How do you test for cations and anions dissolved in water?

To test for aqueous cations, you use flame tests or the sodium hydroxide test. To test for aqueous anions, you use various reactions, such as the tests for carbonate, sulfate, and halide ions. This should help you identify an unknown ion dissolved in solution.

How do you test for anions?

There are three different common tests for anions, that test for carbonate, sulfate, and halide ions respectively. Adding a dilute acid to aqueous carbonate ions releases carbon dioxide. Adding hydrochloric acid and barium chloride solution to aqueous sulfate ions forms a white precipitate. Adding dilute nitric acid and silver nitrate solution to halide ions forms a precipitate. Different halide ions result in precipitates of different colours.

How do you test for cations and anions?

To test for cations, you use flame tests or the sodium hydroxide test. To test for anions, you use various reactions, such as the tests for carbonate, sulfate, and halide ions.

What is an anion gap test used for?

Anion gap tests measure the concentrations of electrolytes (charged mineral ions) in your blood and use this information to tell you if your blood pH balance is in a healthy range. This is important because blood that is too acidic or too alkaline could be an indication of other underlying health conditions.

How do you test for anions in an unknown solution?

To test for anions in an unknown solution, you use various reactions, such as the tests for carbonate, sulfate, and halide ions: Adding a dilute acid to aqueous carbonate ions releases carbon dioxide. Adding hydrochloric acid and barium chloride solution to aqueous sulfate ions forms a white precipitate. Adding dilute nitric acid and silver nitrate solution to halide ions forms a precipitate. Different halide ions result in precipitates of different colours.

Test for Anions: Table, Cations & Examples

Of course, once you know what you are looking for, it is easy to distinguish an alligator snapping turtle from the rocks around it, and you certainly wouldn't mistake its tongue for a worm! These physical traits simply make the turtle even more identifiable.

But some things aren't so easy to tell apart, and this is often the case in chemistry. For example, many negative anions, such chloride ions, bromide ions, and iodide ions, are both colourless and odourless when aqueous. How can we distinguish between them? To help identify negative ions in solution, scientists use tests for anions.

Ah, blackberry picking - that quintessential autumnal British tradition. Wild blackberries are tasty, abundant, easy to spot, and hard to confuse with anything else - they make the perfect forager's treat. But picking other wild foods can come with a little more of a risk. For example, you should never pick mushrooms you find when out and about in the countryside unless you are accompanied by an expert. This is because many toxic types of fungi look much like perfectly edible ones. Two seemingly identical mushrooms could actually be entirely different species - one safe to eat, the other extremely deadly. The consequences of a misidentification could be fatal. Do you really want to risk it?

We often find similar situations in chemistry. Many species look just like each other in certain situations, and it's important to be able to tell them apart. For example, multiple anions, such as sulfate, carbonate, and halide ions, are all colourless and odourless when aqueous - how do we distinguish between them? To help identify negative ions in solution, scientists use tests for anions.

This article is about testing for anions in chemistry.
We'll start with an overview of the tests for anions in aqueous solution, including their importance and uses.
We'll then look more closely at the tests for carbonate, sulfate, and halide anions.
After that, we'll pull together our learning with a handy testing for anions table.
Finally, we'll compare tests for anions and cations.

Test for anions in aqueous solutions

If you've read the article Chemical Analysis, you'll know that analytical techniques play a big role in many areas of our lives. For example, they're used in manufacturing industries, medicine, sewage works, and more. Chemical analysis is used to identify certain species in a compound or solution. This can help us work out the products of a reaction, detect pollutants, and improve the safety of products.

An example of chemical analysis is testing for negative ions, which are known as anions. All the tests involve adding a certain reagent to an aqueous solution and observing the results. Like all suitable analytical techniques, we try to choose tests that give distinct and unique results for just one ion. For example, the test might form a coloured precipitate or release a certain gas, which can then be identified.

Why an aqueous solution? Well, remember that we find many ions in ionic salts. As a solid, the ions are bound tightly together by strong ionic bonds, but when dissolved in water, the ions are free to move about. They can then react with whatever reagents we add to them. In addition, carrying the test out in solution allows us to see any solid precipitates that form or gases that are given off.

