# What is the circle on top of variables in chem?

## Like dS^o, dH^o, and dG^o. Do they represent standard state? Or standard condition? What's the difference between these two anyway?

##### 1 Answer
Dec 20, 2017

Circle refers to standard conditions, Plimsoll symbol refers to standard state.

#### Explanation:

Usually:

The circle superscript refers to standard conditions.

The Plimsoll symbol (circle with a line through) refers to standard state . ⦵

You will find that they are used interchangeably, which is a bit confusing. But as long as you can recognise the difference between standard state and standard conditions, and when to use each, you should be okay.

Standard conditions , represented by the initialism STP (stnading for standard temperature pressure) refers to a temperature of $273.15 K$ (${0}^{\circ} C$) and a pressure of $1 a t m$ (${10}^{5} P a$). STP is normally used in calculations using the Ideal Gas Law to calculate gas volume, pressure etc.

Standard State supplies several different conditions, including:

• The standard state temperature is 25°C ($298 K$). Sometimes ${0}^{\circ} C$ is used for standard state, but if another temperature is not supplied assume $298 K$.
• All gases are at $1 a t m$ pressure.
• All liquids and gases are pure.
• All solutions are at $1 M$ concentration.
• The energy of formation of an element in its normal state is defined as zero.

Standard state is used in thermodynamic calculations for entropy, enthalpy, Gibbs, equilibrium constant etc.