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A device used by the police to calculate a suspect's blood-alcohol level. The suspect is advised to blow into this bag, which contains some sort of science. The science changes the colour of some chemical or other if the suspect is too drunk to drive.

Evasion tactic: circular breathing.

another useful tactic for the mildly drunk driver is to have a can of beer right in your car and take a sip right before they breathalyze you.

this might sound counterproductive at first, but the idea is that breathalyzers are only accurate if there is no residual alcohol in the mouth. this is why the police ask you if you have taken a drink within the last 30 minutes. so, if you take a sip, they have to wait at least another 30 minutes and possibly longer before they can get another reading.

in the meantime, you pray that your body enzymes break down the remaining alcohol in your blood stream fast enough to take you below the legal limit ...

As for the science inside the breathalyzer, it works like this:
Cr2O72- + 8H+ + 3C2H5OH --AgNO3--> 2Cr3+ + 3CH3COOH + 7H2O
Before the test is used, the acidic dichromate solution is yellow/orange, due to the dichromate ions. However, after ethanol is added to the mix, it reacts to produce green/blue chromium(III) ions. A spectrophotometer can be used to compare the reacted sample with an unreacted solution to determine how much alcohol is in the person's blood. The amount of alcohol in 2100 mL of breath is equal to the amount in 1 mL of blood.

Info from a Chemistry 30 class and doublechecked at http://classes.colgate.edu/rbowman/core145/Crime_and_Chemistry_Laboratory_F00.pdf

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