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What is pH?

Some of our favorite Some of our favorite foods make our foods make our tongue curl up tongue curl up because they are because they are SOUR. SOUR.

Acids and Bases 2

Bitter! Some foods have a Some foods have a

“bite” of their own “bite” of their own because they’re because they’re somewhat bitter.somewhat bitter.

WHY?WHY?

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Acidic/BasicThere is a scientific reason for this:There is a scientific reason for this:These foods are either acidic or basic.These foods are either acidic or basic.Other substances besides foods have Other substances besides foods have

these characteristics. these characteristics.

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Acids and BasesChemicals may be classed as acids or Chemicals may be classed as acids or

bases.bases.Things that are neither acids nor bases Things that are neither acids nor bases

are neutral.are neutral.pH measures how acidic or basic a pH measures how acidic or basic a

solution is.solution is.

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AcidsOften taste sour Often taste sour **Strong acids can burn skin & eyesStrong acids can burn skin & eyesStrong acids can dissolve metalsStrong acids can dissolve metalsExamples:Examples:

Lemon juiceLemon juiceVinegarVinegarCar battery acid Car battery acid (dangerous!)

*Never test an unknown acid by tasting it!

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Bases Can taste bitter, sweetish, or salty * Often feel slippery or “soapy” * Strong bases can burn skin & eyes Bases react more easily with protein than

with metal; they are often used for cleaning Examples:

Milk Baking soda Soap Drain cleaner (dangerous!)

*Never test an unknown base by touching or tasting it!

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No I am Not: Some Some

substances substances are not really are not really an acid or a an acid or a base: For base: For example, example, pure waterpure water

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Most Substances: Can be identified as Can be identified as

either acidic or basiceither acidic or basic Like the soil in our Like the soil in our

backyard.backyard.

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pH A special name is A special name is

given to the acid or given to the acid or base characteristic base characteristic that a substance has:that a substance has:

It is called: pHIt is called: pH

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How Do We Measure pH?

We measure pH by We measure pH by using special strips of using special strips of paper called pH paperpaper called pH paper

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How Does It Work? The paper is treated with The paper is treated with

chemicals that change chemicals that change color to show the pH.color to show the pH.

When the paper touches When the paper touches the substance being the substance being tested, it turns a specific tested, it turns a specific color to tell if the color to tell if the substance is an acid or substance is an acid or a base.a base.

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AcidAny substance which has a pH of value of Any substance which has a pH of value of

less than 7 is considered an acidless than 7 is considered an acid

0--------------7---------------140--------------7---------------14 Acid Neutral BaseAcid Neutral Base

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BaseAny substance which has pH value greater Any substance which has pH value greater

than 7 is a basethan 7 is a base

0--------------7---------------140--------------7---------------14 Acid Neutral BaseAcid Neutral Base

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pH 7A pH of 7 is called neutral—neither acid A pH of 7 is called neutral—neither acid

nor base.nor base.

0------------7------------140------------7------------14 Acid Neutral BaseAcid Neutral Base

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Acidic or Basic If the number is less than 7 the soil or If the number is less than 7 the soil or

water is acidic water is acidic If the number is more than 7 the soil or If the number is more than 7 the soil or

water is basicwater is basic

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The pH ScalepH scale ranges from 0 -14pH scale ranges from 0 -14pH 7 is neutral; neither acid nor basepH 7 is neutral; neither acid nor basePure water is pH 7Pure water is pH 7Low pH = acidLow pH = acidHigh pH = baseHigh pH = baseThe closer to the ends of the scale, the The closer to the ends of the scale, the

stronger the solution isstronger the solution is

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The pH Scale18

The pH Scale Each pH unit is Each pH unit is 1010 timestimes as large as the as large as the

previous oneprevious one A change of 2 pH units means A change of 2 pH units means 100 times100 times more more

basic or acidicbasic or acidic

x10x10 x100x100

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The pH ScaleCareful measurement is importantCareful measurement is importantA mistake of A mistake of oneone pH unit means pH unit means 10 times

too much or too little!too much or too little!

x10x10 x100x100

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Why is pH important?

Soil has to be in a certain pH range for plants to grow and stay healthy.

Fish can’t live if the pH is too high or too low

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pH and People

Water that has too high or low pH may contain harmful dissolved chemicals.

Water plant operators keep a careful watch on the pH of our drinking water, to keep it safe.

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Two papers, two charts

The yellow paper reads from pH 6.5-13.0 (base). The colors on this side start with yellow (6.5)

The orange paper reads from pH 0-6.0 (acid). Use the side of the dispenser that begins with pH 0.0. It’s easy to recognize because the colors begin with shades of orange.

Using two different papers gives a more precise reading.

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Rain/Soil pH The pH paper comes in a double roll. Orange is for the

low range (0-6). Yellow is for high range (6.5-14). Since most rainwater and soils have a low pH, begin with

the orange paper. Tear off a 2-inch strip. Do not touch the end you will

place in the mixture. Dip the end of the strip into the rainwater or the

soil/water mixture. Compare the color to the orange color chart (0-6). If none of the colors match or if there is no color change,

repeat the process using the yellow paper and the yellow color chart. Be sure to use the correct color chart!

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Rainfall pH If your school has a 1If your school has a 1stst grade, work with them to grade, work with them to

measure rainfall pH when they measure rainfall measure rainfall pH when they measure rainfall depth. There must be at least 2 mm of rain in the depth. There must be at least 2 mm of rain in the rain gauge to measure rainfall pH.rain gauge to measure rainfall pH.

If your school does not have a 1If your school does not have a 1stst grade, you will grade, you will need to install a rain gauge and define an need to install a rain gauge and define an Atmosphere Study Site to measure rainfall pH. Atmosphere Study Site to measure rainfall pH. Use the “Precipitation pH Lab Guide” and the Use the “Precipitation pH Lab Guide” and the “Documenting Your Atmosphere Study Site Field “Documenting Your Atmosphere Study Site Field Guide” (from the Teacher’s Guide CD or from Guide” (from the Teacher’s Guide CD or from www.globe.govwww.globe.gov) for instructions on how to define a ) for instructions on how to define a study site and measure pH.study site and measure pH.

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What color is it?Have you tried testing the pH of bleach? What happens?

•When you try the orange paper, you get a strange (but pretty) purple color that doesn’t match anything on your chart.

•When you try the yellow paper, you get another shade of purple but it doesn’t match either.

•Plus, in a couple of seconds the color goes away and the paper turns white! What’s going on?

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What color is it?Household bleach (Clorox) is a very strong base (around pH 13). It is so strong that even the orange paper will try to match the color. When you use the yellow paper, the pH is still so high the paper can’t get a good color match.

Plus, the bleach immediately starts to oxidize, or use up, the chemicals in the paper. So after a couple of seconds, the color goes away completely!

Try this: put about 100ml of distilled water in a cup. Measure the pH. Then add about 10 drops of bleach. Swirl or stir and measure the pH. Did you get a reading? What was it?

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