Thick atmosphere on Venus none on Mercury
Higher air pressure forces the mercury farther up the tube.
No, mercury has a higher surface tension.
Venus is hotter because it has a thick atmosphere that traps heat, creating a greenhouse effect. This causes Venus to have a much higher surface temperature than Mercury, despite being farther from the Sun.
All eight planets have a surface temperature, though it varies as the planets distance from the sun varies. Closer planets to the sun generally have a higher surface temperature, while further planets have a colder surface temperature.
You might expect a planet to be hotter if it's nearer the Sun. This is true apart from Venus, which is the exception. Venus has a higher average surface temperature than Mercury. We believe this is because of the "greenhouse effect" of the atmosphere of Venus.
Mercury is less hotter than Venus because it has a thin atmosphere and the heat that is attracted during day time goes away at night and the temperature becomes -170 degrees while Venus has a thick atmosphere and does not allow heat to pass.
The planet is heated from the surface upwards, so the higher you go, the farther you are from the heated surface and thus, the cooler it gets. Even though sunlight passes through the upper parts of the atmosphere, it doesn't warm them as much as it warms the surface.
Look at this extremely similar question answered by me and RR12345: Why_do_areas_of_high_elevations_have_lower_temperature
A mercury thermometer measures temperature based on the expansion or contraction of the liquid mercury inside the narrow tube. As temperature rises, the mercury expands and travels up the tube, indicating a higher temperature, and vice versa. The temperature reading is taken at the point where the mercury level stabilizes.
The surface temperature of Venus is higher than the surface temperatures of the other inner planets because of its thick atmosphere.
That type of thermometer has mercury in it, which is very heat sensitive. When used for taking a temperature, the heat from the body causes the mercury to rise. The amount the level of the mercury rises is determined by the body's temperature; the higher the temp, the higher the mercury rises.
The surface temperature of Venus is higher than the surface temperatures of the other inner planets because of its thick atmosphere.