Does the water you make chai with actually matter?

Yes, but less than the internet thinks, and much less once milk goes in. Water is almost all of a cup of black tea, so hardness and freshness change it measurably. In a cup of chai, milk and sugar cover most of the difference. The one habit worth keeping is filling the pan fresh rather than reboiling what is already sitting in it.

This is one of those subjects where the advice online is confident, technical and mostly aimed at someone brewing a delicate first-flush without milk. Almost none of it is written for a steel pan of kadak chai. Here is what actually changes the cup, in the order it matters.

Does the water affect the tea?

It affects black tea a great deal and chai considerably less, and the reason is arithmetic. A cup of black tea is about 99% water, so anything dissolved in that water is a large part of what you are tasting. A cup of chai is roughly half water and half milk, plus sugar, plus usually ginger or elaichi. Every one of those is louder than the water.

So the honest version is: if you drink your tea black, the water is the second most important ingredient after the leaf. If you drink chai, it sits well down the list, behind the leaf, the boil, the milk and the timing. It is worth understanding, but it is not where a disappointing cup usually goes wrong.

Should you reboil water for tea?

Fresh water carries dissolved air. Boiling drives it off, and water that has been sitting in a kettle or pan through two or three boils has lost most of it. Tea brewed in that water tastes flatter, a little duller, slightly less lively at the front of the sip.

It is a real effect and it is a small one. You will notice it in a plain black cup drunk without milk. You will struggle to find it in chai. But the fix costs nothing at all, so there is no reason not to take it: tip out what is left, fill fresh, boil once.

Is reboiled water dangerous?

You will find claims that reboiled water concentrates arsenic, nitrates or fluoride to dangerous levels. For municipal supply this is scaremongering with a grain of chemistry in it. Concentration only rises as water evaporates, and boiling a pan for the minute or two chai needs removes a trivial fraction of the volume. You would have to boil away most of the pan, repeatedly, to move any of those numbers meaningfully, and you would notice long before the chemistry did.

Reboiled water makes a flatter cup. That is the whole of the case against it.

What does hard water do to tea?

Hardness means dissolved calcium and magnesium, and the kind matters more than the amount.

Temporarily hard water carries calcium bicarbonate. This is the one that scales your kettle: heat it and the bicarbonate comes out of solution as the white crust on the element. It also pushes the water alkaline, and alkaline water darkens tea while flattening it. The liquor goes deeper and duller, the briskness softens, and a good Assam stops tasting brisk and starts tasting merely dark.

Permanently hard water carries calcium sulphate, which does not precipitate on boiling. It leaves no scale and treats tea more kindly than the bicarbonate kind.

Soft water gives the brightest, briskest cup, which is why the same pack of tea can taste noticeably better at a relative’s house in another city, and why the iridescent film on a cup of black tea shows up in one city and not another.

There is a floor, though. Water with almost nothing in it, RO output with no remineralisation, or distilled water, does not make the best tea either. Some mineral content helps extraction behave; strip it out entirely and the cup can come out oddly harsh and hollow at the same time.

Is the water in Mumbai and the Thane belt hard or soft?

Most of the city drinks lake water. The municipal supply for Mumbai, and much of Navi Mumbai and the Thane belt with it, comes from the reservoirs east and north of the city rather than from the ground, and lake-fed surface water is generally on the soft side. That is good news for tea and it is why a plain municipal cup in Mumbai usually needs no help at all.

Groundwater is a different matter. Buildings and towns further out through Dombivali, Ambernath, Badlapur and Karjat that draw on borewells, and anyone on tanker supply, are much more likely to be brewing in hard water. So are plenty of housing societies inside the city that top up from a borewell without mentioning it.

You do not need a test kit to find out. Look at the kettle or the base of the pan you boil water in. A white crust means temporary hardness and it means your tea is being flattened a little every time. Soap that will not lather says the same thing.

What can you do about hard water?

In order of how much difference it makes for the effort involved.

Fill fresh every time. Free, takes no time, and it is the only one of these that applies to everybody.

Descale the kettle or pan. If there is a crust, it is dissolving back into every cup you make. Citric acid or plain lemon, boiled and rinsed, clears it.

If your water is hard, adjust how you brew rather than what you brew with. Hard water mutes briskness, so a slightly shorter boil after the milk keeps what brightness there is. Chasing it with more patti makes the cup heavier, not brighter.

Filtered water, if you already have it. A domestic filter that reduces hardness will give you a visibly brighter cup of black tea. It is not worth buying one for the tea alone, and in chai you would be paying to improve something the milk is about to cover.

Quick reference

• Water is 99% of black tea and about half of chai. It matters proportionally.
• Fill fresh, boil once. Reboiled water tastes flat, and that is the only real objection to it.
• The “reboiling is toxic” claim does not survive contact with the arithmetic.
• Scale on the kettle means temporarily hard water, which darkens and dulls the cup.
• Lake-fed municipal water across Mumbai and the Thane belt is generally soft. Borewell and tanker supply usually is not.
• Descaling the pan is the highest-value fix after filling fresh.
• In a cup of kadak chai, the leaf, the boil and the milk all outrank the water.