I/GCSE Chemistry - Insoluble Salts and Flame Tests

Preparation of an insoluble salt😃

In I/GCSE Chemistry you will be asked to describe the preparation of an insoluble salt.

Barium sulfate is an example of an insoluble salt.

Example question: Describe how you would make a pure, dry sample of barium sulfate (BaSO4), an insoluble salt. You can use any of the following: barium iodide, barium nitrate, potassium chloride, potassium sulfate.

Answer: 

1. Mix solutions of barium nitrate and potassium sulfate (you want a soluble salt of barium 'something' and a soluble salt of 'something sulfate'. Barium nitrate is soluble and potassium sulfate is soluble).
2. Filter out the precipitate of barium sulfate.
3. Wash it with distilled water.
4. Dry it on filter paper.

You may also be asked about the real life uses of insoluble salts like Barium Sulfate.

• Barium sulfate can be used for x-rays.
• Normally, only bones show up when you have an x-ray. However, barium sulfate is opaque to x-rays. When drunk it shows up the gut so that any problems (e.g. blockages) can be seen.
• Barium salts are toxic, but barium sulfate can be safely drunk because it's insoluble. This means that it isn't absorbed into the bloodstream - it just passes through the body.

Flame tests🔥

• Sodium ions (Na+) - yellow/orange flame.
• Potassium ions (K+) - lilac flame.
• Calcium ions (Ca2+) give a brick-red flame.
• Copper ions (Cu2+) give a blue-green flame.

Flame tests don't just work when you've got a sample of a pure element - they also work with a compound that contains that element. So if you stick a sample of copper sulfate in a bunsen flame, you'll also see a blue-green flame because of the Cu2+ ions present.

1. To flame-test a compound in the lab, dip a clean wire loop into a sample of the compound
2. Put the wire loop in the clear blue part of the flame
3. But make sure the wire loop is really clean by dipping it into hydrochloric acid and rinsing it with distilled water
4. You can use this test for sodium, potassium, calcium or copper ions in a solid or a solution. If you're using a powdered solid, you can dip the wire in the hydrochloric acid again to moisten it before dipping it in the solid, so that some of the solid sticks to the wire.

Reference: https://v1.nitrocdn.com/kDXDIJNDOaszRbpagqNqUtquAQQkiLpO/assets/static/optimized/rev-ddee88e/wp-content/uploads/2019/02/Preparation-of-soluble-salts.png

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Drafted by Catrina (Chemistry)