Is Your Tap Water Secretly Alkaline? A BTB Experiment That Will Surprise You

I’m Ken Kuwako, the Science Trainer. Every day is an experiment.

“What color does BTB solution turn when you add it to tap water?”

Many people would probably guess, “It’s drinking water, so it must be neutral… which means green, right?”

Surprisingly, the real answer is quite different. Take a look.

Wait… Tap Water Isn’t Neutral?

It’s blue!

The BTB solution turns blue. This result comes from testing tap water in Chiba Prefecture.

If you’re thinking, “Shouldn’t tap water be neutral?” there’s an interesting piece of chemistry behind the answer.

Tap water is generally close to neutral, but it is often slightly alkaline rather than exactly pH 7. There are two main reasons for this.

First, tap water is disinfected with chlorine-based chemicals (mainly sodium hypochlorite), which tend to make the water slightly more alkaline.

Second, drinking water regulations allow the pH to range from 5.8 to 8.6, meaning it doesn’t have to be exactly neutral. Since BTB solution appears green around pH 6.0–7.6, water that’s only a little above pH 7 can already start looking bluish.

From the Tokyo Metropolitan Government Bureau of Waterworks:

Question: What does the pH of tap water mean?

Answer: pH indicates how acidic or alkaline water is. The scale runs from 0 to 14, with pH 7 being neutral. Lower values are more acidic, while higher values are more alkaline.

Japan’s drinking water standard requires tap water to remain between pH 5.8 and 8.6 to help prevent corrosion in water supply facilities. Tokyo’s tap water typically has a pH of around 7.5, making it close to neutral.

https://www.waterworks.metro.tokyo.lg.jp/faq/qa-22.html

From the Chiba Prefectural Government:

Because lead is less likely to dissolve in alkaline water, Chiba Prefecture has adjusted tap water to be slightly alkaline throughout its water supply system since April 2002.

https://www.pref.chiba.lg.jp/suidou/kyuusui/namari/03qa.html

I also tested the water in my laboratory, and it turned blue as well.

That said, tap water isn’t guaranteed to be alkaline everywhere in Japan.

Although sodium hypochlorite used for disinfection tends to raise the pH, the final value also depends on the original water source—such as rivers, reservoirs, groundwater—as well as local geology and dissolved minerals.

As a result, some regions have water that’s almost perfectly neutral, while others may even be very slightly acidic. So it’s more accurate to say that disinfection generally pushes tap water toward the alkaline side.

Blow Into It… and Watch the Color Change!

Now for the fun part.

Take the blue tap water and slowly blow into it through a straw. As you do, the color gradually changes from blue → green → yellow.

Watch the experiment here.

Comparing the water before and after makes the difference unmistakable.

This happens because the carbon dioxide in your breath dissolves into the water, forming carbonic acid. As more carbon dioxide dissolves, the water gradually becomes more acidic.

Interestingly, this is exactly the same process behind one of today’s major environmental issues: ocean acidification. Carbon dioxide from the atmosphere dissolves into seawater, slowly lowering the ocean’s pH.

It’s amazing to think that the tiny color change happening in a glass of water is driven by the very same chemistry affecting our entire planet.

Does Adding Chlorine Make Water Acidic? Clearing Up a Common Misunderstanding

Several readers asked an excellent question:

“Doesn’t adding chlorine make water acidic?”

The answer is a little more nuanced.

Chlorine gas (Cl₂) does make water acidic when dissolved, but modern water treatment plants generally use sodium hypochlorite instead. Because sodium hypochlorite is the salt of a weak acid and a strong base, its solution is actually alkaline.

Even though both substances contain chlorine, their chemical behavior is completely different.

Chemistry textbooks often describe the reaction between chlorine gas and water:

Cl₂ + H₂O ⇌ HClO + HCl

Since this reaction produces two acids—hypochlorous acid (HClO) and hydrochloric acid (HCl)—chlorine gas does indeed make water acidic.

Many older water treatment plants once injected liquefied chlorine gas directly into the water, so this explanation may be familiar from older textbooks.

Today, however, most treatment plants in Chiba Prefecture and many other regions use sodium hypochlorite (NaClO), a liquid disinfectant instead of chlorine gas.

Sodium hypochlorite is the salt formed from a weak acid (HClO) and a strong base (NaOH). When dissolved in water, it undergoes hydrolysis, producing hydroxide ions (OH⁻), which makes the solution alkaline.

ClO⁻ + H₂O ⇌ HClO + OH⁻

As a result, adding sodium hypochlorite raises the pH rather than lowering it.

It’s a wonderful example of chemistry: two chemicals made from the same element can produce completely opposite effects simply because they exist in different chemical forms.

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