What substances can't dissolve in water?

What Won’t Mix with Water?

Water might be the universal solvent, but it doesn’t get along with everything. Some substances simply refuse to dissolve, no matter how much you stir. Understanding why comes down to chemistry—specifically, the principle that “like dissolves like.”

Vegetable oil, for instance, is a classic example of a nonpolar compound. Since water is highly polar, it won’t mix with oils. That’s why your salad dressing always separates unless you shake it. The oil just floats on top, refusing to dissolve.

Then there’s sand (SiO₂), a covalent solid. Even if you leave it in water for days, it won’t dissolve. Instead, it settles at the bottom. That’s because the strong network of covalent bonds in silica holds firm against water’s attempts to break it apart. You’ll see this in rivers and beaches—sand suspended or resting in water, never truly dissolving.

Somewhat surprisingly, silver chloride, though an ionic compound, is also insoluble. Most ionic substances dissolve in water, but silver chloride is an exception due to its high lattice energy and low hydration energy. It forms a white precipitate in reactions—something often seen in basic chemistry labs when testing for chloride ions.

And then there are metals. Take iron, for example. Drop a nail in water, and it won’t dissolve—at least not quickly. Instead, it might eventually rust due to a chemical reaction with water and oxygen, but that’s corrosion, not dissolution. Pure metallic iron remains intact in water because its metallic lattice is too stable to be pulled apart by water molecules alone.

So, while water dissolves a wide range of substances, things like oils, sand, certain ionic compounds like silver chloride, and metals such as iron stand their ground. Their insolubility isn’t random—it’s all about molecular structure and polarity.