Revision notes for AQA GCSE Chemistry Carbonates. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Carbonates
What you'll learn
- What a carbonate is.
- How to carry out a chemical test to see if an unknown substance contains carbonate ions.
- The chemical equations that explain this test.
What is a carbonate?
In chemistry, negative ions are often found bonded to positive metal ions to form ionic compounds. One of the most common negative ions you will encounter is the carbonate ion.
Definition
Carbonate ion
A carbonate ion is a molecule made of one carbon atom and three oxygen atoms, carrying an overall charge of −2-2−2. Its chemical formula is written as CO32−\text{CO}_3^{2-}CO32−.
You probably already know a few common carbonate compounds. Chalk, limestone, and marble are all mostly made of calcium carbonate (CaCO3\text{CaCO}_3CaCO3). Baking soda is sodium hydrogencarbonate (NaHCO3\text{NaHCO}_3NaHCO3).
The chemical test for carbonates
If you are given an unknown powder or a clear solution in the lab, you can test to see if it contains carbonate ions. The test takes advantage of a simple chemical rule: when carbonates react with acids, they produce carbon dioxide gas.
Because this test relies on identifying the gas produced, it is actually a two-step process.
Step 1: Adding the acid
First, you add a few drops of dilute acid to the unknown solid or solution in a test tube. You can use any common laboratory acid, such as dilute hydrochloric acid (HCl\text{HCl}HCl) or dilute nitric acid (HNO3\text{HNO}_3HNO3).
If carbonate ions are present, the acid will react with them to produce carbon dioxide gas. In the test tube, you will immediately see effervescence (fizzing and bubbling).
Common Mistake
Stopping at the fizzing
A lot of students write "add acid, and if it fizzes, it is a carbonate". This will not get full marks! Other gases can also cause fizzing. You must always carry out the second step to prove that the gas is actually carbon dioxide.
Step 2: The limewater test
To prove the gas bubbling out of the mixture is carbon dioxide, you need to collect it and pass it through limewater.
Definition
Limewater
Limewater is an aqueous solution of calcium hydroxide, Ca(OH)2(aq)\text{Ca(OH)}_2\text{(aq)}Ca(OH)2(aq). It is naturally a clear, colourless liquid.
You usually set this up by placing a rubber bung with a glass delivery tube into the reacting test tube, and feeding the other end of the tube into a second test tube containing limewater.
When carbon dioxide gas bubbles through the clear limewater, it reacts to form tiny, suspended solid particles of calcium carbonate. This causes the limewater to turn milky (or cloudy).
Key Idea
The complete carbonate test
To test for a carbonate, add dilute acid. If a gas is given off, bubble it through limewater. If the limewater turns milky (cloudy), the original substance contained carbonate ions.
The chemistry behind the test
It is important to be able to write the equations for the reaction taking place in the first test tube. The general word equation for any acid reacting with a carbonate is:
Acid+Metal Carbonate→Salt+Water+Carbon Dioxide \text{Acid} + \text{Metal Carbonate} \to \text{Salt} + \text{Water} + \text{Carbon Dioxide} Acid+Metal Carbonate→Salt+Water+Carbon DioxideIf we use dilute hydrochloric acid and solid sodium carbonate, the balanced symbol equation (with state symbols) is:
2HCl(aq)+Na2CO3(s)→2NaCl(aq)+H2O(l)+CO2(g) 2\text{HCl(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \to 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} 2HCl(aq)+Na2CO3(s)→2NaCl(aq)+H2O(l)+CO2(g)Tip
Ionic equations
If you are taking the Higher Tier, you might be asked for the ionic equation. Because the specific acid and specific metal don't actually matter (they are just "spectator ions"), we can write a general ionic equation for any carbonate reacting with any acid:
CO32−(aq)+2H+(aq)→CO2(g)+H2O(l) \text{CO}_3^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \to \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} CO32−(aq)+2H+(aq)→CO2(g)+H2O(l)Applying the test
Let's look at how you might use these observations to identify an unknown compound.
Example
Deducing the identity of an unknown compound
A student is given an unknown white solid, Compound X. She places a small amount of Compound X into a test tube and adds dilute nitric acid. She observes vigorous effervescence. She bubbles the gas produced through limewater, and the limewater turns cloudy. A separate flame test on Compound X produces a yellow-orange flame. Identify Compound X.
- Analyse the first test: The student adds dilute nitric acid and observes effervescence. This suggests a gas is being produced, which is the first step of the carbonate test.
- Analyse the confirmatory test: The gas turns limewater cloudy. This confirms that the gas is carbon dioxide (CO2\text{CO}_2CO2).
- Identify the negative ion: Because adding acid produced carbon dioxide, Compound X must contain the carbonate ion (CO32−\text{CO}_3^{2-}CO32−).
- Identify the positive ion: The yellow-orange flame test indicates the presence of sodium ions (Na+\text{Na}^+Na+).
- Combine the ions: The compound contains sodium ions and carbonate ions. Therefore, Compound X is sodium carbonate.
Exam technique
In the exam
- State both steps clearly: If asked to describe the test for a carbonate, always state the reagent you add ("add dilute hydrochloric acid") AND the test for the gas produced ("bubble the gas through limewater").
- Use the correct descriptive words: In chemistry exams, it is safer to write "limewater turns cloudy" or "limewater turns milky" rather than saying it changes colour.
- Don't forget state symbols: If asked for a balanced equation for a carbonate reacting with an acid, remember that carbon dioxide is a gas
(g), water is a liquid(l), the acid is aqueous(aq), and the salt formed will generally be aqueous(aq).
Self review
Check yourself
- What reagent is added to an unknown substance to test for carbonate ions?
- What would you observe in the first test tube if a carbonate is present?
- What is the chemical formula for the carbonate ion?
- How do you confirm that the gas produced is carbon dioxide?
Identification of ions by chemical and spectroscopic means (chemistry only)
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