Chemical changes Revision Notes

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.

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.

Acids as Sources of Hydrogen Ions

Acids are substances that, when dissolved in a solution (typically water), produce hydrogen ions (H⁺). These H⁺ ions are responsible for the acidic nature of the solution. For instance, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻ ions. The presence of these hydrogen ions is what gives acids their characteristic properties, such as turning blue litmus paper red and reacting with carbonates to produce carbon dioxide.

Alkalis as Sources of Hydroxide Ions

Alkalis are soluble bases that, when dissolved in a solution (typically water), produce hydroxide ions (OH⁻). These OH⁻ ions are responsible for the alkaline nature of the solution. For example, sodium hydroxide (NaOH) dissociates in water to form Na⁺ and OH⁻ ions. Alkalis are typically corrosive and feel slippery to the touch. They turn red litmus paper blue and neutralise acids by reacting with the H⁺ ions.

The Role of Water in Dissociation

Both acids and alkalis require water (or another suitable solvent) to dissociate and release their respective ions. Without a solvent, a substance like hydrogen chloride gas behaves as a covalent molecule, not an acid. Similarly, solid sodium hydroxide does not exhibit alkaline properties until it is dissolved in water, allowing the hydroxide ions to become mobile and reactive.

Understanding Solubility Rules Precisely

Solubility rules predict which ionic compounds dissolve in water. All nitrates, all Group 1 and ammonium compounds are soluble. Most chlorides are soluble except silver chloride and lead(II) chloride. Most sulfates are soluble, but barium sulfate, lead(II) sulfate and calcium sulfate (which is only sparingly soluble) are not. Most carbonates and hydroxides (except Group 1 and ammonium) are insoluble. Most silver compounds are insoluble, but silver nitrate is a common soluble exception.

Making Justified Predictions About Precipitates

Identify all ions in the mixed solutions, then pair cations with new anions to see which new compounds could form. For each new possible compound, apply solubility rules. If a compound is insoluble or only very sparingly soluble, it forms a precipitate. Clearly name any precipitate that forms. If both new compounds are soluble, no precipitate appears—the solution remains clear and the ions stay dissociated.