pH Scale

The pH of a material is a measure of how acidic or alkaline a substance is. Because it is the measure of activities of Hydrogen ions, the initials “pH” stand for “Potential of Hydrogen.” Acids have pH values under 7 and alkalis have pH values over 7. If a substance has a pH value of 7, then it is neither acidic or alkaline and considered neutral. The pH scale is logarithmic, meaning that greater numbers are multiples in strength. pH devices include litmus paper to determine the pH values.

Questions you may have about it include:

• What are the pH values of various materials?
• What is a logarithmic scale?
• What devices show the pH?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

pH of various materials

The following table shows the pH values of various materials, from acids to neutral to bases.

	Battery acid	-0.5
	Stomach acid	2.0
	Lemon juice	2.4
	Cola drink	2.5
	Vinegar	2.9
	Orange juice	3.5
	Acid rain	4.5
	Coffee	5.0
	Milk	6.5
	Pure water	7.0
	Healthy human saliva	6.5-7.4
	Blood	7.4
	Sea water	8.0
	Hand soap	9.0-10.0
	Ammonia	11.5
	Bleach	12.5
	Lye	13.5

Logarithmic scale

The pH scale is logarithmic. That means a difference of one pH unit represents a tenfold or ten times change. For example, the acidity of a sample with a pH of 5 is ten times greater than that of a sample with a pH of 6. A difference of 2 units, from 6 to 4, would mean that the acidity is one hundred times greater, and so on.

The logarithmic scale is handy in being able to list items in a form that is easy to chart. Otherwise going from a pH of 1 to 7 would be like going from 1 to 10,000,000.

pH measurement

A common way to measure the pH of a liquid is by the use of litmus paper. This is a special type of paper containing a chemical that will tell you whether a substance is acidic or alkaline by its color. Acids turn the paper red and bases turn it blue.

There are also more sensitive pH papers available that can give more accurate readings.

There are natural pH indicators such red cabbage juice, which will change its color when subjected to an acid or a base.

Gardeners also use a battery-powered pH meter to measure the pH of the soil.

In conclusion

The pH scale measures how acidic or alkaline a substance is. pH stands for “Potential of Hydrogen.” Acids have pH values under 7 and alkalis have pH values over 7. The pH scale is logarithmic, meaning that greater numbers are multiples in strength. pH is often measured with litmus paper.

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Mini-quiz to check your understanding

QUIZ

Self-check your understanding

1. Why is the pH of pure water equal to 7.0?

   It is 7.0 times as strong as unpure waterSorry, not correct. Try another button. Water has 7 Hydrogen atomsGood try, but not right. Select another answer. 7.0 on the pH scale means neutralThat is correct. You are like a Champion.

2. If the pH of ammonia is 11.5 and bleach is 12.5, how much more alkaline is the bleach?

   10 times, since 1 on the logarithmic scale represents tenfoldCorrect. You know your Chemistry! 1.0, since 12.5 minus 11.5 equals 1.0Oops! Wrong answer. Re-read the question and try again. 10,000,000 timesSorry, not correct. Try another button.

3. How do you use litmus paper?

   You write on it to keep records of acids and basesGood try, but not right. Select another answer. You wet it with a substance to see if it is an acid or baseThat is correct. You are like a Champion. You place it in a substance to change the liquid to an acid or a baseSorry, not correct. Try another button.

Pick an answer to see instant feedback. Re-read the lesson if you miss one.

If you got all three correct, you are on your way to becoming a champion in science. If you had problems, you had better look over the material again.

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