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When we say that “potassium” is reactive, are we referring to a potassium atom or bulk potassium as in the solid metal? In other words does “potassium” refer to a type of atom or an agglomeration of potassium atoms?

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    $\begingroup$ Typically it refers to the bulk, but you can never be sure. For this and other reasons, the word "reactive" is more confusing than meaningful, and hence is better avoided. $\endgroup$ – Ivan Neretin Nov 22 '18 at 6:17
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Reactivity is a vague concept in chemistry. It is rather confusing than useful. Reactivity involves both thermodynamic and kinetic factors, it predicts whether a chemical compound will undergo a chemical reaction (either by itself or with other compounds) or not and if it does how fast is the reaction going to completion i.e., rate of reaction.

Reactivity is also used to compare or study rates of two or more chemical reaction. For example when we say reactivity of HBr is more than reactivity of HCl - here we refer to how fast the compund will give acid-base reaction.

Reactivity is sometimes referred to bulk and sometimes to a single molecule but mostly it is referred as bulk, but it is not predictable.

In pure compounds, reactivity is regulated by the physical properties of the sample. For instance, grinding a sample to a higher specific surface area increases its reactivity. In impure compounds, the reactivity is also affected by the inclusion of contaminants. In crystalline compounds, the crystalline form can also affect reactivity. However, in all cases, reactivity is primarily due to the sub-atomic properties of the compound.

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  • $\begingroup$ Hi there, so I am currently learning about redox reactions. So is the “oxidation state of a potassium atom” the same as the “oxidation state of potassium”? $\endgroup$ – user70490 Nov 22 '18 at 8:09
  • $\begingroup$ The only oxidation state of potassium is +1. It cannot go to higher values, like +6 !!! $\endgroup$ – Maurice Dec 17 '19 at 9:13
  • $\begingroup$ @Maurice Thank you for pointing out, I forgot why I did that there. $\endgroup$ – Saurabh Singh Dec 20 '19 at 16:11
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When we say that potassium is reactive, it is understood that it is more reactive than sodium. This can be easily demonstrated in classrooms by dropping a small piece (max 0.1 g) of those metals into a tub of cold water.

Sodium reacts in a relatively calm way by melting and moving quickly at the water surface, yielding Hydrogen gas H2 which can be set on fire with a match. The H2 flame is one or two millimeters high. It stays and move erratically around the sodium globule. The whole is not dangerous. It is worth the trouble showing it in a classroom. No smoke is produced. $$\ce{2 Na + 2 H2O -> 2 NaOH + H2}$$ $$\ce{2 H2 + O2 -> 2 H2O}$$

Potassium reacts so violently with water that both metal and gas take fire in such a way that it is a dangerous reaction. Intense smoke is produced, made of potassium oxide, hydroxyde and superoxide KO2, which are corrosive and toxic. It should not be shown in an ordinary classroom. $$\ce{2 K + 2 H2O -> 2 KOH + H2}$$ $$\ce{K + O2 -> 2 KO2}$$

Don't try to use more than 0.1 g of these metals. The reaction becomes explosive with higher amounts.

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