Often in the laboratory we would like to have conditions (state functions) that are similar to those used by scientists in other laboratories. As such, as a community, we have decided upon some conditions that we will call "standard". The two state functions we can typically control are temperature ( \(T\) ) and pressure ( \(P\) ), so these are the conditions we standardize. We will call these conditions "Standard Temperature and Pressure" or STP. We will take standard pressure as 1 atm, and standard temperature as 0°C. You should note that these are choices. Like many things in life it is difficult to get everyone to agree. Recently, the International Union of Pure and Applied Chemistry (IUPAC) adopted 1 bar as standard pressure and 25°C as standard temperature. This "new" standard is referred to as Standard Ambient Temperature and Pressure (SATP). However, as this new condition has been slow to catch on, we will stick with the "old" standard (STP) unless told otherwise.
Keep them Straight
STP : 0°C (273.15 K) and 1 atm ←(our default "standard" for gases)
SATP : 25°C (298.15 K) and 1 bar