pH Scale

pH Scale

Middle School

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Learning Objectives

• Define acids, bases, and the pH scale.

• Interpret the pH scale to identify substances as acidic, basic, or neutral.

• Describe the properties of acids and bases.

• Explain the role of buffers in maintaining stable pH levels.

• Identify methods for measuring the pH of a solution.

Key Vocabulary

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pH Scale

The pH scale is a 0 to 14 scale indicating the hydrogen ion concentration in a solution.

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Acid

An acid is a compound that forms hydrogen ions (H⁺) in a solution, having a pH less than 7.

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Base

A base is a compound that produces hydroxide ions (OH^-) in a solution with a pH greater than 7.

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Buffer

A buffer is a compound that prevents sharp, sudden pH changes by reacting with strong acids or bases.

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Neutral

A neutral solution has a pH of 7, where hydrogen and hydroxide ion concentrations are exactly equal.

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Acids and Bases: A Comparison

Acids

• ​Forms hydrogen ions (H⁺) in a solution and has a pH of less than 7.

• ​​They are known for their sour taste and corrosive properties, reacting with many metals.

• ​In many cases, the chemical formula for an acid will start with an H.

Bases

• ​Produces hydroxide ions (OH^-) in a solution and has a pH of more than 7.

• ​​They are often characterized by a bitter taste and a slippery or soapy feel.

• ​In many cases, the chemical formula for a base will end with OH.

Multiple Choice

A substance that tastes bitter, feels slippery, and has a pH greater than 7 is known as what?

• An acid

• A base

• A neutral substance

• A buffer

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The pH Scale

• The pH scale (0-14) measures the concentration of hydrogen ions (H⁺) in a solution.

• Acidic solutions have a pH from 0 to 6.9; neutral solutions have a pH of 7.

• Basic solutions, also called alkaline, have a pH ranging from 7.1 to 14.

• Each one-step pH change represents a tenfold change in the substance's acidity.

Multiple Choice

A solution with a pH of 3 is how many times more acidic than a solution with a pH of 4?

• 2 times

• 100 times

• 10 times

• 1 time

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What are pH Buffers?

• Buffers are compounds that prevent sharp, sudden changes in a solution's pH.

• They are weak acids or bases that neutralize added strong acids or bases.

• Controlling pH is essential for homeostasis, which maintains stable internal conditions.

• Buffers in human blood keep its pH stable, between 7.35 and 7.45.

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Multiple Choice

What is the primary function of a buffer in a biological system like human blood?

• To make the solution strongly acidic

• To make the solution strongly basic

• To cause sharp changes in pH

• To resist sharp and sudden changes in pH

How to Measure pH

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Litmus Paper

• This is a type of paper that contains a special dye.

• Blue litmus paper will turn red when it is dipped in an acid.

• Red litmus paper turns blue when placed in a basic solution.

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pH Paper

• This paper has a range of different indicator dyes on it.

• It changes to a specific color when dipped into a solution.

• You compare this color to a chart to determine the pH value.

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Phenolphthalein

• This is a chemical indicator that starts out as a colorless liquid.

• It remains colorless when it is added to acidic or neutral solutions.

• The solution will turn a bright pink color if it is a base.

Multiple Choice

If you add a few drops of phenolphthalein to a test tube and the solution turns bright pink, what can you conclude about the substance?

• It is a strong acid

• It is a neutral substance

• It is a base

• It is pure water

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Common Misconceptions

Misconception	Correction
A pH of 0 means there is no acidity.	A pH of 0 indicates the strongest possible acidity.
A change from pH 8 to pH 9 is a small, insignificant change.	The solution has become ten times more basic.
Only strong acids are dangerous.	Strong bases can be just as corrosive and harmful.

Multiple Choice

If you have a solution of tomato juice (pH 4) and a solution of black coffee (pH 5), what does this tell you about the concentration of hydrogen ions (H⁺)?

• Coffee has 10 times more H⁺ ions than tomato juice.

• Tomato juice has 10 times more H⁺ ions than coffee.

• They have the same concentration of H⁺ ions.

• Coffee has 100 times more H⁺ ions than tomato juice.

Multiple Choice

Why is it critical for human blood to contain buffers?

• To make the blood as acidic as possible for digestion.

• To ensure the blood pH remains exactly 7.0 at all times.

• To prevent large, sudden pH swings that would stop proteins from working correctly.

• To turn the blood blue when it is exposed to air.

Multiple Choice

A scientist tests an unknown clear liquid. Blue litmus paper turns red in it, and phenolphthalein remains colorless. The scientist then adds a buffer to the liquid and adds a few drops of a strong base. What is the most likely outcome?

• The liquid's pH will increase dramatically.

• The liquid's pH will not change significantly.

• The liquid will turn bright pink.

• The liquid was a strong base to begin with.

Multiple Choice

Which statement best describes the role of buffers in human blood?

• Buffers raise the blood’s pH to 8.5–9.0

• Buffers keep the blood’s pH stable, between 7.35 and 7.45

• Buffers lower the blood’s pH to 6.0–6.5

• Buffers completely stop any pH change in the blood.

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Summary

• The pH scale measures if a solution is an acid, a base, or neutral.

• The scale is logarithmic, with each step being a tenfold change in acidity.

• Buffers resist pH changes, which is essential for homeostasis in living things.

• pH changes cause environmental issues like acid rain and ocean acidification.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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