Barium meals and flame tests

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: 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).
Filter out the precipitate of barium sulfate.
Wash it with distilled water.
Dry it on filter paper.

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…