Blog Archives

Iodine Clock Reaction

2/16/2018

In chemistry, kinetics is the term we use when measuring and studying the rates of reactions. Chemical (or reaction) kinetics involves investigations of how different experimental conditions impact the rate of a chemical reaction. We also use kinetics to make reaction equations that can describe the characteristics of a chemical reaction, such as the physical states of the product(s)!

One classic experiment that showcases two chemical reactions taking place is the iodine clock reaction, discovered by Hans Heinrich Landolt in 1886. He took two colorless solutions and mixed them together. After a little time had passed, the solution suddenly turned dark blue! When he repeated the experiment, he found that the solution turned blue after the same amount of time had passed. Clock reactions (or oscillating reactions) are labeled as such because there is a sudden property change after a predictable amount of time.

In this case, the change in property happens to be a colorless solution abruptly becoming a much darker color.

Recreate the Experiment at Home!

Before we get into the reaction equations, it might be easier to see what’s going on instead of trying to imagine it. Luckily, we can recreate this famous experiment with a few items you might have in your medicine cabinet! All of the chemicals you’ll be using are completely safe, but iodine is very good at staining your skin and clothes, so you may want to wear some latex gloves just to be safe. All of the materials can be found in a drugstore or supermarket!

What You’ll Need:

Distilled (or tap) water
A few disposable cups
One 1000 mg vitamin C tablet
Tincture of Iodine (2%)
Hydrogen peroxide (3%)
Liquid laundry starch

First, prepare three solutions: a vitamin C stock, an iodine solution, and a hydrogen peroxide solution. For the vitamin C stock, crush the tablet and dissolve it in 2 oz (59 mL) of water.

The next solution will be one of the reactants. Mix 1 tsp (5 mL) of the vitamin C stock with 1 tsp of iodine and 2 oz of water. Label this as “Solution A”.

To prepare the second reactant, add 2 oz of water to 1 tbsp (15 mL) of hydrogen peroxide and 1/2 tsp (2.5 mL) of liquid starch solution. This will be “Solution B”.

Now you’re ready to begin the reaction! Carefully pour Solution A into Solution B. Then pour the entire solution into the now empty cup. Continue transferring the solution back and forth until you see the color change!

What is Happening?

Because the laundry starch contains sulfuric acid, we created the first reacting solution: hydrogen peroxide with sulfuric acid. Then we added it to a solution containing potassium iodide, sodium thiosulfate, and starch. When these are combined, it creates both the elemental and ion forms of iodine.

First Reaction:

In this reaction, iodide ions react with hydrogen peroxide to produce elemental iodine, which is blue in the presence of starch. However, before that can happen, the vitamin C quickly reacts and consumes the elemental iodine (that's what is happening in the second reaction below). That’s why we don’t immediately see the change of color!

Second Reaction:

The second reaction is much quicker than the first, so the reactions can proceed for a short amount of time before the thiosulfate (S4O62-) ion is exhausted in the experiment and the iodine-starch complex’s blue color is visible.
You can repeat this experiment as many times as you’d like! You could even try timing the reaction to see if the blue color appears at the exact same time in each trial.

References:

“Iodine Clock Reaction”. Imagination Station, www.imaginationstationtoledo.org.

Image Credits:

“Iodine Clock”. Released into the public domain under the Creative Commons Attribution-Share Alike 3.0 Unported license. Uploaded on 2/12/18 from commons.wikimedia.org

Hodan, George. “Laboratory Glassware”. Released into the public domain. Uploaded on 2/13/18 from publicdomainpictures.net

0 Comments

How Snow Forms

2/6/2018

0 Comments

It’s winter up here in the northern hemisphere, and with the chilly weather often comes snow! Snow is precipitation in the form of ice crystals, or frozen water. We know that water freezes at 32℉ (0℃), but contrary to popular belief, the temperature doesn’t need to be below freezing in order for snow to form. In fact, the heaviest snowfalls often occur when the air is between 35 and 32℉! If it’s warmer than about 35.6℉, the snow will melt and fall as sleet instead of ice crystals.

In last year’s blog about crystallization, we learned that snow can also be classified as a mineral because ice is an inorganic homogeneous solid. If something is inorganic, that means its chemical composition doesn’t include the element carbon. We also learned that in order for snowflakes to form, moisture in a cloud needs to freeze around a tiny particle of dust. From there, more water molecules must freeze and stick to that ice crystal to make the six arms of a snowflake.

Wet and Dry Snow

The best kind of snow to make snowballs or snow men out of is what we call “wet” snow, because it sticks together and holds its shape for a long time. But what is the difference between “wet” snow and “dry” snow?

The size and shape of a snowflake depends on how many ice crystals stick together, which is determined by the temperature of the air. Snowflakes that fall through dry, cool air will be small and powdery. They won’t stick together very much. This is what we call dry snow. It’s great for snow sports like skiing and snowboarding, but is more likely to drift in windy weather. This is dangerous while driving because it obscures the road lines from vision and often causes quite a few car accidents!

When the temperature is slightly warmer than 32℉, the snowflakes will melt around the edges and stick together to become big, heavy flakes. This creates wet snow that sticks together easily and is good for making snow sculptures, but is much more difficult to shovel due to its weight!

Make a Snow Storm in a Jar

With a few household materials, we can create suspended mixture that looks like a snow storm has been captured in a jar! As an added bonus, most of these items you can find at a dollar store so you don’t have to break the bank to have a little fun.

Here’s what you’ll need:

A mason jar or similar container
Canola oil
1 tsp white paint
1 cup water
Iridescent glitter
Alka Seltzer
Blue food coloring (optional)

The first step is to fill the jar about three quarters of the way with canola oil and set it aside. In a small or medium sized bowl, combine the white paint and water. Stir the mixture until the paint is dissolved, leaving you with white water. Sprinkle in as much glitter as you’d like and add some blue food coloring to provide a color contrast. Pour the paint mixture into the jar of oil, leaving a little bit of room at the top.

Wait for the glitter and paint mix to settle at the bottom of the jar.

To make the “snow storm” break up a tablet of alka-seltzer and drop the pieces into the jar. Watch carefully to see what happens! Once your “storm” has settled, you can add more pieces of alka-seltzer to start the process over again and again.

Image Credits:

Bentley, Wilson. “Snowflakes”. Released into the public domain. Uploaded on 1/28/18 from commons.wikimedia.org

Griffin, Peter. “Building Snowman”. Released into the public domain. Uploaded on 1/29/18 from publicdomainpictures.net

Snow in a Jar images property of Discovery Express Kids LLC.

0 Comments

Iodine Clock Reaction

How Snow Forms

Archives

Categories