Ascientist adds different amounts of salt to 5 bottles of water. he then measures how long it takes for the water to boil. what is the independent variable in this experiment?

An independent variable is the one which is under your control. Its variation does not depend on any other factor. You can assign any value to it and manipulate. A change in independent variable causes changes in dependent variable.

Here, the amount of salt added to the water bottles is the independent variable. The temperature at which water will boil depends on the amount of salt added(impurity).

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