Sodium hydroxide (NaOH) is a strong base. When you add universal indicator to a solution of sodium hydroxide, it will turn a deep purple color.

Here's why:

* Universal indicator is a mixture of dyes that change color depending on the pH of a solution.

* pH is a measure of acidity or alkalinity.

* Strong bases like sodium hydroxide have a very high pH (typically above 11).

* High pH solutions cause the universal indicator to turn purple.

So, the deep purple color indicates that the solution is strongly alkaline due to the presence of sodium hydroxide.