What happens to the water in the small tube as the water in the test tube absorbs heat?

As the water in the test tube absorbs heat, its temperature increases, causing it to expand. This expansion can create a pressure difference that may cause the water in the small tube to either rise or flow, depending on the setup. If the small tube is connected to the test tube, the heated water may push some of the water in the small tube upward, demonstrating the effect of thermal expansion.