Last updated 1-31-08
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Air We Breathe: Experiment #2:
Detecting water vapor and CO2 in Car Exhaust

Car Exhaust contributes a great deal to pollution. It also contributes to global warming by the emission of carbon dioxide and water vapor. The warming isn't from the hot exhaust (which does contribute some) but from the fact that carbon dioxide and water soak up heat from the sun and from heat radiating from the ground.

In this experiment we want to detect water vapor and carbon dioxide coming from car exhaust.

For now you will just confirm that car exhaust contains water vapor.  I'm postponing the experiment for detecting carbon dioxide. However, I did the experiment for you, which you will study here.

The source of the water and the carbon dioxide is from the burning of the gasoline in the engine cylinders. Gasoline is a mixture of hydrocarbons. Hydrocarbons are chains of carbons with hydrogen atoms attached. In the picture carbon atoms are gray and hydrogen atoms are white.

Gasoline and oxygen enter the cylinder and a spark ignites the mixture. The spark begins the break up of the gasoline molecules and the oxygen molecules. Oxygen starts combining with the carbon atoms to form carbon dioxide (CO2) and oxygen also combines with the hydrogen atoms to form water (H2O). Both of these reactions give off energy. Some of the energy starts the break up of other gasoline molecules, which further allows more oxygen to combine with the freed up carbon and hydrogen atoms. In a flash a trillion trillion gasoline molecules are disintegrated and now just water and carbon dioxide.

All of the energy released as the gasoline molecules break up and combine with oxygen creates high pressure of carbon dioxide and water vapor in the engine cylinder. After this pressure is used to push the cylinder down and make the car go, the hot gases are released to the exhaust system shown below...
In the above picture, the exhaust manifold is the first set of pipes that combine the exhausted gases from the cylinders into one pipe. The exhaust is then routed to the muffler to reduce the explosive sound that came from the gasoline exploding in the cylinders. A catalytic converter (not shown) comes before the muffler and converts unburned gasoline to carbon dioxide and water. The exhausts exits the tail pipes. It's at the tail pipe where we want to detect the water vapor.

The key to detecting water vapor from car exhaust is to start with a car with a cold engine.

No, it doesn't have to be this cold.

Evidence #1 (cloud): However, if the air is cold, the normally invisible water vapor turns to droplets of water and a cloud forms behind your tail pipe. If this is the case, just take a picture of your car (or someone's) whose exhaust is showing the presence of water by condensing into visible water droplets (a cloud).
By the way, if this was burning oil, it would be blue smoke. If it was excess unbent gasoline, then it would be black smoke.

Evidence #2a (liquid water): Again, detecting water is best done when a car engine has just started up. (Note: The water is always there, but when the exhaust is very hot or the weather is hot, then it's harder to get the water to condense). When I started up my Jaguar, in a matter of seconds water was dripping from the tail pipe. If the engine was hot, then the water would not have condensed on the tail pipe and could not have been seen. Warning: turn off engine before taking pictures. If you see water on the tail pipe, take a picture.

Evidence #2b (liquid water): On my friend's Honda, I noticed water spitting from the tail pipe. I first noticed it by feeling the exhaust from about 1 foot away. Drops of water was hitting my hand and felt cool as they evaporated. I also could see drops of water on the driveway. If you see drops of water on the ground coming from tail pipe, take a picture. Again, turn off engine before taking pictures. I don't want you getting run over by your own car.

Evidence #3 (Condensation on cool object): If you can't see liquid water on tail pipe or on the ground, you can try holding a cool object near the tail pipe. Make sure car is in park! Here I'm using the 250mL beaker to show that it fogs up when it came near the exhaust. Actually I got quite a bit of water. A shiny cooking pan might work better. Note: The condensation may only last for a few seconds. Just place the glass or metal in the exhaust for a few seconds and remove it after you see condensation. You might have time to take a picture before it evaporates. In hot weather, you may have to put some ice in the beaker. Of course, there will be some condensation even without the exhaust. However, you should see a lot more condensation when you bring it near the exhaust pipe.

Evidence #4 (Feeling of moisture): Even when there's no condensation, you can feel moisture in the air. Car in Park! In this picture there are water drops on my hand from the tail pipe, so it was easy. But even without liquid water, you can sense the moisture. If the other methods fail, this might work for you. If so, take a picture of your hand. It doesn't need to be near the tail pipe.

