unit 5 lesson 3 measuring ph copyright houghton mifflin harcourt publishing company

Unit 5 Lesson 3 Measuring pH

Copyright © Houghton Mifflin Harcourt Publishing Company

What’s Your Number?


What is the pH scale?• The pH of a solution is a measure of how acidic or

basic a solution is. A solution that has a pH value of exactly 7 is neutral—neither acidic nor basic.

• A solution with a pH value of less than 7 is acidic. There are more H3O+ ions than OH– ions in the solution.

• A solution with a pH value greater than 7 is basic. There are more OH– ions than H3O+ ions in the solution.


What is the pH of some common solutions?

• Lemon juice has a pH of about 2. Milk has a pH of about 6.5, but sour milk has a pH of about 4.

• Pure water is neither acidic nor basic; it is neutral. It has a pH of 7.

• A solution of ammonia has a pH of around 12. Antacid tablets dissolve in water to form a basic solution of about pH 8.



What is the pH of some common solutions?

• Locate lemon juice, milk, pure water, and ammonia on the pH scale.



How is pH measured?• An acid-base indicator is a dye that changes color

at a certain pH range. The pH can be estimated by placing a few drops of indicator in a solution.

• The pH of a solution can be checked by placing a drop of the solution on a strip of universal pH paper, which contains several acid-base indicators.

• An electronic pH meter has a probe that is placed into the solution being measured. A precise pH value is measured.




A Natural pH Meter


• Hydrangea plants act as acid-base indicators in nature. The flowers change color depending on the pH of the soil.

• Pink flowers indicate that the soil pH is greater than 6.5. It is basic.

• Blue flowers indicate that the soil pH is less than 5.5. It is acidic.

In Our World


What role does pH play in the environment?

• If soil pH is too acidic or too basic, the nutrients plants need do not dissolve into the water in the soil.

• As a result, plants do not grow well, and the animal population that depends on the plants also suffers.

• The pH of ocean water is usually about 8. If the water absorbs too much carbon dioxide, the pH may be lowered, harming sea animals.


What role does pH play in the environment?

• How would a decrease in soil pH affect the ecosystem shown?



What causes acid rain?• Acid rain is any form of precipitation, such as rain,

sleet, or snow, that has a lower pH than the normal range between 5 and 6.

• The burning of fossil fuels releases sulfur oxides and nitrogen oxides, which combine with water in the air to produce sulfuric acid and nitric acid.

• These acids in the atmosphere form acid rain when they dissolve in precipitation such as raindrops.



What are the effects of acid rain?• Acid rain can lower the pH of lakes, ponds, and

streams, harming aquatic species and other organisms that rely on the water for survival.

• Acid rain can harm plants by damaging their leaves.

• It can also harm plants and other living things by lowering soil pH.



What are the effects of acid rain?• Acid rain can damage buildings and sculptures

made of limestone and marble, by reacting with the stone.

• Acid rain can also damage paint and cause metals to corrode faster.



Body in Balance


Where does pH play an important role in the human body?

• Stomach pH is usually about 2.5. But if food that is difficult to digest is eaten, more stomach acid is made and the acidity can drop as low as 1.2.

• Difficulty digesting food and a low stomach pH can cause discomfort called indigestion.

• Stomach pH can be restored back to a normal level by consuming antacid tablets, which are basic.



• To maintain good health, blood pH needs to be between 7.35 and 7.45.

• If blood pH falls below 7.35, it is too acidic. This condition is called acidosis.

• If blood pH rises above 7.45, it is too alkaline. This condition is called alkalosis.

• Both conditions make a person feel ill.




• How does blood pH affect a person’s health?



unit 5 lesson 3 measuring ph copyright houghton mifflin harcourt publishing company

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