What is Mixture?

Example 8.12 (Reading a mineral-water label)

A label reads: calcium $78\,\mathrm{mg}/\mathrm{L}$, magnesium $24\,\mathrm{mg}/\mathrm{L}$, sodium $5\,\mathrm{mg}/\mathrm{L}$, hydrogencarbonate $357\,\mathrm{mg}/\mathrm{L}$, sulfate $10\,\mathrm{mg}/\mathrm{L}$, chloride $4\,\mathrm{mg}/\mathrm{L}$. (These figures are an example.) In one litre there are $78 + 24 + 5 + 357 + 10 + 4 = 478$ mg of dissolved substances, about half a gram: tiny beside the $1000\,\mathrm{g}$ of water, but enough to make it a mixture.