The Famous Dry Ice Bomb

Our final topic for the year was gas laws and for the final experiment we made dry ice bombs. One of the students had brought a small Coke bottle to use for the experiment so we half filled it with water, added some dry ice, put the lid on tight, placed it on the verandah outside the lab and retreated to a safe distance.

It seemed like nothing was happening so we started another activity and after about 10 minutes we heard a big bang. When we went out to look, we could see a few little bits of plastic but most of the bottle had disappeared. Then we looked up.

The bootle had gone straight up and was embedded into the ceiling. We took this photo because we thought people might not believe us.

If you want to know what made the bottle explode, do the calculations. We added about 20g of dry ice and it had an airspace of about 112 mL. That day, the temperature was about 25 degrees C and atmospheric air pressure about 101 kPa.

Gases in Real Life

Were you disappointed at the end of our gas laws topic, to find that the laws do not work all the time after all? Don’t get too concerned though. The laws are still useful for a wide range of purposes as you would have found from the review questions. This blog will consider a few practical cases where pressure, temperature and volume are significant.

Aircraft

If you watch the news you would have noticed that QANTAS has been getting some bad press lately about lapses in maintenance standards. This is particularly significant to QANTAS. In the 1988 movie “Rain Man” QANTAS was mentioned as the "airline that never crashed" and QANTAS wants to keep a clean record.

On 13 August 2008, QANTAS admitted to a maintenance lapse with their Boeing 737s which meant that cracks in the forward pressure bulkhead could have caused an explosive decompression. Fortunately, this did not happen to any of their 737s but in 1985 this did happen to a Japan Airlines 747. JAL flight 123 subsequently lost control and crashed into a mountain. What does this have to do with gas laws?

When an aircraft is sitting on the ground, the atmospheric pressure is balanced by the air pressure in the aircraft. As the aircraft ascends to cruising altitude of around 10,000 m what changes? The outside pressure decreases significantly so the pressure inside the cabin is much more than the pressure outside. This net pressure would try to push the walls of the aircraft outwards. Airlines actually reduce the pressure in the aircraft as it ascends to reduce this net outwards pressure, but they need to maintain enough pressure so passengers can breathe easily.

Have you ever wondered why your ears pop when the plane goes up? Now you know why. It also happens as the plane descends. The air pressure outside increases and the cabin pressure is increased gradually to compensate and this increase in air pressure explains why your ears pop again and why babies often scream in pain as the plane descends. We can allow the pressure in the inner ear and the pressure in the outer ear to equalize by a swallowing action which opens the Eustachian tubes allowing outside air to move into the inner ear. Babies have thinner Eustachian tubes which can easily be blocked by congestion. Because they can not equalize the pressure they experience a sharp pain in the ears. Adults sometimes experience this too if they have a lot of congestion, such as from a cold.

Hot Air Balloons

Hot air balloons are popular for joy rides, but how do they work? They are not sealed and have an opening at the bottom. Wouldn’t the warm air leak out?

To inflate the balloon, hot air from a gas burner is blown into the balloon which then rises and lifting the balloon. Because the hot air is at the top and wants to rise, it doesn’t leak out. So how does the hot air balloon pilot control the altitude? The pilot just turns up the heat. When the air gets hotter, it expands and gives greater lift. Conversely, to go down, the pilot just reduces the heat gradually for a gentle descent.

It is basically about Charles’ Law. The pressure outside is atmospheric pressure and changes very little. The temperature is adjusted to change the volume of hot air.

Smells

If someone produces an unpleasant smell at the back of the room, how long does it take for those at the front to notice? Let’s assume there is no wind blowing at this stage. The gas molecules can be travelling hundreds of metres per second, so on the basis if this it should be only a fraction of a second before everyone in the room notices the odour.

The reason for the delay is that the gas molecules keep colliding with other molecules and changing direction. On hot days you get a double whammy, especially after lunch. Those who forgot their deodorant bring in their BO and because it is hot, the diffusion occurs quicker. Aren’t you glad we have air conditioners now?

Air Conditioners

Have you ever wondered how air conditioners work? The theory behind their workings relates more to the heat of vaporisation and heat of condensation than on gas laws. Remember that vaporisation is endothermic and condensation is exothermic. To make the air conditioner cool a room, you have to ensure that the endothermic reaction occurs inside the room and the exothermic reaction occurs outside. In winter we have to reverse this cycle.

