The number of substances that should be tested at a time is : one ( 1 )
When preparing an experiment in the laboratory the substances/conditions related to the experiment should be tested one at a time for optimal result.
Substances used in an experiment
Substances used in an experiment can be grouped into solutes and solvents and should be kept in their appropriate containers and tested one at a time before been used for the experiment.
Hence we can conclude that The number of substances/conditions that should be tested at a time is : 1
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