In this article, we'll look at how you test for the following anions:

Carbonate ions (CO₃^2-).
Sulfate ions (SO₄^2-).
Halide ions. In particular, we'll focus on identifying chloride, bromide, and iodide ions (Cl^-, Br^-, and I^-).

Test for anions: Pros and cons

The following tests for anions that we'll explore in this article are all cheap, simple to carry out, and use readily accessible reagents. However, they aren't perfect. Here are some of their downsides:

Tests for anions aren't as accurate, sensitive, or precise as other chemical analysis techniques, such as flame emission spectroscopy.
They can't be used on trace samples and aren't good at determining concentration.
They can also give ambiguous results.
For example, it can be hard to distinguish between bromide ions and iodide ions, as you'll find out later on. This means that you need to carry out further tests to tell the two apart.
Many tests for anions form a precipitate or release a gas. You could theoretically measure the mass of the precipitate formed or the volume of the gas released, and use this to work out the levels of anions in your solution. However, the procedure is tricky and there are many opportunities for error. As a result, tests for anions are better used qualitatively, not quantitatively.
- On the other hand, because the tests are only qualitative, it is OK if you don't measure your ionic solutions and reagents out with 100% accuracy!

Test for anions: carbonates

First up, let's explore the test for carbonate ions (CO₃^2-). Most carbonate salts are insoluble, and can't be identified in this way, but some carbonates do dissolve in aqueous solution and so can be tested for. Soluble carbonate salts include sodium, lithium, potassium, ammonium, and many uranium carbonates.

Here's the process:

Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
Use a pipette to add a few drops of a dilute acid, such as hydrochloric acid (HCl), to the test tube.
If the solution contains carbonate ions, the reaction will give off bubbles of carbon dioxide gas, which we can test for:
- Attach a bung and tubing to the top of the test tube to collect the gas given off.
- Bubble the gas through clear limewater.
- If the gas is carbon dioxide, the limewater will turn cloudy.

The release of carbon dioxide gas, which we confirm using the limewater test, tells us that our solution contains carbonate ions.

Test for Anions carbonate test StudySmarter Fig. 1: A diagram showing the test for carbonate ions. Carbon dioxide gas is released, which turns clear limewater cloudy.StudySmarter Originals

Test for anions: sulfates

Next up: testing for sulfate ions (SO₄^2-). Instead of releasing a gas, this test makes use of a precipitation reaction. We add specific reagents to an ionic solution, and if a white precipitate forms, we know that we have sulfate ions in our hands.

To carry out the test for sulfate ions:

Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
Use a pipette to add a few drops of hydrochloric acid (HCl) to the test tube.
Next, add in a few drops of barium chloride solution (BaCl₂).
If the ionic solution contains sulfate ions, a milky white precipitate will form.

Test for Anions sulfate ions test StudySmarter Fig. 2: A diagram showing the test for sulfate ions. A white precipitate indicates a positive result.StudySmarter Originals

Explaining the test for sulfate ions

Wondering what the white precipitate is? It is actually barium sulfate (BaSO₄), an insoluble ionic compound. BaSO₄ forms when barium ions from aqueous BaCl₂ react with the sulfate ions in your solution, and we use this precipitate to confirm the presence of sulfate ions. However, barium can also form white precipitates with other ions, such as carbonate ions. This is why we add HCl before pipetting in our BaCl₂ - it removes the carbonate ions from solution so that they can't give a false positive result, using the same reaction as the test for carbonates that we looked at just a moment ago.

You don't actually have to stick to barium chloride and hydrochloric acid, you know. In fact, you can use any soluble barium compound, as long as you use an acid with the same ion. For example, you could use barium nitrate and dilute nitric acid. Pretty cool, huh?

Test for anions: Halides

Last up, let's consider how you identify halide ions in solution. You can do this in multiple ways, but today we'll learn about their reaction with silver nitrate solution. This particular method is another example of a precipitation reaction and tests for chloride (Cl^-), bromide (Br^-), and iodide (l^-) ions. We can even use the test to distinguish between the three!

