Video: EC17-18-S2-Q33



Video Transcript

Magnesium sulfate and sodium bicarbonate can be mixed in solution without forming a precipitate. After heating, a precipitate forms. Why is this?

Let’s start by presenting this information in a visual form. So, what this question is telling us is that if we mix magnesium sulfate and sodium bicarbonate in solution, we don’t see any real change. But then, if we heat the solution, we get a white precipitate forming. And it’s asking us what’s happening to cause this precipitate.

So, let’s look at this in two stages. First, what’s happening in the flask on the left-hand side? And then, what happens once we heat it up? It might be easiest to consider this in terms of reaction equations. Magnesium sulfate is MgSO₄, while sodium bicarbonate is NaHCO₃. So, what happens when we mix these two compounds together in water?

While we may not be able to see any visible change, there is, in fact, a chemical reaction going on. Both the magnesium sulfate and sodium bicarbonate will dissociate and effectively swap their counter ions. So, what we’re left with is magnesium bicarbonate and sodium sulfate. Importantly, we can’t see anything occurring because all of these compounds are soluble in water. So, they all exist in their aqueous form. Clearly upon heating something changes. So, let’s think what that might be.

When we apply heat, one of the two products from reaction one must change in some way and become insoluble. What we now have to decide is which one is likely to change? It turns out that the compound which changes is the magnesium bicarbonate. The bicarbonate group can easily decompose producing both water and carbon dioxide. So, what we’re left with is magnesium carbonate. This is not very soluble in water at all. And that’s why we see it form as a precipitate.

Just to be thorough, let’s balance equation one. There we go. Now we have two complete equations for what’s going on in these flasks. Now we just need to explain this in words. Let’s do it in two steps.

So the question asks why is it that after heating we get a precipitate? The answer is because the product of the reaction of magnesium sulfate and sodium bicarbonate, magnesium bicarbonate, is soluble in water. To finish our answer, let’s explain what happens after heating. After heating, the magnesium bicarbonate decomposes to magnesium carbonate which is sparingly soluble in water and precipitates. So, here we have the answer to our question.

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