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Name: Dawn
Status:  student
Grade: 6-8
Country: USA
Date: February 2009


Question:
The experiment was to try three acids against three bases to see which one was the most effective. The acids were: coke, lemon juice and vinegar. The bases were Mentos candies, baking soda and Milk of Magnesia. Based on a Zoom (PBS kids show) we used the amount of 1/2 cup of the acid in a 1/2 liter water bottle then filled the bottle with water to the half way mark. Then we added 1 tsp. of the base. We then corked the bottle shook it and waited for any results. The strongest (most explosive) reaction was with lemon juice and baking soda. It shot the cork out higher than my two story house. The second was vinegar and baking soda...Most of others did little to nothing.

Since it is for my science fair project I would like to know why the weaker acid when mixed with the strongest base was the most explosive. We hypothesized that the strongest acid when mixed with the strongest base would be the most explosive. I know that the two materials neutralize each other and try to come back to 7 on the pH scale and that they eventually form a salt + water. During the neutralization time they form a gas...I suppose the answer to my question is probably in what kind of gas it forms during the neutralization process and the nature of that gas....I just thought you might know why.



Replies:
Largely the reactions you are describing are not strictly acid-base neutralisation reactions. When baking soda (Sodium Bicarbonate) is reacted with acid, it decomposes to produce carbon dioxide as a gas. Milk of magnesia (Magnesium Hydroxide) is a very effective base, in that it will neutralise a significant quantity of acid, but it produces no gas in the reaction. Mentos (whether mint or fruit flavoured) are mostly sugar and I do not imagine they would have a strong base reaction of any sort.

The explosiveness of the reaction is no indication of the effectiveness of the base at neutralising the acid. I would suggest you do some investigation about acid-base indicators - such as Litmus (or any of several flower juice extracts - e.g. hydrangeas) which will allow you to "see" the strength of the acid or base.

Nigel Skelton
Tennant Creek High School
AUSTRALIA


Dawn,

You are already on the right track with your reasoning. However, you need to refine your terms a bit so your own answers become clear to you.

First, you need to make sure to distinguish between "strong" acid/base versus "high concentration" acid/base. pH essentially measure the concentration of H(+) present in solution. So it is a measure of how high the acid concentration is.

A quick look on the Internet tells me that the pH of most commercial vinegar is around 2.5 - 2.8. Lemon juice on the other hand range from 1.8 - 2.3. This tells me that (low numbers mean higher H[+]) that the lemon juice is a much more highly concentrated acid solution than vinegar.

Since baking soda is the common factor in the two experiments (it is the constant), then we can discount its relative strength and concentrate on the two acids. I think at this point you can figure out the reason of why more gas was produced with lemon juice.

The next issue is why the other bases did not produce as much gas. Here you want to make sure again about your terms. Strongest, as you define it, is the one that shot the cork the highest. All this means therefore is that the reaction that produce the most gas the quickest is a "strong" reaction.

In this case, therefore, you need to look at the chemical reaction between an acid and Mentos (sugar), baking soda (NaHCO3), milk of magnesis (Mg[OH]2) - and decide, based on the chemical reaction possible whether sugar+acid or magnesium hydroxide + acid, will produce a gas.

You have done good work, Dawn. Just make sure you define your terms more precisely, and you will be on your way to answering questions for yourself.

Greg (Roberto Gregorius)


Hi Dawn,

The explosiveness of the mix is not just a question of the strength of the acids and bases mixing, it is also related to the speed of the reaction. For example ground up mentos powder would react quicker than a complete mentos tablet, because its entire content is quickly exposed to the acid, rather than just the outside of the tablet.

The cork popping power comes from the formation of gas pressure inside the bottle, which in the case of baking soda (sodium bicarbonate) is carbon dioxide.

The height the cork pops is dependent on things other than just the strength of acid and base, for example, if on one bottle you're a tiny bit slow on corking some of the gas will escape before you cork it and you will lose some of the power. Likewise if the cork fits tighter or looser after soaking up water, or the corks are slightly different sizes, this will affect results. Ideally you would repeat the experiment multiple times for each combination and take the average of results, which would even out some of the variation in corking speed and other things that would change the results.

Because your experiment depends on generation of CO2, some information to support your hypothesis would be to calculate the amount of CO2 you could theoretically generate from the amount of base (e.g. baking soda) added, and the amount of acid you added (which you could find by determining how much base it takes to get to neutral pH).

Don Yee



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