# Question f3be3

Aug 21, 2017

Here's my explanation.

#### Explanation:

Bromothymol blue (HBtb) is a weak acid in which the molecular form and the ionized form have different colours.

color(orange)("HBtb") + "H"_2"O" ⇌ color(blue)("Btb"^"-") + "H"_3"O"^"+"#

The unionized form is yellow (below pH 6.0), while the ionized form is blue
(above pH 7.5).

When you place $\text{HBtb}$ in a solution of sodium hydroxide, the $\text{OH"^"-}$ ions react with the $\text{H"_3"O"^"+}$ ions and convert them to water.

Le Châtelier's Principle states that, when a stress is applied to a system at equilibrium, the system will respond in such a way as to reduce the stress.

When you add $\text{NaOH}$ to $\text{HBtb}$, you are removing a product ($\text{H"_3"O"^"+}$) from the equilibrium reaction.

The system responds by producing more $\text{H"_3"O"^"+}$.

At the same time, it produces more $\text{Btb"^"-}$, so the solution turns blue.