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In a science exhibition in 2011 , I and my friend made a volcano that used vinegar and baking soda for the 'effects'. There was another group that made volcano too and they had brought their own supplies. But before the exhibition started, the members of other group came to us and asked us if we could share our baking soda because their baking soda wasn't 'working'. When I asked them why it wasn't working they told me that there wasn't 'salt' in it. (We were only 8-9 years old). When I tried using it it was forming bubbles but very slowly. Eventually our supplies ran out before the exhibition was over. One of my friends suggested that we should use chalk powder instead of baking soda(I don't know how he figured it out at age 9). So we ground up chalks and tried using it. In this case , it was just like the baking soda that didn't work , it was producing bubbles ,but very slowly.

What could be the issue in the two cases? Was it a matter of purity? We had used synthetic vinegar used for food purposes(5-10%)(There was no issue with the vinegar because it worked well with our own baking soda). The chalks were ordinary chalks used in classrooms. Regarding the other group's Baking soda I do not have any other information cause it happened 10 years earlier.(And I'm no longer in contact with them)

Was reaction less fast in case 2(Chalk powder) because of the low solubility product of Calcium carbonate? But Solubility product is a thermodynamic factor and not a kinetic factor right ?

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  • $\begingroup$ It must just have been a question of dimension of the grains. It is well known that when powders reacts, the reaction rate is proportional to the contact surface, So the contact surface is biggest when the grains are thinnest. If the baking soda had been ground with care, it must react quicker with vinegar. $\endgroup$
    – Maurice
    Feb 28 at 13:37
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    $\begingroup$ It always works properly with real baking soda. No salt is needed. Calcium carbonate has poor solubility in water, e.g., white cliffs of Dover, etc., so you would need to grind it to fine powder to get high surface area contact with the vinegar. Even then, the bubbles impede contact between the vinegar and powdered chalk. Sodium bicarbonate has reasonable water solubility, and vinegar is just a dilute aqueous solution of acetic acid. So mixing them always works well. Not much more to say: use inexpensive fresh baking soda. $\endgroup$
    – Ed V
    Feb 28 at 13:41
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Baking soda is sodium bicarbonate NaHCO$_3$. It is stable in air indefinitely. Baking powder is a mix of sodium bicarbonate and a powdered acid, typically an aluminum or calcium hydrogen phosphate, which, in the absence of moisture, is stable for a long time (maybe a year? maybe more). After exposure to moisture, it will release CO$_2$ and become inactivated.

The third item, "chalk", has many definitions. Blackboard chalk is not just a stick of CaCO$_3$. A stick of CaCO$_3$ could be marble, or limestone - oh - no, you would like it to be nice and powdery but sticking together. Yep, it can be done: glue. Nah, too scratchy. Plaster of Paris! Mix with water, add some CaCO$_3$ chalk, maybe some white clay... You get the idea. Blackboard chalk is not just CaCO$_3$. Not even that, but CaCO$_3$ reacts very fast with hydrochloric acid (but that's too hazardous for grade school experiments), and its reaction with vinegar (5% CH$_3$CO$_2$H) is slower initially, and slows down markedly as the reaction progresses because the acetate ion produced inhibits the ionization of the remaining (weak acid) CH$_3$CO$_2$H.

The key lesson is that demonstrations are very temperamental, and depend on doing the same thing with the same materials many times, to see how the "same" experiment can vary so much. You may even begin to doubt that any experiment can ever be duplicated - but it can, if you keep everything constant.

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