The Magical White Cloud Born from “Neutralization”: The Surprising Chemistry Between Limewater and Carbon Dioxide
I’m Ken Kuwako, a science communicator. Every day is an experiment!
Do you remember blowing into a straw filled with limewater during science class? Watching the clear liquid suddenly turn milky white felt almost like performing a magic trick.
But why does simply adding carbon dioxide make the liquid cloudy? The answer is surprisingly interesting. In chemistry, this reaction is actually considered a type of neutralization reaction in the broad sense. Let’s take a closer look at the fascinating exchange of substances happening behind that mysterious white cloudiness.
When students first learn about neutralization, it’s usually introduced as:
“Acid (H⁺) + Base (OH⁻) → Salt + Water”
The limewater experiment is a little different. There’s one important “culprit” hiding behind the scenes.
The Culprit: Tiny Rocks That Won’t Dissolve
Limewater is an alkaline solution containing dissolved calcium hydroxide. When carbon dioxide, which behaves like an acid after dissolving in water, is bubbled through it, the following reaction takes place.
The newly formed calcium carbonate is exactly what causes the white cloudiness.
Calcium carbonate is the same substance that makes up seashells, coral, classroom chalk, and even marble. Its most important property is that it is almost insoluble in water.
Imagine countless microscopic grains of stone suddenly appearing and floating throughout the previously clear water. As light strikes these tiny particles, it scatters in every direction, making the liquid appear white and cloudy.
- Base: Calcium hydroxide (limewater)
- Acid: Carbon dioxide (acts as an acid after dissolving in water)
- Salt: Calcium carbonate (the tiny “rock” responsible for the cloudiness)
- Water
The alkaline limewater meets acidic carbon dioxide, their chemical properties neutralize each other, and salt plus water are produced. It perfectly fits the definition of neutralization.
A Surprising Twist: Keep Blowing… and It Turns Clear Again!
The story doesn’t end there.
Continue bubbling carbon dioxide through the already cloudy limewater, and something unexpected happens—the liquid gradually becomes clear once again.
That’s because the insoluble calcium carbonate reacts with excess carbon dioxide to form calcium bicarbonate, which dissolves readily in water.
This repeated cycle of dissolving and solidifying is actually the same process responsible for creating magnificent limestone caves filled with stalactites and stalagmites. It’s amazing to think that the same chemistry happening with your breath also shapes entire cave systems over tens of thousands of years.
Why Doesn’t Hydrochloric Acid Make It Cloudy?
You might be wondering, “If neutralization causes the cloudiness, wouldn’t hydrochloric acid do the same thing?”
That’s an excellent question—but the answer is no. This is where chemistry gets especially interesting.
When hydrochloric acid is added to limewater, a neutralization reaction certainly occurs.
The product is calcium chloride.
Unlike calcium carbonate, calcium chloride is highly soluble in water. Once dissolved, it separates into invisible ions, so the solution remains perfectly clear.
The Secret Requirement for a Cloudy Solution
The key takeaway is simple:
For a chemical reaction to produce a cloudy white appearance, the newly formed substance must be insoluble in water, creating a precipitate.
- Carbon dioxide + Limewater: Produces insoluble calcium carbonate, so the solution turns cloudy!
- Hydrochloric acid + Limewater: Produces highly soluble calcium chloride, so the solution stays clear!
That’s exactly why limewater is such an effective “detective” for identifying carbon dioxide.
Interestingly, the calcium chloride that stays invisible is far from useless. It’s commonly used as a road de-icer during winter and as the active ingredient in many household moisture absorbers.
If you’d like to see another spectacular white precipitate formed by neutralization, try combining sulfuric acid with barium hydroxide.
The product, barium sulfate, is also completely insoluble in water. In fact, it’s the same white substance used in the “barium swallow” examination during medical X-ray imaging.
Chemistry is full of hidden substances that simply refuse to dissolve!
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