1. Acid value of oil:

The acid number is defined as the number of milligram of $\text{KOH}$ required to neutralize one gam of oil or fat

Fat may become rancid after long storage

Fat or oil is hydrolysed by different microorganisms with the formation of fatty acid.

Acid value of oil gives information about age of oil sample, also it signifies due to attack of atmospheric oxygen, hot moist air or microorganisms how much generation of free fatty acid has taken place leading to rancidity.

1. Cloud and pour point:

When an oil is cooled slowly, the temperature at which it becomes cloud-like or hazy in appearance, is called its “cloud-point”; while the temperature at which the oil ceases to flow or pour, is called its pour-point.

Cloud and pour-points indicate the suitability of lubricants in cold conditions.

Lubricant used in a machine working at a low temperature should possess low pour-point; otherwise solidification of lubricant will cause jamming of the machine.

1. Flash and Fire point:

Flash-point is “the lowest temperature at which the oil lubricant gives off enough vapours that ignite for a moment, when a tiny flame is brought near it”; while

Fire-point is “the lowest temperature at which the vapours of the oil burn continuously for at least five seconds, when a tiny flame is brought near it.”

The flash and fire points do not have any bearing with the lubricating property of the oil, but these are important when oil is exposed to high temperature service.