Instrumental Methods & RP7 - Indentifying Unknown Ionic Compounds

What is Required Practical 7 (Unkown Compounds)?

Use chemical tests to identify the ions present in unknown single ionic compounds.

Which types of tests are included (Unkown Compounds)?

Flame tests, sodium hydroxide precipitation tests, carbonate tests, halide tests and sulfate tests.

Why should separate samples be used for different ion tests?

To avoid contamination or interference from reagents used in previous tests.

How can an unknown ionic compound be identified?

Identify the positive ion (cation) using flame or hydroxide tests. -- Identify the negative ion (anion) using the appropriate chemical test. -- Combine the identified ions to determine the compound's formula.

How do you work out the formula of an ionic compound from its ions?

Balance the positive and negative charges so the compound has no overall electrical charge.

Why must the observations be recorded carefully (Unkown Compounds)?

The colour of a flame or precipitate is evidence used to identify the ions present.

Why is a white precipitate alone sometimes insufficient to identify a metal ion?

Aluminium, calcium and magnesium ions all produce white precipitates with sodium hydroxide, so further testing is required.

What are instrumental methods of analysis?

Methods that use scientific instruments to detect and identify elements or compounds.

What are three advantages of instrumental methods?

They are accurate, sensitive and rapid.

What does accurate mean?

Producing results close to the true value.

What does sensitive mean in chemical analysis?

Able to detect very small amounts or concentrations of a substance.

What does rapid mean?

Results can be obtained quickly.

Why are instrumental methods useful when only a small sample is available?

They are sensitive enough to detect very small quantities of substances.