Why is sodium chloride solution a mixture?
Solutions and mixtures are both composed of two or more substances.
Recall that a solution is a homogeneous mixture composed of two or more substances . It appears to be uniform throughout. One cannot be able to determine its composition just by taking a look at the solution.
In your case, in order for sodium chloride solution to be a mixture, it must contain sodium chloride (salt) and another substance, mixed together such that it appears as if it were only composed of one substance.
For example, if the sodium chloride solution was composed of sodium chloride and water, it would be considered a mixture since sodium chloride and water are in the solution, and it appears to be uniform throughout, that is, you can't tell its composition just by looking at it. For instance, if I didn't tell you that this cup contained salt water, would you be able to guess what was in it?