Required Practical 4: Identifying Cations and Anions

This practical aims to carry out simple test-tube reactions to identify cations and anions in aqueous solutions. Students will perform tests for the presence of various ions and make accurate observations of the results.

Definition: Ions are charged particles formed by the addition or removal of electrons from neutral atoms or molecules. Cations are positively charged ions, while anions are negatively charged.

The practical covers eight different tests to prove the presence of various cations and anions in different substances. These chemical tests rely on simple reactions where the products give visual clues about the reactants present.

Example: If universal indicator turns red, it indicates the presence of H+ ions in the substance being tested. Similarly, if a gas turns limewater milky, it suggests the production of calcium carbonate precipitate, indicating the presence of carbon dioxide.

The equipment required for this practical includes test tube racks, test tubes, forceps, stoppers, and plastic graduated dropping pipettes. Solutions needed include 0.1 mol dm^-3 barium chloride, 0.6 mol dm^-3 sodium hydroxide, 0.1 mol dm^-3 calcium bromide/nitrate or potassium bromide, 0.1 mol dm^-3 magnesium chloride, and 0.1 mol dm^-3 strontium chloride.

Highlight: When using dilute sodium hydroxide to test for Group 2 metal cations, calcium bromide and magnesium chloride are expected to produce white precipitates due to their low solubility. Strontium will result in a slightly white solution, while barium chloride will remain visibly unchanged.

Chemical Reactions and Observations

This section focuses on the chemical reactions involved in testing for Group 2 metal cations using dilute sodium hydroxide. The reactions and expected observations are crucial for accurately identifying ions in the Required Practical 4 AQA Chemistry A level write up.

Example: The reaction between barium chloride and sodium hydroxide: BaCl2(aq) + 2NaOH(aq) -> 2NaCl(s) + Ba(OH)2(aq)

When barium chloride reacts with sodium hydroxide, a white precipitate forms after a short time. This precipitate is barium hydroxide, which is slightly soluble in water.

Highlight: The formation of a white precipitate is a key indicator in many test tube reactions to identify cations and anions. The appearance, timing, and behavior of the precipitate can provide valuable information about the ions present.

For other Group 2 metal cations, the reactions and observations may vary:

• Calcium bromide: Expected to form a white precipitate of calcium hydroxide
• Magnesium chloride: Should produce a white precipitate of magnesium hydroxide
• Strontium chloride: May result in a slightly white solution due to the formation of strontium hydroxide

These observations are essential for completing the AQA A level Chemistry Required Practical 4 results table and for understanding the behavior of different Group 2 metal cations in alkaline solutions.

Vocabulary: Precipitate - A solid that forms and separates from a solution during a chemical reaction.

Students should carefully note the timing, appearance, and any changes in the precipitate formation for each test. These details will be crucial for accurately identifying unknown cations in future experiments and assessments.