Like all labs, I want at least one photo with your face in the picture. For this lab, either sit in or stand by the vehicle that you detected water vapor.

Would you believe I was testing the original Batmobile?
(this was me before shaving my mustache)

DETECTION OF CARBON DIOXIDE: Below I show how I detected carbon dioxide by measuring the pH of a solution before and after it was exposed to the carbon dioxide in the exhaust. (Don't do this part of the lab). You will do the following part of the lab. Let's first discuss what happens when carbon dioxide makes contact with water.

Whether carbon dioxide comes out of the tail pipe of the car or out the nozzle of soda fountain machine, it reacts the same with water. When water and carbon dioxide gas combine, they form a compound called carbonic acid. This is where we get the name "carbonation" or "carbonated." Carbonic acid is what makes carbonated drinks acidic. It's an acid because the two hydrogen atoms (the small white spheres) come off. When a hydrogen atom comes off, it leaves its one and only electron behind, which makes the hydrogen positively charged.

In your mouth the free hydrogen atoms (H+) give your taste buds a feeling of tartness. The fizz you feel or see is the reaction reversing (going from carbonic acid to CO2 and water)

Since carbon dioxide makes an acid when it contacts water, we can use the acids to neutralize an alkaline solution. Alkaline is just the opposite of acid.

Lime water is the alkaline solution I will neutralize. No, it's not make from the lime fruit.

Lime water gets it's name from the mineral known as lime (chemically called calcium oxide / CaO) not the lime fruit. When lime (CaO) is added to water, it becomes calcium hydroxide [Ca(OH)2]. The picture shows what calcium hydroxide looks like. In water the OH- (hydroxide) breaks away . The black minus sign represents an extra electron that the hydroxide pulls off the calcium and keeps. Hydroxide neutralizes acids H+. Or you can say the reverse, "acids neutralize hydroxides." Either way, when neutralization occurs, the OH- and the H+ combine to make water.

This graphic shows what is happening when exhaust hits the lime water. Hot CO2 and water vapor come from the exhaust pipe (top). 1) A CO2 molecule slams into the water molecule and joined they make carbonic acid (H2CO3). 2) The hydrogens on the carbonic acid break away leaving their electrons behind and becoming H+ ions (circle with plus sign). The H+ ions are attracted to the apposite charged OH- ions (hydroxide ions). 3) Combined they make water. 4) The carbon with 3 oxygen atoms is called carbonate. It has a negative 2 charge because the hydrogens left their electrons behind. The negative carbonate (CO3)2- is attracted to the positively charged calcium ion (Ca2+). Together they form a powder called calcium carbonate also know as chalk, which can make the water look cloudy. Unfortunately, I never saw much cloudiness, so I started up my gas guzzler; the Jaguar XJS with a V12 engine.

In my experiment, I will start with a calcium hydroxide solution (lime water). By using the 0-14 pH test strip, I found the pH to be near 10.


This is how I set up the experiment (you don't do this part). I placed about 50 mL of lime water, Ca(OH)2, in a beaker. I placed an air stone in the beaker. The air stone was connected to the portable air pump. The air pump was placed in the path of the exhaust coming from the tail pipe of my friend's Honda. The air pump brings in air around it and pumps it to the air stone. I let it run for 10 minutes. I wasn't seeing much so I decided to use my Jaguar that had much more exhaust. (I don't drive the Jaguar much because it's such a gas hog.)
The tail pipe on the Jaguar shot straight back so I had to set the pump and beaker on a short stool. Here I knew the exhaust had water in it because it felt like a wet sauna sitting behind the Jaguar with the engine running. (I didn't sit there long). I let the engine and air pump run another 15 minutes. ( I had also moved the lime water to a smaller beaker to make the liquid deeper and more likely to absorb the carbon dioxide.)
Using the pH test strips for 6.5 to 10 range, I tested the lime water after car exhaust had bubbled through it and found that the pH dropped from 10 to about 7. That means it completely neutralized the hydroxides in the lime water. At pH 7, water is neither acid or alkaline because the concentration of both the H+ and the OH- are equal and both are quite low.
As a double check of the pH test paper strips, I also used red cabbage indicator solution (something you will make in experiment #10). The greenish color of the pure lime water with the indicator indicated a pH of 11. The pale purple color of the lime water after exposure to exhaust looks close to pH 7. This confirms that the pH dropped significantly and that carbon dioxide is a component of car engine exhaust.
pH Scale for Red Cabbage indicator.
DETECTION OF CARBON DIOXIDE USING A DYE (This part of the lab you do): A compound called phenolphthalein is used commonly in laboratories to indicate when a solution goes from acid to alkaline or the reverse. Above we showed carbon dioxide neutralizing lime water. When that happens a phenolphthalein solution will go from pink to clear.

