# Specific Gravity and Temperature?

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### manohman

##### Full Member
7+ Year Member
Im trying to understand how or why specific gravity changes with temperature.

It is unitless ratio between the density of a substance at a certain temperature over the density of water at the same temperature.

So if temperature changes the density would change for both the substance and water. But the reason why the specific gravity doesnt stay the same is that the degree of change is different for different substances?

IS that correct? Makes sense because the bonds holding substances together may be different and so same energy would result in different changes in volume/spreading out. I think?

Specific gravity is a relative measure of density, since you are calculating the ratio of a density of an unknown substance to the density of a reference substance (usually water) at a specified temperature and pressure. This is because density changes with temperature and pressure due to properties of intermolecular interactions.

The temperature dependence of specific gravity can be seen by measuring the changes in the ratio of density of water at changing temperatures to density of water at a fixed temperature (usually at 4 °C). Doing so, you'll get something like this:

When calculating specific gravity of an unknown substance, it's important to keep the temperature and pressure for both the unknown substance and water the same to minimize variability. Because of differences in intermolecular interactions, the density of the substance can change at different rates compared to the density of water, which is why the specific gravity also changes with changing temperature and pressure.

Specific gravity is a relative measure of density, since you are calculating the ratio of a density of an unknown substance to the density of a reference substance (usually water) at a specified temperature and pressure. This is because density changes with temperature and pressure due to properties of intermolecular interactions.

The temperature dependence of specific gravity can be seen by measuring the changes in the ratio of density of water at changing temperatures to density of water at a fixed temperature (usually at 4 °C). Doing so, you'll get something like this:

When calculating specific gravity of an unknown substance, it's important to keep the temperature and pressure for both the unknown substance and water the same to minimize variability. Because of differences in intermolecular interactions, the density of the substance can change at different rates compared to the density of water, which is why the specific gravity also changes with changing temperature and pressure.
i see! thank you that was so helpful.

i had one more question related to density. and specific gravity.

why is the density the same on the earth for a substance as it is on the moon. Given the same temperature and pressure, the gravity would definitely be different on the earth vs on the moon.

I guess what im wondering is, wouldnt gravity affect the degree to which something is "pulled together". Or is that taken into account by pressure.

If we have a ball of a substance, and put it on the moon or on earth given the different gravities i cant help but imagine the density being different becuase of the volume being different due to the pull of gravity. Unless pressure takes that into account (i.e. atmospheric pressure)

sorry I know this is very specific but i'm so curious

i see! thank you that was so helpful.

i had one more question related to density. and specific gravity.

why is the density the same on the earth for a substance as it is on the moon. Given the same temperature and pressure, the gravity would definitely be different on the earth vs on the moon.

I guess what im wondering is, wouldnt gravity affect the degree to which something is "pulled together". Or is that taken into account by pressure.

If we have a ball of a substance, and put it on the moon or on earth given the different gravities i cant help but imagine the density being different becuase of the volume being different due to the pull of gravity. Unless pressure takes that into account (i.e. atmospheric pressure)

sorry I know this is very specific but i'm so curious

Not really. Gravity doesn't change the mass nor the volume of an object; gravity is just an interaction between different masses, so mass contributes to gravitational force/field/acceleration rather than the converse (this is why every object has a gravitational field). The reason why the density of an object may differ across different planets/moons/satellites etc. is due to differences in atmospheric pressure. The atmospheric pressure of the moon is very small and essentially negligible (Moon Fact Sheet), so a liquid like water will evaporate away. A solid object will maintain roughly the same volume (this is why astronauts maintain roughly the same density in the moon as in Earth; the minor differences are due to physiological processes).

Last edited:
1 user
Members don't see this ad :)
Not really. Gravity doesn't change the mass nor the volume of an object; gravity is just an interaction between different masses, so mass contributes to gravitational force/field/acceleration rather than the converse (this is why every object has a gravitational field). The reason why the density of an object may differ across different planets/moons/satellites etc. is due to differences in atmospheric pressure. The atmospheric pressure of the moon is very small and essentially negligible (Moon Fact Sheet), so a liquid like water will evaporate away. A solid object will maintain roughly the same volume (this is why astronauts maintain roughly the same density in the moon as in Earth; the minor differences are due to physiological processes).
cool okay thank you! and atmospheric pressure is affected by both gravity and the density of the gas above, yes?

i guess the way i thought of it was of gravity pulls an object down, then if gravity from a planet is stronger it will "pull" on the object more almost squeezing it against the ground if that makes sense, affecting volume. if that makes any sense. i know thats taking enormous liberties conceptually. but what im describing in effect is also what atmospheric pressure does.

cool okay thank you! and atmospheric pressure is affected by both gravity and the density of the gas above, yes?

i guess the way i thought of it was of gravity pulls an object down, then if gravity from a planet is stronger it will "pull" on the object more almost squeezing it against the ground if that makes sense, affecting volume. if that makes any sense. i know thats taking enormous liberties conceptually. but what im describing in effect is also what atmospheric pressure does.

Atmospheric pressure depends on a lot of things including the planet's gravity. But gravity here plays a very indirect and insignificant role to cause changes in an object's density (remember, the object itself also creates a gravitational field that interacts with planet's gravity). An object in a black hole will have a significantly different density to the point it will cease to exist.

Gravity acts between the center of mass of two objects and pulls those center of masses towards each other. (At least that's how we best understand it - gravity is the odd force out, as it's the one that physicists haven't been able to reconcile with the other fundamental forces) The gravitational force felt by an object will be pulling all particles within that object towards the center of mass of the other object with more or less the same force (marginally different but negligible). So it will not have the effect of compressing or dilating the space between particles within the first object (volume). You're confusing a push/pull force with a compressing force.