Meanings of undissolved residue in English

Heat at a temperature just below boiling until the undissolved residue is white or until solvent action has ceased.

The undissolved residue is dried, and after the filter has also been burned in due manner and the ash added, the weight is ascertained.

The undissolved residue is by filtering separated from the dissolved, the filter washed with distilled water, and the wash water added to the filtrate.