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Author Topic: How do you know if its a gas, solid, or liquid?  (Read 11483 times)

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lhunt

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How do you know if its a gas, solid, or liquid?
« on: December 10, 2011, 09:22:30 AM »

Just a general question here. Or perhaps not so general. Or perhaps we just haven't gotten to that point yet.
We started doing chemical reactions, and these notations are in the equations, s, g, l. I know what they mean, but in a very early set of problems they ask us to predict how certain reactants would pair up or change in the products. And I could figure out mostly how the reactants would pair up, but really had no idea if it would be a gas, liquid, or solid. Is that still to come? Will we be able to tell if SO3 (e.g) is a gas or a liquid or a solid?

Thanks!
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Arkcon

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Re: How do you know if its a gas, solid, or liquid?
« Reply #1 on: December 10, 2011, 09:50:51 AM »

Well, experience, or you can look each product up in a reference book.  When you have a stack of problems, you should see what you'll have to know.  But no, there's generally no way to know with out having seen the products.  That's one of the reasons you have a laboratory assignment.  Example, if I tell you that I've burned sulfur in oxygen to generate SO2, and react the SO2 with oxygen and a catalyst to get SO3 and then I bubble SO3 into sulfuric acid to make oleum ... do you now know what phase SO3 has?  That's how you'll learn what to expect.
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lhunt

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Re: How do you know if its a gas, solid, or liquid?
« Reply #2 on: December 10, 2011, 12:46:18 PM »

Ok, so I guess I wouldn't be expected to know those at this stage. They were simple problems like this:

Combination: Mg(s) + Cl2 (g) yields what?

The book said try your hand at predicting, focus on equation. I knew the products, I just wasn't sure at all which form it would be. The answers were in the book, I just was starting to get worried that all the problems in this chapter have these notations, and at some point would I have to know not only the reaction equation, but also the state in which the products exist. But sounds like not now.

Thanks for relieving anxiety. Now onto bigger things like conversion factors using the Avogadro number and moles.
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Arkcon

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Re: How do you know if its a gas, solid, or liquid?
« Reply #3 on: December 10, 2011, 01:59:35 PM »

Well, let's try that one.  What do you think we're likely to get?
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lhunt

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Re: How do you know if its a gas, solid, or liquid?
« Reply #4 on: December 11, 2011, 07:19:16 AM »

Well the answer is magnesium chloride. Sounds like a gas. Chloride is mostly a gas. There are two chlorides for every magnesium. Gas?

So let me try another.

Decomposition: HBr(g) yields... So balance the equation
 2 HBr (g) yields H2 (g) + Br2 (g), because of decomposition. I know Hydrogen is a gas. Br (halogen, is a gas).

Single replacement: Mg(s) + Zn(NO3)2 (aq) yields...

Mg(NO3) + Zn....OK so for this one, I would have no idea what magnesium nitrate would be. Zn and Magnesium are metals, so the zinc would be a metal. The book says the magnesium nitrate is an aqueous. OK so that makes sense. This is a single replacement. So we switch out the Mg for the zinc.

Double: K2S (aq) +Pb(NO3)2 (aq) yields...
2KNO3 + PbS. So if by going by the names, Potassium Nitrate, no idea, maybe a liquid. Lead sulfide, solid? Here is what I know. Because of the potassium metal, KNO3 has substance, so not a gas. Lead sulfide, metal and group 6. Lead is a solid, so not a gas. So we started with an aq and aq. Is this a start of how to think through it? 
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fledarmus

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Re: How do you know if its a gas, solid, or liquid?
« Reply #5 on: December 11, 2011, 08:54:03 AM »

And now it is time for some experience...

Salts are usually solids - anything with ionic bonds tends to be solid. If water is present, many of them are water soluble (aq), but others aren't and you will eventually learn which ones are which.

So look again at your MgCl2 - what sort of bonds are those?
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Arkcon

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Re: How do you know if its a gas, solid, or liquid?
« Reply #6 on: December 11, 2011, 09:18:56 AM »

You need more experience lhunt:, try this: look at the periodic table here: http://0.tqn.com/d/chemistry/1/0/X/u/PeriodicTableNaturalState.jpg and see the states of pure elements, look for patterns, or heck just compare the numbers of liquid, gas and solid elements.

Here's another though experiment:  List all the pure solids, liquids and gasses -- not mixtures. alloys or solutions.  So soda pop, milk, orange juice are all one thing liquid water.  And count those.

You'll see, liquids and gasses are a bit less likely than your first guesses seem to be.

Note:  once you have good instincts, you'll still find surprises, uranium hexafluoride is a gas, this allows us to isolate isotopes for weapons enrichment easier.  There are advanced physical chemistry concepts that allow us to predict the melting point of compounds.  But that sort of thing is well beyond my level.  See the discussion here:  http://www.chemicalforums.com/index.php?topic=29205.msg111261#msg111261
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lhunt

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Re: How do you know if its a gas, solid, or liquid?
« Reply #7 on: December 11, 2011, 10:27:31 AM »

OK based on this information, MgCl2 has ionic bonds and probably a solid.

Potassium sulfide-solid


I understand. Looks like there are some ways to figure it out, but still a novice guess in most instances until you become more familiar with substances and properties.

So before we close this topic, or we can continue, What about the -ates, -ites? Or does it just depend?

Thanks!
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fledarmus

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Re: How do you know if its a gas, solid, or liquid?
« Reply #8 on: December 12, 2011, 10:08:51 AM »

That depends some on which part of chemistry you are in and which "-ates" and "ites" you are talking about.

In general chemistry and inorganic chemistry, things like sulfate, sulfite, nitrate, nitrite, chlorate, phosphate, are all complex anions containing multiple oxygen atoms bonded to an atom of another element (sulfur, nitrogen, chlorine, and phosporus for these examples). If the cation is a proton, you get acids - for example, sulfuric acid (H2SO4, could also be called hydrogen sulfate) - which are mostly liquids. With any other cation, you get salts - for example, sodium sulfate (Na2SO4) - which are mostly solids.

It will take you a while to get familiar with all the possibilities, but for now, "-ate" or "-ite" indicating a salt and a solid will be the right answer most of the time.
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