour teacher.

Page 3: 8.3 Textbook

298 Chapter 8

pH and Soil

Have you ever seen someone adding lime to soil? Lime (CaO) can beadded to acidic soil to make the soil more basic. As you learned in Chapter3, the pH of soil determines the fertility of the soil. Plants differ in theirpreference for soil acidity. For example, many leguminous crops preferneutral to slightly alkaline soils (pH of 7 to 10); corn and small grainsprefer slightly acidic soil (pH of 5 to 6); and potatoes and blueberriesprefer acidic soils (pH of less than 5). The sources of soil acidity includeorganic matter decay, naturally occurring acids, and acid precipitation.The correct management of soil pH is important for providing optimalgrowing conditions for plants, maintaining optimal levels ofnutrients, and using fertilizers efficiently.

pH and Cosmetics

Have you ever heard pH mentioned inadvertisements for skin and hair products?Many products advertise that they are “pHbalanced” or that they “restore your hair’snatural pH” (Figure 3). Should youbelieve such claims? What do they mean?

The acidity of a shampoo can have adefinite effect on hair. The clear outerprotein layer of hair is called the cuticle.Very basic (high-pH) shampoos cause theinner hair shaft to swell and push apart thecuticle. The harsh bases used in hairpermanents and hair colouring have evenstronger effects. They can dissolve the cuticleand damage the hair, making it dull and dry.

Acidic (low-pH) shampoos shrink theinner shaft, causing the cuticle to lie flat and restoring the hair’s originalflexibility and shine. However, low-pH shampoos are not for everyone.People with thick, curly hair who would prefer their hair to be straightercan benefit from more alkaline (higher-pH) shampoos, which soften andstraighten the hair.

The pH of soaps and other cosmetic products can also affect skin cells.The pH of skin normally ranges between 5 and 6. Using basic (high-pH)products can remove the outer layer of dead skin cells, which containprotein molecules. The skin will look brighter and clearer. But frequent useof basic products may remove too many layers of cells. Moreover, the toplayer of the skin contains natural acids that protect the skin from infection.Too much use of very basic soaps can remove these protective acids.

Many acids are used in cosmeticproducts. Alpha hydroxy acids(AHAs) are found in manypopular face creams. Salicylicacid is a white powder used inwart remover.

Did You Know ?

1 Some personal care productshave a particular pH. Does thepH of the product you havechosen have any effect on itsmarketability or its usefulness?

Challenge

Figure 3

Some cosmetic products advertise their pH.

Page 4: 8.3 Textbook

Acids and Bases 299

Red TornadoTry This

ActivityPut on your apron and safety goggles.

Add red cabbage juice to a tall clearcontainer. Add a small amount of vinegar untilyou get a red colour. Stir the mixture with a

spoon until it swirls like a tornado. Drop anAlka-Seltzer tablet into the mixture. Describe andexplain what you observe.

Understanding Concepts1. In your own words, explain the meaning of the term “pH.”

2. What would you expect as an approximate pH value foreach of the following:

(a) a very concentrated base;

(b) a dilute basic solution;

(c) a very concentrated acid;

(b) a dilute acid solution;

(e) tap water.

3. How much more acidic is a solution with a pH of 4.5 than asolution with a pH of

(a) 5.5?

(b) 6.5?

4. How much more basic is a solution with a pH of 12.5 than asolution with a pH of

(a) 10.5?

(b) 8.5?

5. Look at Figure 1. Roughly how much more acidic is

(a) stomach acid than tomatoes?

(b) lemon juice than apples?

6. Look at Figure 1. Roughly how much more basic is

(a) borax than baking soda?

(b) bleach than blood?

7. What happens to the pH of an acid when water is added to it?

8. Look in your bathroom at home or in the local pharmacy tofind some examples of products that include informationabout pH levels. Record your observations in a table withthe headings: Product, pH, and Use. What connections canyou see between a product’s pH and its use?

9. Toothpastes are usually slightly basic (alkaline). Why doesthis make sense? (Hint: Consider the types of compounds inour mouths that may damage teeth.)

Exploring 10. Use the pH formula to calculate the pH of solutions in

which the hydrogen ion concentration (in mol/L) is

(a) 0.1

(b) 0.01

(c) 0.001

(d) 1 x 10–5

(e) 1 x 10–4

11. Summarize your calculations in question 10 as

(a) a table

(b) a graph

(c) a statement describing the relationship between con-centration of hydrogen ions and pH.

12. You can investigate another way of looking at the acidity ofsolutions using the following equation:

[H+] = 10–pH

Calculate the concentrations of hydrogen ion for thefollowing pH values: 1.0, 2.0, 2.5, 3.0, and 5.0.