The water out of your tap is an ingredient with a composition

The one ingredient nobody lists

Recipes specify the flour, the fat and the spice, and then say water as though water were a constant. It is not. Water arriving from a limestone aquifer and water arriving from a granite catchment carry very different quantities of dissolved mineral, and those minerals participate in the cooking rather than sitting there inertly.

This is one of the more satisfying explanations for a common frustration: the recipe that worked in one house and doesn’t work in another, cooked by the same person from the same notes. Sometimes the variable is the tap.

Hardness is a measure of dissolved rock

Water picks up whatever it passes through. Rain falling on chalk or limestone dissolves calcium and magnesium compounds and carries them into the supply; rain running over hard igneous rock picks up comparatively little. That difference is what hardness means, and it varies enormously not only between countries but between neighbouring towns.

The everyday signs are familiar even to people who have never thought about the chemistry. Scale inside a kettle, a film on the inside of a pan, soap that does not lather easily, a faint chalkiness in a glass of plain water. All of them point the same way.

It is also worth knowing that the supply to a single house isn’t necessarily constant through the year. Where water comes from more than one source, or where a borewell supplements a piped supply during shortages, the mineral content can shift between seasons — which is one reason a cook may find a familiar dish behaving oddly for a few weeks and then returning to normal without explanation.

Calcium makes plant tissue firmer, and that cuts both ways

Calcium ions interact with the cement between plant cells, cross-linking it into a structure that resists breaking down under heat. In practice that means vegetables and pulses cooked in hard water stay firmer for longer, and in some cases stubbornly so.

For pulses this is usually a nuisance, since it adds to a softening problem that ageing has often already started. For vegetables it can be an advantage: something that would otherwise collapse into a mush may hold its shape better, which is exactly why some traditional preserving methods deliberately add a calcium source to keep pieces firm.

Soft water does the opposite. Things break down faster, which suits a dal and can be a genuine problem for a vegetable you wanted to remain distinct. Neither water is correct. They simply place you at different points on the same scale, and the correction is timing rather than technique.

Alkalinity changes colour and flavour in ways you can see

Hardness usually travels with alkalinity, and alkaline conditions have visible effects. Green vegetables keep their bright colour longer in slightly alkaline water, because the pigment responsible is degraded by acid — but the same conditions accelerate softening of the tissue, so the vegetable looks better and holds together worse.

The effect on brewed drinks is more noticeable still. The compounds extracted from tea leaves react with dissolved minerals, which is why the same tea gives a dull, heavy cup in one city and a bright one in another, and why the film that forms on the surface of tea appears in hard-water areas and not elsewhere.

Legumes and lentils that are already coloured can shift as well, and cooks in hard-water areas sometimes find that dishes simply look different from the pictures without any obvious explanation.

Doughs, batters and the practical response

Water chemistry affects doughs too, though more subtly than it affects vegetables. Minerals in the water have a strengthening effect on the gluten network, so a dough made with very hard water can feel tighter and less extensible, while very soft water can give a slack, sticky dough that resists being shaped.

For a fermented batter, the mineral content is one variable among many and rarely the dominant one — temperature matters far more. But it isn’t nothing, and a batter that ferments differently after a move to a new city is not necessarily being imagined.

The practical response is unexciting and effective. Learn what your water does, adjust timing rather than recipes, and use filtered or boiled and cooled water for the few things that are genuinely sensitive. Boiling drops out some of the hardness as scale, which is why the water from a kettle isn’t chemically the same as the water from the tap that filled it.