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Name: Jeffrey
Status: student
Grade: 6-8
Location: SC
Country: N/A
Date: October 2006

Question:
I wanted to know if I can safely work with dry ice to use it in a gardening experiment to see if it will help plants to grow. I'm thinking of a way to melt it and use the liquid to nourish the plants with like water. I am wanting to do this as a science project this year.



Replies:
It is possible to work safely with dry ice--gloves, ventilation, etc. with an adult present. However, you may not get what you are looking for that way. Under normal conditions, dry ice will turn into a gas---you won't get a liquid if you "melt" it. I wonder if you could use a carbonated beverage like tonic water or a soda water of some sort---you'd have to watch for other nutrients in there. Also, do some research about how a plant brings carbon dioxide into itself. You may find that you want to do a different sort of experiment---either changing your carbon dioxide delivery system or studying a different nutrient delivered through the root system. Good luck and have fun!

Patricia Rowe


When dry ice melts, it converts directly to gas, so it wouldn't be possible to add as a liquid to plants. Although melting dry ice will often develop a liquid crust, that is only water ice condensing and freezing from the air.

Plants are able to get their carbon dioxide directly from the air, so if you want to test the effects on plant growth, you can add CO2 at different levels to the air of plants growing in closed chambers, compared to some in a chamber of ordinary air. You want to keep all other factors as similar as possible (temperature, soil, water, light, etc).

Very high CO2 concentrations may in some cases have a negative effect on plants, so separate chambers with different CO2 levels would help identify beneficial concentrations.

Don Yee


Hi Jeff, unfortunately you won't be able to "melt" the dry ice to a liquid. Dry ice is actually solid carbon dioxide, and when it changes phases at usual temperatures and pressures it goes directly to the gas phase. Greenhouses control the environment they plants grow in, and they can control things like temperature and CO2 levels (plants convert CO2 and water using the energy of light to oxygen and glucose). If you're interested in how CO2 affects plant growth you can contact a local nursery for ideas. As a safety issue, dry ice isn't really that dangerous, you can hold it in your hand for short periods of time without getting burns, although I would recommend getting some supervision when you're working with it.

Ethan Greenblatt
Ph.D. Candidate
Stanford Department of Chemistry


Jeffrey, This is an interesting idea, but unfornately, not feasible. Dry ice (solid carbon dioxide) does not have a liquid phase at normal atmospheric pressure. It goes through a phase change from a solid directly to a gas (this phase change from solid to gas is called sublimation). There is no way you will have liquid carbon dioxide to "water" your plants.

For the sake of discussion, let's look at what might happen IF you could do this. Maybe, for example, you could grow the plants in a pressure chamber so that the dry ice actually did melt to form a liquid (not practical, but let's continue). You "water" the plants with liquid CO2; but plants DO need H2O, and now they are not getting any. Not good for the plants; they will die. On the other hand, plants DO need CO2, but they take this in as a gas through tiny holes in the leaves called stomata. Some interesting experiments have been done looking at plants grown in a chamber with a higher than normal concentration of CO2 gas. Maybe there's a science project in there somewhere. Good luck.

Paul Mahoney, PhD


Jeffery,

Dry ice is nothing more than carbon dioxide. It has the special property of sublimation, which means that it can transform directly from a solid to a gas (or gas to solid) without actually going through the liquid phase. Dry ice has a temperature of -78C and can only only become a liquid under very high pressures. DO NOT try to contain the dry ice in a sealed vessel because the vessel WILL most likely explode and hurt you. Carbon dioxide can also be dissolved in water to make soda water. When dissoltion of CO2 in water occurs, it becomes acidic because the carbon dioxide forms carbonic acid (H2CO3). It is not good to change the pH of your soil unless you take a measurement of the pH first and then determine what the optimal pH for your particular garden is. Nitrogen, in the form of ammonia, is used heavily in fertilizers, and is usually quite basic. A basic pH is usually perfered over an acid pH for most plants. Even if you try to use dry ice directly in your experiment and put it deep in the soil, the very low temperature would freeze the roots. When the roots freeze, the water in the cells becomes ice. You know that ice floats in water and this is becaue it is less dense. Water is the only substance that expends when it freezes (at normal temperatures and pressures). When the water inside the cell freezes, it's like sticking a soda in the freezer--it will explode. When this occurs with a cell it is called lysis and destroys the cell. If you kill the roots, the plant will obviously not grow.

If you want another idea for a science project, then why don't you test out growing plants in different types of soil. I would grow four plants per soil type so that you can start to eliminate bad seeds etc. You can grow in top soil, mulch, Mirical Grow, clay and whatever you can get your hands on at the local grocery garden shop or home improvement store. You can also compare fertilizers in a signle soil type to see which is best. You can compare the nitrogen content of each and take soil pH samples to see what the optimal soil pH is for a particular plant.

Matt Voss


Unfortunately, melting dry ice will not give you a liquid. Dry ice goes directly from a solid to a gas in a process called sublimation. You could float the dry ice in water and it will melt turning the water into a very weakly acid solution. That might spur plant growth depending upon the soil type and plants. For example, strawberries seem to give sweeter fruit in an acid soil.

R. Avakian


You won't be able to melt dry ice - that's why it's "dry". It passes directly from the solid to the gas phase. Dry ice would be a good way to get concentrated carbon dioxide gas, but there won't be a liquid to work with.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


Dry ice normally sublimes (converts directly from a solid to a gas at 1 atm. at about -78.5 C. It melts, in the usual sense of a solid converting to a liquid at -56C., but here the pressure of the gas is about 5 atm. So it is difficult to handle without special high pressure equipment. In addition, at such a high pressure the carbon dioxide will dissolve in water making he soil very acidic, so separating the effects of carbon dioxide pressure from a change in pH would be difficult. You need to re-think your experiment in such a way so that you won't have to handle "liquid" carbon dioxide. That is a task for more experienced hands.

Vince Calder


I'm afraid melting it won't work. Dry ice is frozen Carbon Dioxide, which is never a liquid at normal atmospheric pressures. As it warms u ptowards room temperature, it chages states directly from solid to gasseous, hence the name "Dry ice".

As for safety, there are some serious things to keep in mind. The first is frostbite. NEVER touch the dry ice directly. Never never never!

Now, in the interest of helping your plants grow, a piece of dry ice nearby would be an excellent source of the carbon dioxide plants need, as it evaporates. Just how much of an effect this has would be a good basis for an experiment in it's own right!

Ryan B.



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