Here's how:

Add approximately 10 cm³ of an aqueous ionic solution to a test tube.
Use a pipette to add a few drops of dilute nitric acid (HNO₃) to the test tube.
Next, add in a few drops of silver nitrate solution (AgNO₃).
If the ionic solution contains halide ions, a precipitate will form. The precipitate generally clouds the whole test tube, and its colour depends on the halide ion itself:
- A white precipitate means that the solution contains chloride ions.
- A cream precipitate means that the solution contains bromide ions.
- A yellow precipitate means that the solution contains iodide ions.

Test for Anions halide ions test chloride iodide bromide StudySmarter Fig. 3: A diagram showing the test for halide ions. A precipitate indicates a positive result. We can use the colour of the precipitate to identify the halide ion.StudySmarter Originals

You might find it tricky to tell the creamy bromide precipitate apart from the slightly yellow iodide precipitate. Fortunately, there are other tests we can carry out to distinguish between the two. You'll learn about these if you study chemistry at A level.

Explaining the test for halide ions

In this test, our precipitate is an ionic silver halide. If our solution contains chloride ions, we produce AgCl; if our solution contains bromide ions or iodide ions, we produce AgBr or AgI respectively. These ionic compounds are insoluble and so precipitate out of solution.

Like with the sulfate test, the acid is used to remove any carbonates. These would also form a white precipitate. But note that we can't use hydrochloric acid here - it contains chloride ions, and so would give a false positive result!

Test for anions table

Can you remember all the tests for anions that we've learned today? If not, don't worry - here is a handy test for anions table that summarises the three tests. We've included the name of the ion, the reagents needed for the test, the positive result you should expect to see, and any further notes.

Ion		Reagent(s)	Positive result	Notes
Carbonate		Dilute acid (such as hydrochloric acid)	Bubbles of carbon dioxide given off	Carbon dioxide gas turns clear limewater cloudy
Sulfate		Hydrochloric acid + barium chloride solution	White precipitate formed
Halide	Chloride	Dilute nitric acid + silver nitrate solution	White precipitate formed
	Bromide		Cream precipitate formed	Carry out further tests to distinguish between the two
	Iodide		Yellow precipitate formed	Carry out further tests to distinguish between the two

Test for anions and cations

This article has focused on testing for anions, such as those found in an aqueous solution of a dissolved ionic compound. Anions are negative ions. But remember that ionic compounds contain positive ions, too. These are known as cations. How can we test for these?

There are two main methods used to test for cations:

We can identify some metal cations using a flame test. This involves dipping a nichrome wire in an ionic solution and holding it in a Bunsen burner flame. The flame changes colour depending on the metal ion within the solution. For example, potassium ions cause the flame to burn a brilliant purple, whilst calcium ions produce a brick-red flame.
Not all metal cations change the colour of a flame. However, we can detect other cations using the sodium hydroxide test. This test involves gradually adding sodium hydroxide (NaOH) to an ionic solution until it is in excess. The hydroxide ions react with the metal ions in solution and form a precipitate. We can use the colour and solubility of these precipitates to help determine the metal cation's identity.

We won't look at flame tests and the sodium hydroxide test here. However, you need to know about them for your GCSE exams. Check out blah blah and Sodium Hydroxide Test for more information.

Test for Anions - Key takeaways

Tests for anions are a form of chemical analysis used to identify negative ions in aqueous solution.
Scientists have come up with different ways to test for carbonate, sulfate, and halide anions. The tests all release a gas or form a precipitate, which allows us to identify the negative ion.
- Adding a dilute acid to aqueous carbonate ions releases carbon dioxide gas.
- Adding hydrochloric acid and barium chloride solution to aqueous sulfate ions forms a white precipitate.
- Adding dilute nitric acid and silver nitrate solution to aqueous halide ions forms a precipitate. Different halide ions result in precipitates of different colours.
Tests for anions are cheap and simple to carry out. However, they are less accurate and sensitive than other analytical techniques. In addition, they are qualitative, not quantitative.

Test for Anions

StudySmarter Editorial Team

Test for anions in aqueous solutions

Test for anions: Pros and cons

Test for anions: carbonates