Phthalic acid is a dicarboxylic acid with the formula C₆H₄(CO₂H)₂. It exists in three isomeric forms:

* ortho-phthalic acid (also called phthalic acid)

* meta-phthalic acid (also called isophthalic acid)

* para-phthalic acid (also called terephthalic acid)

Terephthalic acid is the para-isomer of phthalic acid. It has the following properties:

* Formula: C₆H₄(CO₂H)₂

* Appearance: White, crystalline solid

* Melting point: 300 °C (572 °F)

* Solubility: Insoluble in water but soluble in hot alkaline solutions

Uses:

* Production of polyester fibers and resins: Terephthalic acid is a key ingredient in the production of polyethylene terephthalate (PET), which is used for clothing, bottles, and other plastic products.

* Other applications: It is also used in the production of paints, coatings, and food additives.

Note: The term "para-phthalic acid" is not commonly used. The correct term for the para-isomer of phthalic acid is terephthalic acid.