Final answer:
When heated from 10°C to 90°C, the pressure at the bottom of a beaker of water increases due to the expansion of the water and the higher column of water above each point at the bottom, despite a decrease in water's density.
Step-by-step explanation:
When a beaker of water is heated from 10 degrees Celsius to 90 degrees Celsius, the pressure at the bottom of the beaker does indeed increase. This happens because water expands when heated, which means that its density decreases; however, the effect of expansion is more than compensated by the increased height of the water column due to the thermal expansion. Therefore, even though the density is lower, the pressure at the bottom, which is a product of the height of the water column, the density of the fluid, and the acceleration due to gravity (P = h⋅ρ⋅g), actually increases.
Furthermore, as water temperature approaches its boiling point, water starts to form bubbles due to the increasing vapor pressure. The bubbles are filled with water vapor and other gases dissolved in water. At 100°C, the vapor pressure of water is equal to the atmospheric pressure, and bubbles form more easily throughout the liquid, eventually leading to boiling. However, the increase in pressure described here pertains to the pressure increase before reaching the boiling point, where the liquid water itself is expanding and increasing the hydrostatic pressure at the bottom of the beaker.