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The pH scale runs from 0 to 14, and each decrease of 1 pH unit means a 10 times higher hydrogen ion concentration. Free notes and 10 questions answered.
pH Scale and Hydrogen Ion Concentration is part of Reactions of acids in GCSE AQA Higher Chemistry.
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GCSE AQA Higher
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The pH scale runs from 0 to 14, and each decrease of 1 pH unit means a 10 times higher hydrogen ion concentration. Free notes and 10 questions answered.
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Use this lesson to stay inside Reactions of acids while connecting the detail back to the wider Topic 4: Chemical changes topic.
Read the core explanations from pH Scale and Hydrogen Ion Concentration before testing yourself from memory.
Every water-based solution contains hydrogen ions (H⁺) and hydroxide ions (OH⁻). The balance between these ions determines whether a solution is acidic, neutral, or basic. When there are more H⁺ ions than OH⁻ ions, the solution is acidic. When they're equal, it's neutral. When OH⁻ ions outnumber H⁺ ions, it's basic (alkaline).
But here's the problem - hydrogen ion concentrations are tiny numbers like 0.0001 mol/dm³. These are awkward to work with in calculations and comparisons. Scientists needed a simpler system.
The pH scale transforms those awkward decimal numbers into simple whole numbers. A solution with pH 3 is more acidic than pH 5, and much more acidic than pH 7. The lower the pH, the higher the hydrogen ion concentration.
Think of the pH scale as a ruler for measuring acidity. Just like a ruler has numbered marks, the pH scale runs from 0 to 14 with specific meanings at key points.
pH 0-6: Acidic solutions (stomach acid is around pH 1, lemon juice is pH 2)pH 7: Neutral (pure water)pH 8-14: Basic/alkaline solutions (soap is around pH 9, household ammonia is pH 11)
At pH 7, the solution contains equal numbers of H⁺ and OH⁻ ions. This is neutrality - neither acidic nor basic. Pure water at room temperature always has pH 7 because it contains tiny but equal amounts of both ions.
Here's where it gets interesting. The pH scale isn't just a simple counting system - it's logarithmic. This means each unit change represents a massive change in hydrogen ion concentration.
When pH decreases by 1 unit, the hydrogen ion concentration increases by a factor of 10. When pH decreases by 2 units, it increases by a factor of 100. This is the key relationship that catches many students out in exams.
Worked example: Calculating Hydrogen Ion Concentration Changes
A solution changes from pH 6 to pH 3. By what factor has the hydrogen ion concentration increased?
Answer: The hydrogen ion concentration increased by a factor of 1000
This logarithmic relationship explains why small pH changes have huge effects. A solution with pH 2 isn't just slightly more acidic than pH 3 - it's 10 times more acidic. This is why acid spills are so dangerous and why your stomach acid (pH 1-2) is so effective at breaking down food.
Not all acids are created equal. Some acids are strong, others are weak. But what does this actually mean in terms of hydrogen ions and pH?
For acids of the same concentration, the stronger the acid, the lower the pH. This is because strong acids release more hydrogen ions into solution. Hydrochloric acid (HCl) is strong - it completely breaks apart in water. Ethanoic acid (vinegar) is weak - only some molecules release their hydrogen ions.
This relationship is crucial for predicting behaviour. If you have two 0.1 mol/dm³ acid solutions and one has pH 1 while the other has pH 3, you immediately know the first acid is stronger because it produced more hydrogen ions.
pH questions appear regularly in GCSE chemistry papers, and they follow predictable patterns. Master these techniques and you'll never lose marks on pH again.
Worked example: Exam-Style pH Comparison Question
Solution A has pH 2, Solution B has pH 5. Both are 0.01 mol/dm³ acid solutions. Compare their hydrogen ion concentrations and acid strengths. [4 marks]
Answer: Solution A has 1000 times higher hydrogen ion concentration and is the stronger acid
The key to pH success is recognising the patterns. Low pH = high H⁺ concentration = strong acid. High pH = low H⁺ concentration = less acidic solution. The factor of 10 rule connects pH changes to concentration changes. Master these connections and pH questions become straightforward.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The pH scale is a logarithmic scale from 0 to 14 that measures hydrogen ion concentration: pH 7 is neutral, lower values are acidic and higher values are basic (alkaline). The lower the pH, the higher the hydrogen ion concentration.
A pH of 8 to 14 is alkaline (basic), pH 7 is neutral and pH 0 to 6 is acidic. In alkaline solutions hydroxide ions outnumber hydrogen ions; soap is around pH 9 and household ammonia is pH 11.
Each decrease of 1 pH unit means the hydrogen ion concentration increases by a factor of 10. A decrease of 2 units means a factor of 100, and a decrease of 3 units, such as pH 6 to pH 3, means a factor of 1000.
A solution is acidic when it has more hydrogen ions (H⁺) than hydroxide ions (OH⁻). When they are equal the solution is neutral, and when OH⁻ ions outnumber H⁺ ions it is basic, or alkaline.
pH 7 is neutral because the hydrogen ion concentration equals the hydroxide ion concentration. Pure water at room temperature has pH 7 because it contains tiny but equal amounts of H⁺ and OH⁻ ions.
Stomach acid is around pH 1, lemon juice is pH 2, pure water is pH 7, soap is around pH 9 and household ammonia is pH 11. Solutions from pH 0 to 6 are acidic and from pH 8 to 14 are basic or alkaline.
For acids of the same concentration, the stronger the acid, the lower the pH, because strong acids release more hydrogen ions. If two 0.1 mol/dm³ acid solutions have pH 1 and pH 3, the pH 1 acid is the stronger acid.
The pH scale is logarithmic because each unit change represents a factor of 10 change in hydrogen ion concentration. This turns awkward tiny numbers like 0.0001 mol/dm³ into simple whole numbers, while showing that small pH changes have huge effects.
A solution of pH 2 is 10 times more acidic than one of pH 3, because each decrease of 1 pH unit increases the hydrogen ion concentration by a factor of 10. A drop of 3 units, such as pH 6 to pH 3, is a factor of 1000.
The pH scale is used because hydrogen ion concentrations are tiny numbers like 0.0001 mol/dm³, which are awkward to work with in calculations and comparisons. pH turns these into simple numbers from 0 to 14, where a lower pH means a higher hydrogen ion concentration.