Color Changing Liquids with Cabbage Juice and Dry Ice

How Does It Work

Chemically speaking, when you add dry ice to water, the result is a slightly acidic solution. While you can taste the tangy acidity of soda pop, you need something like universal indicator (or red cabbage juice) to actually detect the change in pH (by seeing the different colors). Remember that water usually has a neutral pH of 7.0. Acidic solutions have a pH range from 1.0 to 6.9 and basic solutions are 7.1 to 14.0. Adding dry ice to water results in a solution that has an acidic pH of about 4.5.

Universal indicator is actually a combination of many different acid/base indicators that produce different colors as the pH changes. Adding universal indicator to plain water gives you a good benchmark of the color of a “neutral” solution. Adding dry ice lowers the pH and results in a color change related to chemistry, not temperature.

Ammonia water is a basic solution (high pH). It works well for this demonstration but other bases like a weak solution of sodium hydroxide (washing soda) or sodium bicarbonate (baking soda) also work. Adding universal indicator (or red cabbage solution) to a solution of water and ammonia causes the indicator to turn a beautiful purple (a basic pH of about 11.6). Adding dry ice to this solution results in many color changes as the solution drops from basic down to neutral down to acidic.

Safety Information

NOTE: Whenever you use dry ice, always be aware of the rules for handling it safely.

• This is not a toy. It’s for demonstration purposes only.
• Use dry ice only with adult supervision.
• Dry ice must be handled using heavy gloves or tongs. It will cause severe burns if it comes in contact with bare or unprotected skin.
• Always wear safety goggles when handling dry ice. The debris and shards are extremely dangerous to your eyes. When tapping dry ice with a hammer, first cover it with a towel to keep the pieces in one place.
• Never put dry ice in your mouth.
• Never store dry ice in an airtight container. As the dry ice sublimates, gas pressure will build and the container will explode. Make sure your container is ventilated or has a loose-fitting lid.
• Do not store dry ice in your freezer. It will cause your freezer to become too cold and the freezer may shut off. On the other hand, if you lose power for an extended period, dry ice is a good way to keep things cold if you can get it.
• In the unlikely event of a dry ice burn, treat it the same as you would a heat burn. See a doctor if the skin blisters or comes off. Apply antibiotic ointment to prevent infection and bandage mild burns.

What is Dry Ice?

Dry ice is not frozen water – it’s frozen carbon dioxide (CO₂). Unlike most solids, dry ice does not melt into a liquid as the temperature rises, but instead, changes directly into a gas. This process is called sublimation. The temperature of dry ice is –109.3°F (-78.5°C). Dry ice is particularly useful for keeping things cold because of its temperature. Dry ice does not last very long, however, so it’s important to purchase the dry ice you need for these science activities as close as possible to the time you need it. The best place to store dry ice is in a Styrofoam ice chest with a loose fitting lid that allows the CO₂ to escape as the ice sublimates.

Some grocery stores and ice companies will sell dry ice to the public especially around Halloween. Dry ice is typically sold as flat, square slabs a few inches thick or as cylinders that are about three inches long and about a half-inch thick. Either size will work fine for these experiments.

Remember the science when purchasing dry ice. Dry ice in a grocery bag will vanish in about a day. The experts tell us that, depending on weather conditions, dry ice will sublimate at a rate of 5 to 10 pounds (2.3 to 4.5 kg) every 24 hours even in a typical Styrofoam chest. So, again, it’s best to purchase the dry ice as close to the time you need it as possible. Last minute shopping is necessary. If you are planning to perform a number of dry ice demonstrations or have a lot of people involved, purchase 5 to 10 pounds  (2.3 to 4.5 kg). A little dry ice does go a long way in these activities.

How is Dry Ice Made?

The first step in making dry ice is to compress carbon dioxide gas (CO₂) until it liquefies while at the same time removing excess heat. The CO₂ will liquefy at a pressure of approximately 870 pounds per square inch (4500 cmHg)  at room temperature. Once liquid CO₂ is formed, the CO₂ is sent through an expansion valve and enters a pressure chamber. This pressure change causes the liquid to flash into a solid and causes the temperature to drop quickly. About 46% of the gas will freeze into “dry ice snow.” The rest of the CO₂, about 54%, is released into the atmosphere or recovered to be used again. The dry ice snow is collected in a chamber where it is compressed into block, pellet, or rice-sized pieces using hydraulics. It’s complicated but really cool science – really cool.

Can you make your own dry ice? Sure, anything is possible, but it’s not practical (unless you have a huge tank of compressed CO₂ sitting around and lots of extra time and equipment on your hands). For around $2 US a pound, it’s hard to beat the convenience of just purchasing it at the store.