Identifying Carbonates, Halides & Sulfates

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Watching what happens in chemical reactions can help identify unknown compounds. In this lesson, we will learn how to identify carbonates, halides, and sulfates.

Identifying Unknown Compounds

Let's say that you are making up a batch of cookies and it calls for baking soda. You notice that there are three containers with white powder, but they don't have a label on them. You can't tell which one is baking soda by looking at it, feeling it, or smelling it. So, how are we supposed to know which white powder is baking soda?

There are several different tests that can be used to identify compounds within an unknown mixture. Particularly carbonates can be identified using dilute acids and limewater. Halides can be identified using silver nitrate and dilute nitric acid. Sulfates can be identified using barium chloride and hydrochloric acid.

Identifying Carbonates

Let's get back to figuring out which white powder is the baking soda for our cookies. Baking soda is sodium bicarbonate, so we know that it is a carbonate or a compound with a carbon connected to two oxygen atoms. So let's use a test to determine which container holds the carbonate. We start out with a dilute acid. This can actually be found in most kitchens, in the form of vinegar (dilute acetic acid). We can also use dilute sulfuric acid, dilute hydrochloric acid, or any other dilute acid.

When we combine a sample of each of the white powders with the vinegar we notice that two of the samples bubble up, but one of them has no visible reaction. We know that the sample with no visible reaction is not baking soda (that it contains no carbonates) because no gas was formed. We now need to determine which of these samples created carbon dioxide as a by-product, as this will be the container with the carbonate.

Baking soda and vinegar form bubbles when they react together, further testing can show that these bubbles are carbon dioxide
Baking soda and vinegar

In order to identify the carbon dioxide, we need lime water. This isn't lime water like the green limes with water. Instead, this form of lime water is calcium hydroxide. Calcium hydroxide is used in canning pickles, thus it can be found in the canning section of many grocery stores. We simply need a container with the lime water, and we can bubble the gas formed through the lime water. If the lime water turns cloudy (with a white precipitate forming) then the gas bubbled through it was carbon dioxide.

When carbon dioxide reacts with lime water it forms a cloudy, white precipitate as shown here
Carbon dioxide in lime water

As you bubble both gases through the lime water you determine that the second container produced the carbon dioxide, thus it must contain the carbonate, and this is the baking soda! You have just identified carbonates using things found in your home!

Identifying Halides

While we can't identify halides, or compounds in group 17 of the periodic table, using compounds found in the grocery store, it still uses many of the same principles - we add chemicals to the unknown sample and watch what happens in the reaction. We can identify if a halide is chloride, bromide, or iodide based on the color of the precipitate formed at the end of the reaction.

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