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SQA National 5 Chemistry The pH scale

The pH scale

This page covers the following topics:

1. pH scale

The pH scale is used to rank the acidity or alkalinity of aqueous solutions, ranging from 0 to 14. A solution with pH = 7 is neutral, anything below pH = 7 is acidic and anything above pH = 7 is alkaline. Acidic solutions have a higher concentration of hydrogen ions and alkaline solutions have a higher concentration hydroxide ions than pure water. The pH scale is logarithmic, so if the concentration of hydrogen ions is increased by a factor of 10, then the pH is decreased by 1 unit, and vice versa. Indicators can be used to determine the pH of solutions. The universal indicator undergoes several colour changes as it goes from acidic to neutral to alkaline, from red to green to violet.

pH scale

1

3 beakers have unknown substances and universal indicator in them. Describe what happens on the image.

3 beakers have unknown substances and universal indicator in them. Describe what happens on the image.

2

An unknown substance has a pH of 8. Is it acidic or alkaline?

An unknown substance has a pH of 8. Is it acidic or alkaline?

3

Which of the following substances is the most alkaline: zinc hydroxide, beryllium hydroxide, potassium hydroxide?

Which of the following substances is the most alkaline: zinc hydroxide, beryllium hydroxide, potassium hydroxide?

4

We have an acid with [H⁺] = 0.1 and with pH = 1. It is diluted until the pH is equal to 3. What is the new concentration of the hydrogen ions?

We have an acid with [H⁺] = 0.1 and with pH = 1. It is diluted until the pH is equal to 3. What is the new concentration of the hydrogen ions?

5

An acid with a concentration of hydrogen ions of 0.25 and a pH of 2 is diluted until [H⁺] = 0.00025. What is the pH of the new solution?

An acid with a concentration of hydrogen ions of 0.25 and a pH of 2 is diluted until [H⁺] = 0.00025. What is the pH of the new solution?

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