Cooking in a Pressure Cooker

When you cook in a normal saucepan, what temperature does the food cook at? If you live close to sea level, it will be 100°C. What stops the temperature from rising above 100°C even if you turn the element on full? The latent heat of vaporisation is the limiting factor. Once all the water is boiled, the temperature can rise way above 100°C and your food is dry and most likely burned black!

What is different in a pressure cooker? It is a sealed unit and the water vapour can’t escape so the water can’t boil at 100°C. The temperature goes well above 100°C and the water remains liquid. The higher temperature cooks the food quicker.

Water only boils at 100°C if the pressure is 10.1 kPa.

A similar thing happens in a car’s engine. Water, with come chemical additives is used to cool the engine. The cooling system is pressurised so that the engine can operate at a temperature above 100°C and not lose water.

If you really want to examine this topic further, you could examine how internal combustion engines work, what causes cyclones, how air pumps and vacuum cleaners work, and whether the suction pads they used in Mission Impossible are realistic. However, you probably do not have time to look at these in detail now, but they are worth thinking about.

Carbon in the Atmosphere

Carbon Dioxide

When we study the carbon cycle, we typically focus in carbon dioxide when we look at the atmospheric portion of the cycle. This is simply because carbon dioxide is the major carbon containing component of the atmosphere even though it constitutes less than 0.04% of the atmosphere. How can such a small amount be important?

Carbon dioxide is a greenhouse gas which means that it has the ability to act like a blanket around the earth and trap heat. (For an extreme example of carbon dioxide causing a greenhouse effect read about the atmosphere of Venus.) However, carbon dioxide is not the only natural substance to do that. Water vapour is also responsible for contributing to the greenhouse effect, but of course we would not want to reduce the amount of water vapour in the atmosphere to significantly because we would risk another problem - less rainfall. This has happened too often already when areas have become arid because of too much land clearing.

What can you do as an individual to reduce carbon dioxide emissions?

Methane

Carbon is also present in methane, another of the gases implicated for increasing the greenhouse effect. While the quantity of methane in the atmosphere is much less than that of carbon dioxide, one tonne of methane has the effect of 20 to 25 tons of carbon dioxide. SO where does the methane come from?

Some methane is produced by decomposition, such as marsh gas. Some is produced by industry but a significant amount is produced by livestock. "According to a new report published by the United Nations Food and Agriculture Organization, the livestock sector generates more greenhouse gas emissions as measured in CO2 equivalent – 18 percent – than transport." (FAO Newsroom, 2006).

A vegetarian diet has been shown to have significant health benefits but now there is another good reason to eat less meat, or none at all. It helps the environment! (See National Vegetarian Week: Environmental Benefits.) The typical Aussie who loves his steak probably won't like this idea too much but it will also save money.

Carbon Monoxide

Fortunately, there is no one likely to oppose efforts to reduce carbon monoxide emissions. Even though it is important for a variety of industrial processes, there is a strong incentive to ensure that it is not released to the atmosphere because we don't like poisons. How can we reduce carbon monoxide production?

Carbon monoxide is produced by incomplete combustion so we need to ensure that cars are well tuned and that furnaces are correctly adjusted. This in ceases the heat efficiency of the combustion as well as reducing carbon monoxide production.

Solid Carbon

These days, we don't hear a lot about solid carbon in the atmosphere but there are both health and environmental concerns about the levels of soot in the atmosphere. See Scientists warn of soot effect on climate. One of the most infamous episodes involving smog occured in the northern winter of 1952 and was called the Great London Smog when about 4000 deaths were attributed to the effects of the soot, sulphur dioxide and other pollutants from burning coal.

Industries using combustion of coal and other fossil fuels have been forced to clean up their act in industrialised countries but unfortunately, there are many places where there is still much to be done in this regard. As final preparations are put in place for the Beijing Olympic Games, there is still concern about the smog. (See Beijing Olympics: 1.15m cars banned from roads in last-ditch smog effort.)

Questions

1. The information above suggests that carbon in the atmosphere is responsible for many problems. Is this a balanced view?
   
2. List some things that you can do to reduce the amount of greenhouse gases resulting from your daily activity. Explain why your ideas will be benficial. Try and list them in decreasing order of importance. ie Start with the most important ones.
3. A common response is, "What I do will not make a difference." How would you respond to someone who says this.
4. The carbon cycle suggests recycling of carbon in the environment. Explain which parts of the carbon cycle have true recycling and where the system has been upset.
5. Former prime minister John Howard said he wanted to avoid responding to the greenhouse issue in a way that would upset the economy. How would you respond to this?