Why is Venus called the hottest planet of our Solar System even though it is in the second place from the Sun?

Venus is the second planet from the sun in our Solar System. It is a rocky planet and a neighboring one of Earth.

Venus is second in terms of position from the sun but yet it is the hottest planet in our Solar System. It's surface temperature i.e. 735 Kelvin or around 462°C or 872°F, always remains constant whether it is daytime or night. In fact, the temperature of the poles of Venus is also the same at that of the equator.

Venus imaged by Magellan Image Credit: NASA/JPL

So, why Venus has such a high temperature?

The answer is, all due to its atmosphere. Venus's atmosphere comprises of 96.5 % Carbon Dioxide and 3.5 % Nitrogen and other gases. As we all know that CO2 is a greenhouse gas that traps the heat of the Sun and prevents it to reflect back to space. That's why Sun's heat radiation falls on Venus's surface but the greenhouse gases don't let them escape back. 
The pressure also due to carbon dioxide is a huge pressure and therefore atmospheric pressure of Venus is as much as you feel pressure under 1 kilometer of ocean water.
The pressure and trapping of heat both simultaneously increase the temperature of Venus making it the hottest planet of our Solar System.
Mercury being an airless atmosphere doesn't make these conditions and hence it is not the hottest planet of our Solar System.