A variable is just a mathematical symbol for a measured (or measurable) property or quantity.
Consider the ideal gas equation which regularly makes an appearance in chemistry and physics. It is not important what this equation says; we are interested only in how it represents the information.
We say that the pressure, p, is the dependent variable and that there are three independent variables; the amount of gas, n, the termperature, T, and the volume of gas, V.
As the name implies dependent variables depend upon other quantities. On the other hand, independent variables have no such dependence. They are free to take whatever values they like and are uninfluenced by the other quantities we may be considering.
In the ideal gas equation, the symbol R is not a variable. It is a constant. Its value does not change if we change the amount, temperature or volume.
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