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Okay, if you have ever been inside a massive, underground cave, you have probably seen those crazy, jagged rock spikes hanging from the ceiling like giant stone icicles. They are called Stalactites.
They look incredible, but they have one massive problem: they take thousands and thousands of years to grow. A single drop of mineral water drips from the cave ceiling, leaving a microscopic grain of rock behind. Drop by drop, century by century, it forms a massive stone spike.
But what if you don’t have ten thousand years to wait? What if you want to grow cave rocks right now, in your kitchen, in just a few days?
You actually can. By hacking the way water holds onto salt, you can force water to violently drop its minerals, growing a beautiful, hanging, solid white rock formation between two cups in less than a week. This is the Baking Soda Stalactite experiment, and it is a masterpiece of slow-motion chemistry. Let me explain how it works.
The Super-Saturated Sponge
To grow a rock, we have to start by completely breaking water.
If you take a glass of cold water and pour a spoonful of baking soda into it, the baking soda dissolves and disappears. The water acts like an invisible sponge, soaking up the baking soda.
But if you keep pouring baking soda in, eventually, the water gets “full.” The baking soda will just fall to the bottom of the glass in a cloudy pile. The water simply cannot hold any more powder.
But here is the science hack: Hot water is a much bigger sponge than cold water!
If you boil the water on the stove, the heat forces the water molecules to spread out, creating massive gaps between them. While the water is boiling hot, you can dump an insane amount of baking soda into it, and the water will hold all of it! This heavily loaded water is called a Super-Saturated Solution.
If you want to read the hardcore chemistry math behind how temperature affects molecular solubility, the American Chemical Society (ACS) has amazing charts on the subject.
The Slow Dripping Magic
So, you have two cups filled with hot, super-saturated baking soda water. You place the cups a few inches apart.
Then, you take a piece of thick yarn or string, dip one end in the left cup, and dip the other end in the right cup, letting the string hang down like a “U” shape between the two cups.
The yarn acts like a bridge. The heavy, loaded water slowly travels up the string from the cups and travels to the lowest point in the middle of the “U” shape. The water forms a tiny drop, and it slowly drips down onto a plate below.
Here is where the magic happens.
As the hot water travels along the string, it hits the air and cools down. Remember how cold water is a smaller sponge? Because the water is getting colder, it physically cannot hold all that baking soda anymore! It is forced to spit the baking soda out!
As the water drop hangs from the string, the water evaporates into the air, leaving behind a tiny, solid crust of white baking soda. The next drop travels down the string, hits the crust, evaporates, and leaves another crust. Drop by drop, hour by hour, the crust grows downward, forming a solid, hanging white rock spike!
You can read more about the amazing geology of cave formations over at National Geographic Education.
Setting Up the Perfect Cave
Because this trick relies on evaporation, it is going to take a few days, and it is going to get crusty. You absolutely need to place a heavy Silicone Drip Mat under the cups, or you will accidentally grow a rock directly onto your mom’s kitchen table.
Also, to make the water travel up the string perfectly, you need a very thick, absorbent piece of Natural Cotton Yarn. If you use cheap plastic string, the water won’t soak into it, and the rocks will never grow!
To learn more about how geologists study mineral deposits to understand the history of the earth, check out the archives at the National Science Foundation (NSF).
Quick Cave Rock Summary
What you need:
– A large box of Baking Soda
– Hot boiling water
– Two tall drinking glasses
– A piece of thick cotton yarn
– Two paperclips
– A small plate
Step-by-step guide:
1. Boil some water and carefully pour it into the two tall drinking glasses until they are almost full.
2. Stir baking soda into each glass until it absolutely stops dissolving and starts piling up at the bottom. You want it super thick!
3. Place the two glasses a few inches apart, and put your small plate perfectly in the middle between them.
4. Tie a paperclip to each end of your cotton yarn to act as heavy anchors.
5. Drop one paperclip into the left glass, and the other paperclip into the right glass. Let the yarn hang down in a “U” shape right over the plate.
6. Leave it totally alone for 5 days! The water will drip down the string, evaporate, and leave behind an awesome hanging rock!
10 Crusty Brain Teasers
Is your brain super-saturated? See if you can drip out the answers to these 10 riddles!
1. The Riddle: I am the fancy science word for the massive rock spikes that hang straight down from the ceiling of a cave. What am I?
The Answer: Stalactites.
2. The Riddle: I am the everyday white kitchen powder you dissolve into the water to grow your own rocks. What am I?
The Answer: Baking soda.
3. The Riddle: I am the fancy chemistry term for hot water that has been totally forced to hold way too much powder. What am I?
The Answer: Super-Saturated.
4. The Riddle: When the hot water gets to this temperature, the “invisible sponge” shrinks and forces the baking soda out. What temperature?
The Answer: Cold (or cooler).
5. The Riddle: I am the thick, absorbent material used as a bridge to move the water from the cups to the middle of the air. What am I?
The Answer: Yarn (or string).
6. The Riddle: I am the small metal office supply you tie to the ends of the string to anchor it deep inside the water. What am I?
The Answer: A paperclip.
7. The Riddle: I am the slow, natural process where liquid water turns into invisible gas, leaving the solid rock crust behind. What am I?
The Answer: Evaporation.
8. The Riddle: I am the flat kitchen item you must put under the dripping string to catch the water so you don’t ruin the table. What am I?
The Answer: A plate.
9. The Riddle: I am the specific type of scientists who actually study how real rocks and cave formations grow over thousands of years. What am I?
The Answer: Geologists.
10. The Riddle: If stalactites hang down from the ceiling, what do you call the rock spikes that grow up from the cave floor?
The Answer: Stalagmites.
The Wrap Up
Baking soda stalactites are the perfect experiment for anyone who wants to see how the earth actually builds itself.
By tricking hot water into carrying too much cargo, and then forcing that water to evaporate in mid-air, you are essentially speeding up 10,000 years of geology into a single weekend. If you want to dive deeper into how water carves out entire mountains and underground caverns, bookmark the American Physical Society (APS). Patience is the key to awesome rocks!
Cited Sources & Evidence
- American Chemical Society (ACS)
- National Geographic Education
- National Science Foundation (NSF)
- American Physical Society (APS)