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Where do lowercase k and uppercase K make appearances in general chemistry, and what do they signify?

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    $\begingroup$ M is out, K is out; I bet the next letter is going to be L :D $\endgroup$ – andselisk May 12 '19 at 15:55
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    $\begingroup$ @andselisk - one letter per course is all I can offer. $\endgroup$ – Karsten Theis May 12 '19 at 16:26
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In addition to the original answer I also allowed myself to invite lowercase Greek letter kappa κ to the table as typographically it has a similar-looking appearance and add some more definitions:

Lowercase k

Uppercase K

  • $\ce{K}$ is a symbol for potassium.
  • $\pu{K}$ is a symbol for Kaiser, unit of energy $(\pu{1 K} = \pu{1 cm-1})$.
  • K is a symbol for aminoacid lysine.
  • K is used in spectrometry to denote spectral lines.
  • $K$ is sometimes used as a symbol for decadic absorption coefficient.

Lowercase Greek letter kappa κ

Uppercase Greek letter kappa Κ

Both lowercase k and lowercase Greek letter kappa κ

Both lowercase k and uppercase K

  • $k$ and $K$ is in general may refer to a coefficient (German "Koeffizient"); indices are often used to denote which one, e.g. mass transfer coefficient $k_\mathrm{d}$, octanol/water partition coefficient $K_\mathrm{ow}$.
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Lowercase k

  • $k$ is used as the symbol for rate constants (dimensions vary, concentration to some power per time).
  • k is an SI-prefix that stands for kilo ($\pu{e3}$), for example in kJ (kilojoule).

Uppercase K

  • $K$ is used as symbol for the thermodynamic equilibrium constant (dimensionless quantity). Other versions of the equilibrium constant sometimes have units (e.g. dissociation constant $K_\mathrm{d}$ in biochemistry).

  • K is an SI-unit symbol for kelvin (unit for absolute temperature).

Other useful information

  • Make sure your lowercase k and uppercase K look different when you write or type, otherwise you are confusing your readers. If you type, quantities should be italicized and units should not. That also helps to distinguish the different meanings of k.
  • You can't cancel K with K if they don't have the same meaning. It is fine to cancel the kilo in kJ/kV to obtain J/V, but do not cancel kJ/(mol K) to obtain J/mol.
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