In the fountain experiment, if blue litmus is used, what color will it turn?

The fountain experiment demonstrates the high solubility of a gas in water. As the gas inside the flask dissolves in the water from the trough, the pressure inside the flask drops significantly. The atmospheric pressure then pushes the water from the trough up into the flask, creating a fountain.

The water in the trough contains blue litmus. Blue litmus is an acid-base indicator that turns red in acidic solutions and remains blue in neutral or basic (alkaline) solutions.

For a dramatic color change in a fountain experiment using blue litmus, the gas inside the flask is typically an acidic gas (e.g., hydrogen chloride, $\text{HCl}$). When an acidic gas dissolves in water, it forms an acid.

When the blue litmus solution is drawn into the flask and comes into contact with the dissolved acidic gas, the solution becomes acidic. An acidic solution turns blue litmus red.

The final answer is $\text{C}$.