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Like, in the equation between rubidium and water:

2Rb(s) + 2H2O(l) → 2RbOH(?) + H2(?)

How do we know what state symbol to use in 2RbOH and H2? I'd like to do this just by looking at the chemical formula, but it's OK if I have to memorize a list of stuff to be able to do this.

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  • $\begingroup$ If you don't get that compounds can be in different states depending on temperature... $\endgroup$ – Mithoron Nov 21 '17 at 16:02
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You can't do this by:

just by looking at the chemical formula

But instead look up the properties of the compounds in question (Wikipedia is a good source of data).

$\text{RbOH}$ is highly soluble in water, so assuming there's an excess of water present, it will end up being dissolved in the water. So its state symbol will be $\text{(aq)}$.

Hydrogen on the other hand is a gas (at room temperature and above) and highly insoluble in water: it will leave the reaction as gas and its state symbol will be $\text{(g)}$.

So without knowing the physical/chemical properties of the reagents/reaction products, the state symbols cannot be determined.

Note that the state symbols are often idealisations: solid or liquid compounds are rarely completely soluble or completely insoluble. The same is true for gases. Even water is a case in point: liquid, $\text{(l)}$, at room temperature but with significant vapour pressure.

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  • $\begingroup$ In many cases, it helps to know either something about the type of reaction occurring, or the temperature at which the reaction is taking place. For example: water is generally a product of combustion reactions, but since these take place at high temperature (we are burning something after all) the water will be present as a vapor (g). $\endgroup$ – Michael Lautman Nov 21 '17 at 15:53

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