Chemical changes Revision Notes

pH Scale and What It Shows

The pH scale runs from 0 to 14. A pH of 7 indicates a neutral solution, like pure water. Acids have pH values less than 7, and bases (alkalis) have values greater than 7.

Classifying Solutions by pH

You can classify any solution by its pH. If the value is below 7, the solution is acidic. If it's above 7, it's alkaline. Exactly 7 means the solution is neutral.

Litmus Indicator

Litmus is a well-known indicator used to distinguish acids from alkalis. In acidic solutions, litmus turns red. In alkaline solutions, it turns blue. For most practical school purposes, it is not used to distinguish neutral solutions, as the litmus colour is more subtle near neutrality.

Methyl Orange Indicator

Methyl orange is used in titrations and gives clear colour changes: it is red in acidic solutions and yellow in alkaline solutions. It also appears yellow at neutral pH. It does not turn orange in a genuinely neutral (pH 7) solution; orange is only seen within its pH transition region between strong acid and neutral.

Phenolphthalein Indicator

Phenolphthalein is colourless in acidic and neutral solutions, and it becomes pink in the presence of an alkali. Its sudden colour change makes it a useful indicator when testing for alkaline conditions, especially with bases.

Summary Table

Litmus: red in acid, blue in alkali; Methyl orange: red in acid, yellow in alkali or neutral; Phenolphthalein: colourless in acid and neutral, pink in alkali. Remember these changes when identifying unknown solutions.

Hydrogen Ion Concentration and pH

In acidic solutions, an increase in hydrogen ion (H⁺) concentration causes the pH to decrease, making the solution more acidic. pH is inversely related to [H⁺]. For example, a solution at pH 2 has a higher hydrogen ion concentration than a solution at pH 5. This does not necessarily mean a stronger acid; strength refers to how completely the acid dissociates, whereas pH simply measures the current concentration of hydrogen ions.

Hydroxide Ion Concentration and pH

In alkaline solutions, an increase in hydroxide ion (OH⁻) concentration raises the pH, making the solution more alkaline. The pH is directly related to [OH⁻]. A solution at pH 12, for example, contains a much higher hydroxide ion concentration than one at pH 9. Again, the concept of strength applies to how fully the alkali dissociates in water, not the actual concentration of hydroxide present.

The pH Scale Explained

The pH scale runs from 0 to 14: acids have a pH below 7, alkalis (standard spelling) have a pH above 7, and neutral solutions are exactly 7. Each movement of one pH unit represents a change in hydrogen or hydroxide ion concentration by a factor of 10. Understanding this relationship is key for recognising how acids and alkalis behave in solution.

The pH Scale Basics

pH is a measure of how acidic or alkaline a solution is. The scale typically ranges from 0 to 14. A pH of 7 is neutral (e.g., pure water). A pH value less than 7 indicates an acidic solution, while a pH value greater than 7 indicates an alkaline (basic) solution. Lower pH means higher acidity.

Relationship with Hydrogen Ion Concentration

The pH scale is logarithmic and inversely related to the concentration of hydrogen ions ([H⁺]) in a solution. An increase in [H⁺] makes a solution more acidic, lowering its pH. Crucially, a tenfold increase in [H⁺] leads to a decrease of 1 pH unit. For instance, if a solution's [H⁺] is 10 times greater, its pH will be 1 unit lower.

Understanding Logarithmic Change

Because pH is logarithmic, a small change in pH represents a large change in [H⁺]. For example, moving from pH 3 to pH 2 means the hydrogen ion concentration has increased by a factor of 10. Moving from pH 3 to pH 1 means the hydrogen ion concentration has increased by a factor of 100 (10 x 10).