Video: Applying Knowledge of Good Laboratory Practice for the Mixing of Concentrated Sulfuric Acid and Water

For statements I and II, state for each if they are true or false. I) In the laboratory, water should never be added to concentrated sulfuric acid. II) When water is added to concentrated sulfuric acid, the heat produced may cause the mixture to boil vigorously. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) In the laboratory, water should never be added to concentrated sulfuric acid. II) When water is added to concentrated sulfuric acid, the heat produced may cause the mixture to boil vigorously. If both are true, state if II is a correct explanation for I.

This question is about safe laboratory practice when dealing with concentrated acid and their dilution. Let’s imagine we want to make a dilute solution of sulfuric acid for a different experiment. We have to make our dilute solution using a mixture of water and concentrated sulfuric acid. Concentrated sulfuric acid is an extremely dangerous chemical. It reacts quickly with organic matter, often leaving behind a carbon skeleton, turning the rest into water and other biproducts. It’s commonly supplied in concentrations up to 98 mass percent.

The first statement says that water should never be added to concentrated sulfuric acid. What this means is that if we have concentrated sulfuric acid in a container, we shouldn’t try and dilute the acid by pouring water on top of it. Let’s see what happens if we do. Let’s say we start off by measuring out the amount of concentrated sulfuric acid we need. And we start pouring water over the top. And let’s for the moment imagine we only pour on enough to form a thin film over the top of the concentrated sulfuric acid. Sulfuric acid is much more dense than water. And it’s quite thick and viscous.

So the first thing we’ll notice is that the liquids don’t seem to mix very well. The water layer just sits around on top. The next thing we might notice is that where the water meets the sulfuric acid, there seems to be a lot going on, lots of individual density fluctuations, indicating that, little by little, the concentrated sulfuric acid is mixing with the water and dissolving. And what might happen next could be very dangerous. The dissolution of sulfuric acid into water releases lots of heat. The water starts to boil and starts to fling fragments of solution all over the place, perhaps into your eye, perhaps over the desk. So statement I is true. Water should never be added directly to concentrated sulfuric acid. Instead, we should do it the other way round.

Statement II tells us that when water is added to concentrated sulfuric acid, the heat produced may cause the mixture to boil vigorously. That’s also true. Since both statements are true, we address the last part of the question, which says whether II is a correct explanation for I. And indeed, it’s true. II explains I. The risk when we add water to concentrated sulfuric acid is that the water will heat up and boil.

So what should you do? If you start with water and add your concentrated acid slowly, it’s much safer. The dense concentrated sulfuric acid will fall through the water, resting at the bottom. As it falls through the water, it’s going to mix an awful lot more than if we do things the other way round. And not only are we going to have better mixing, there’ll be more water to absorb the heat being given out. We can also stir the mixture or use a thermometer to make sure it doesn’t get too hot.

As a rough estimation, one milliliter of concentrated sulfuric acid added to 100 milliliter of water will raise its temperature by three to four degrees Celsius. So you could imagine if you’re adding 20, 30, or even 50 milliliters, you’re going to have to wait for the mixture to cool down between each addition. However, even with this better procedure, concentrated sulfuric acid should always be handled using appropriate acid-resistant gloves, lab coat, goggles, and appropriate training.

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