[srinivas23578]

Plz some one help me

I need answer to why specific gravity of liquid is checked at 20°C and why not other than 20°C?

[P]

What happens to volume at inreased or decreased temperatures?

Then ask - how is specific gravity measured?

Hope this helps!

P.

[Borek]

I need answer to why specific gravity of liquid is checked at 20°C and why not other than 20°C?

Unfortunately, that's not true! Specific gravity can be checked at any temperature, but you have to report this temperature for your result to make any sense.

http://en.wikipedia.org/wiki/Specific_gravity

[P]

Unfortunately, that's not true! Specific gravity can be checked at any temperature, but you have to report this temperature for your result to make any sense.

http://en.wikipedia.org/wiki/Specific_gravity

20C being standard temperature, I would assume that if no temperature was quoted with an s.g. value that the temp and pressure would be stp.  If the temperature was 10 C then the volume of the liquid is likely to be less due to contraction of the liquid and the s.g. reading would be higher. (less for hotter temp for opposite reason).

[Borek]

1. http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure - in general there is no one and widely accepted definition of STP. 20 deg C is as good as 0 deg C or 25 deg C.

2. Dependence between specific gravity and temperature depends on properties of two liquids. Depending on the ratio of contractions you may have specific gravity being smaller or higher when moving with temperature in either direction. For example, pure water has a density of 0.99973 g/mL at 10 deg C and 0.988807 at 50 deg C.

what	temp (deg C)	density (g/mL)	specific gravity
10 % NaOH	10	1.1132	1.1135
	50	1.0942	1.1074
mercury	10	13.5708	13.5744
	50	13.4729	13.6356

As you see for NaOH solution specific gravity grows with temperature, while for mercury it gets lower. That's not the case when you look for density - it gets lower with increasing temperature, as expected


3. In older books specific gravity is sometimes given with two indices, like d²⁰₄ (these should be one above the second, html limitiations doesn't allow that). This means specific gravity is ratio of of liquid density at 20 deg C compared to water density at 4 deg C (if water is at 4 deg C, it means the result is in g/mL, but you may see also other numbers, like d¹⁵₂₀). In a way it also means it is not a specific gravity, as SG calls for identical temperature

As far as I remember specific gravity was introduced as an easy way of making measurements of mineral density in field. It is pretty easy to submerge piece of rock in water to find out how its density relates to that of water. You can easily make sure they have both the same temperature, but it is pretty hard to make it 20 deg C (or any other standard). That's why it was defined this way. As it often happens, simple definition that works in field, becomes a problem when nitpicking specialists in white coats and well equipped labs start to look at fine details