Chemistry · Thermodynamics · NEET
It is the pure form of that substance at a pressure of 1 bar. NCERT says it clearly: the standard state of a substance at a specified temperature is its pure form at 1 bar. For example, the standard state of liquid ethanol at 298 K is pure liquid ethanol at 1 bar. It just fixes the conditions so that everyone reports the same enthalpy value.
The modern NCERT value is 1 bar, not 1 atm. Older books used 1 atm (101325 Pa). 1 bar = 100000 Pa, which is very slightly less than 1 atm. For NEET, always answer 1 bar. If a question gives you the old 1 atm, treat it as the outdated version, but the correct current answer is 1 bar.
The standard state does NOT fix a temperature by itself. You must state the temperature separately. By convention, most data are given at 298 K (25 C), so people say 'standard state' and mean 298 K. But 273 K (0 C) is used for STP, which is a different idea. So: standard state pressure is fixed (1 bar), temperature is chosen (usually 298 K).
That superscript circle (⊖ or a plimsoll/theta sign) means 'standard'. So ΔH⊖ means the enthalpy change when every substance is in its standard state (pure, 1 bar). Without the circle, ΔH just means the enthalpy change under whatever conditions were used. The circle is a promise that standard conditions were used.
They are different. Standard state is about defining a reference for enthalpy: pure substance at 1 bar, with temperature stated (usually 298 K). STP (Standard Temperature and Pressure) is a fixed gas condition used for gas volume calculations: 273.15 K and 1 bar (giving 22.7 L per mole). Do not mix them up. Standard state lets temperature be chosen; STP fixes both temperature and pressure.
For any substance, solid, liquid or gas, the standard state is its pure form at 1 bar. For solid iron at 500 K, the standard state is pure iron at 1 bar. For liquid ethanol at 298 K, it is pure liquid ethanol at 1 bar. The physical state (solid/liquid/gas) is just whatever the pure substance is at that stated temperature and 1 bar.
Because the enthalpy change of a reaction depends on the conditions. If two people run the same reaction at different pressures, they get slightly different ΔH values. To compare data fairly, chemists agreed on one fixed condition (pure, 1 bar). This is why formation, combustion and reaction enthalpies are all tabulated as standard values (ΔH⊖). For NEET, standard enthalpy data all use this reference.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 bar (100000 Pa). This is the modern NCERT value. The older value 1 atm is no longer used for the answer.
No. Standard state only fixes the pressure at 1 bar and requires the substance to be pure. Temperature must be stated, and 298 K is the usual choice for data.
It is the standard enthalpy change. The circle superscript tells you every substance in the reaction is in its standard state (pure, 1 bar).
No. STP fixes both 273.15 K and 1 bar for gas volume work. Standard state fixes only 1 bar and lets you name the temperature (usually 298 K).
Pure liquid water at 1 bar pressure, at 298 K. The physical state is just whatever the pure substance naturally is at that temperature and 1 bar.