Your kit has a bottled labeled, "Phenolphthalein." Phenolphthalein is pronounced "fee nol they lean." The middle "ph" is silent.


In this experiment we need to place 3 milliliters (3 mL) of distilled water into the 10 mL graduated cylinder on the left. Pouring from a big jug into the small 10 mL graduated cylinder is not a good idea. One way is to pour some distilled water into a beaker and then pour from the beaker to the smaller bottle labeled "Purified Water." The small bottle has a dripper nozzle that let's you control the amount going into the graduated cylinder.

Your bottle of "Purified Water" is probably already filled.  If not, then refill as mentioned here.

The Purified Water bottle holds about 30 ml of water, so place enough in the beaker to fill up the Purified Water bottle. After pouring remember to put the dripping nozzle back onto the Purified Water bottle.
Add 3 ml of purified (distilled) water to the graduated cylinder. Look closely at the graduated cylinder to see where the 3mL mark is.

Locate the lime water test tube in your kit. Lime water is water that has the maximum amount of calcium hydroxide dissolved in it. That's about 0.1% w/v (meaning 0.1 grams calcium hydroxide in 100 ml of water). It's a weak alkaline solution.

Get one of the plastic squeeze bulb pipettes from your kit and use it to transfer 1 mL of lime water to the graduated cylinder.

The graduated cylinder should read close to 4 mL because there was already 3 mL of water in it.

We will then have a solution of lime water that is about 25% the strength of normal lime water. We could have used the full strength lime water for this experiment, but it's better if it is weaker because the car exhaust will neutralize it faster.

Now you can pour the 4 mL of the diluted lime water out of the graduated cylinder and into the 50 mL beaker.

Add two or three drops of the phenolphthalein solution to the beaker. You will see a pinkish purple appear. That means the phenolphthalein is changing colors because its in an weak alkaline solution.

If the solution doesn't turn pinkish purple, then it's possible that the lime water got neutralized by carbon dioxide (CO2) in the air. That can happen if the seal to the test tube isn't tight enough. If you see no color, take just a few grains of baking soda from the test tube labeled "Sodium bicarbonate (baking soda)" and add that to the solution and stir. That should make the solution turn purple. If not, add a little more baking soda.

Take a picture of your beaker with the solution showing a pinkish purple color.

Take your beaker out to your vehicle. Start up the vehicle and hold the 50 mL beaker somewhat close to the exhaust pipe (don't burn any fingers).

If our theory is right, carbon dioxide is a by-product of gasoline and will produce carbonic acid in the water in the beaker. That will begin to neutralize the calcium hydroxide (lime water) or the baking soda if you had to add that.

About 30 seconds later I noticed the pink color was getting fainter. Apparently the hydrogen ions (H+) coming off of the carbonic acid being created was neutralizing the hydroxide ions in the lime water.

Another 30 seconds and the pink color disappeared. Apparently, enough carbon dioxide had entered the water and, in essence, carbonated the water with carbonic acid. The acid (hydrogen ions=H+) neutralized all of the hydroxide ions (OH-) and forms water (H2O). What remains is calcium ions and carbonate ions. These combine to make calcium carbonate, which is chalk. You might see a bit of cloudiness because of this.

If you had to use some baking soda, it's possible that it won't turn clear but turn from a dark pink or purple into light pink.      If you don't get a color change after 30 seconds, then stop anyway.  There's no need to breath too much exhaust.     

Take a picture of your beaker after the carbon dioxide from the exhaust caused the pink phenolphthalein to go clear (or pink if using baking soda).

Now we have confirmed carbon dioxide three ways. One using pH paper, one using red cabbage pH indicator, and the last one of using phenolphthalein.

Remember the beaker, graduated cylinder, and the plastic pipette need to be cleaned. Rinse with tap water first followed by some of the distilled water. A further rinse with some of the isopropyl alcohol will help with drying.

Also, be sure to place the cap back on the lime water test tube or carbon dioxide in the air will slowly neutralize it.

Congratulations on finishing the lab.

Phoenix College students should send pictures to

Since Feb 25, 2004
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