## Parachor:

Macleod in 1923 gave the following relation between the surface tension (γ) and density (D) for a normal liquid-

C=γ^{1/4}/(D −d)

where d is vapour density of the liquid at given temperature and C is constant.

In 1924, Sugden modified the above equation as-

γ^{1/4}/(D −d)=MC=P

where M is the molecular weight of the liquid and P is the parachor.

At ordinary temperature, d is very small in comparision to D then-

M γ^{1/4}/D=P

If γ=1 at a particular temperature, then-

M/D=P

Thus, at a particular temperature, the molar volume of a liquid having surface tension unity is called **Parachor**.

If two liquids having the same surface tension are taken whose molecular weights are M_{1} and M_{2} and their densities are D_{1} and D_{2} respectively, then-

M_{1} γ^{1/4}/D_{1}=P_{1}

and

M_{2} γ^{1/4}/D_{2}=P_{2}

P_{1}/P_{2}=(M_{1}/D_{1})/ (M_{2}/D_{2})

So, the ratio of parachors of two liquids having the same surface tension is equal to the ratio of molar volumes. Parachor is both an additive and constitutive property.

### Parachor Value of some Elements and Groups:

Element | P Value | Group | P Value |
---|---|---|---|

C | 8.6 | C=O | 44.4 |

H | 15.7 | OH | 30.2 |

N | 12.5 | COOH | 73.7 |

O | 19.8 | NO_{2} | 73.8 |

Cl | 55.2 | Double Bond | 19.9 |

Br | 68.8 | Triple Bond | 40.6 |

I | 90.3 | Six Membered Ring | 1.4 |

## Application of Parachor:

Parachor data are used to determine the structure of molecules and the nature of bonds.

Example:

Two structure proposed for QUINONE-

**Structure-1**

6C=6 X 8.6=51.6

4H=4 X 15.7=62.8

2O=2 X 19.8=39.6

Four double bonds=4 X 19.9=79.6

one 6 membered ring=1 X 1.4=1.4

So, Calculated Parachor=235 **Structure-2**

6C=6 X 8.6=51.6

4H=4 X 15.7=62.8

2O=2 X 19.8=39.6

Three double bonds=3 X 19.9=59.7

Two 6 membered ring=2 X 1.4=2.8

So, Calculated Parachor=216.5

Since the experimental Parachor is 236.8. Hence structure-1 is correct for QUINONE.