Hydrometer for Rainbow Jar Question

[derekp83]

I have a question regarding the use of a hydrometer for a density/rainbow jar activity in a summer program. The activity involves a jar filled with water, which will be followed by additional substances, some of which we brew with. Corn syrup, food coloring, honey, and a few others.

I'd like to involve the concept of gravity/density in the activity. I want the kids to use hydrometers to analyze the gravity/density of substances as they are added. However, they will not be shaking the jar to mix the substances, and as a result there will be layers.

So, to my question: will the hydrometer read the gravity properly given the rainbow layers, or do the substances need to be mixed?

Thanks!

 

[Trox]

If you want to see the difference that each item has on the density it will need to be mixed, otherwise you'll just read the density of which ever layer the lower portion of the hydrometer will be resting in. Of course that won't be 100% accurate either since the additional layers will have a very small affect on the hydrometer also; just not as much since it will only be able to affect the small glass stem portion. I would also think the further away from the primary layer you get the less affect the new substance will have on the hydrometer.


Edit: Of course not being a scientist I may be off on my assessment, but just going off my basic understanding of how the hydrometer works I don't think I am.

 

[JimRausch]

Here's my guess:
Each layer will have a different density which is why they settle into layers.
A gravity reading of the whole I don't think will give you a useable reading.
However, they can take a reading of each solution before mixing them together and take a guess as to which layer it will be.
I'd experiment ahead of time and see if your predictions are correct.
Good luck!

 

[derekp83]

Thanks for the prompt and helpful responses! We will likely do readings separately and before the substances are added to the jar.

 

[johngaltsmotor]

If you add different amounts of each liquid it will make a difference also. The hydrometer works based on the mass of the hydrometer vs the volume of fluid displaced. If it displaces a different volume of each fluid the various fluids will have a different ratio of influences.
I would try to keep the volume of the layers similar.

 

[derekp83]

If you add different amounts of each liquid it will make a difference also. The hydrometer works based on the mass of the hydrometer vs the volume of fluid displaced. If it displaces a different volume of each fluid the various fluids will have a different ratio of influences.
I would try to keep the volume of the layers similar.

Will do. Thanks!