Acids and Carbonates

Carbonates are compounds that contain the carbonate ion, CO₃²⁻. They include familiar substances such as calcium carbonate, which is found in limestone, chalk, and marble, and sodium hydrogen carbonate, commonly known as baking soda.

Carbonates react readily with acids. One of the easiest ways to recognize the reaction is the fizzing or bubbling that occurs.

The general reaction is:

acid + carbonate → salt + water + carbon dioxide

The bubbles are carbon dioxide gas, CO₂, escaping from the reaction mixture.

What Is a Carbonate?

A carbonate contains the carbonate ion:

CO₃²⁻

This ion contains:

• one carbon atom
• three oxygen atoms
• an overall charge of 2−

Common metal carbonates include:

• calcium carbonate — CaCO₃
• magnesium carbonate — MgCO₃
• sodium carbonate — Na₂CO₃
• potassium carbonate — K₂CO₃

There are also hydrogen carbonates, such as sodium hydrogen carbonate:

NaHCO₃

Both carbonates and hydrogen carbonates react with acids and can produce carbon dioxide gas.

Calcium carbonate is especially common in nature. It is a major component of:

• limestone
• chalk
• marble
• seashells
• many types of coral

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The Acid-Carbonate Reaction

The reaction pattern is extremely useful to remember:

acid + carbonate → salt + water + carbon dioxide

There are three products:

salt + water + carbon dioxide

This makes the reaction different from the acid-metal oxide reaction:

acid + metal oxide → salt + water

With a carbonate, there is an additional product:

carbon dioxide gas

This gas causes the characteristic fizzing.

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Example: Calcium Carbonate and Hydrochloric Acid

Calcium carbonate reacts with hydrochloric acid.

Word equation

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

Symbol equation

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

During the reaction, carbon dioxide escapes from the mixture as bubbles.

This bubbling is called effervescence.

If pieces of marble or limestone are placed in dilute hydrochloric acid, bubbles quickly appear on their surfaces.

The solid calcium carbonate gradually becomes smaller as it is consumed by the reaction.

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Recognizing Carbon Dioxide

Seeing bubbles tells us that a gas is being produced, but bubbles alone do not prove that the gas is carbon dioxide.

Chemists use a simple chemical test to identify CO₂.

The gas is passed through limewater.

If carbon dioxide is present:

limewater turns cloudy or milky.

This happens because carbon dioxide reacts with calcium hydroxide in the limewater and forms insoluble calcium carbonate.

Word equation

carbon dioxide + calcium hydroxide → calcium carbonate + water

Symbol equation

CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

The tiny particles of solid calcium carbonate make the liquid appear cloudy.

Testing the Gas in the Laboratory

A typical experiment can be arranged so that the acid and carbonate react in one container.

A delivery tube carries the gas into another test tube containing limewater.

The sequence of observations is:

Acid added to carbonate

↓

Fizzing occurs

↓

A gas is produced

↓

Gas is passed through limewater

↓

Limewater becomes cloudy

↓

The gas is identified as carbon dioxide

This provides much stronger evidence than simply observing bubbles.

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Why Does Carbon Dioxide Form?

The carbonate ion reacts with hydrogen ions from the acid.

At the particle level, the overall reaction can be represented as:

CO₃²⁻ + 2H⁺ → CO₂ + H₂O

The carbonate ion is therefore broken down as it reacts with the acid.

The products are:

• carbon dioxide
• water

Meanwhile, the metal ion from the carbonate combines with the negative ion from the acid to form the salt.

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Predicting the Salt

Just as with other acid reactions, the acid determines the type of salt produced.

The carbonate provides the metal ion.

Example

Consider:

magnesium carbonate + hydrochloric acid

Magnesium carbonate provides:

magnesium

Hydrochloric acid produces:

chloride

Therefore, the salt is:

magnesium chloride

The complete word equation is:

magnesium carbonate + hydrochloric acid → magnesium chloride + water + carbon dioxide

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More Examples

Magnesium Carbonate and Sulfuric Acid

Sulfuric acid produces sulfate salts.

Therefore:

magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide

Symbol equation:

MgCO₃ + H₂SO₄ → MgSO₄ + H₂O + CO₂

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Calcium Carbonate and Nitric Acid

Nitric acid produces nitrate salts.

Therefore:

calcium carbonate + nitric acid → calcium nitrate + water + carbon dioxide

Symbol equation:

CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂

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Sodium Carbonate and Hydrochloric Acid

Hydrochloric acid produces chloride salts.

Therefore:

sodium carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide

Symbol equation:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

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Worked Example: Predicting the Products

Predict the products when zinc carbonate reacts with hydrochloric acid.

Step 1: Identify the metal

Zinc carbonate contains zinc.

Step 2: Identify the acid

The acid is hydrochloric acid.

Step 3: Determine the salt family

Hydrochloric acid produces chlorides.

Step 4: Name the salt

Zinc + chloride gives:

zinc chloride

Step 5: Remember the other two products

Every normal acid-carbonate reaction also produces:

water + carbon dioxide

Therefore:

zinc carbonate + hydrochloric acid → zinc chloride + water + carbon dioxide

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Acid-Carbonate Reactions Are Neutralization Reactions

Carbonates can act as bases because they react with acids and reduce their acidity.

The carbonate ion reacts with H⁺ ions from the acid.

Therefore, carbonates can be used to neutralize acids.

However, acid-carbonate neutralization looks somewhat different from a reaction involving a metal oxide or hydroxide because carbon dioxide is also released.

Compare:

acid + metal oxide → salt + water

and:

acid + carbonate → salt + water + carbon dioxide

The production of CO₂ provides a useful way to distinguish the two reactions.

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Using Carbonates to Neutralize Acids

Because carbonates react with acids, they have several useful applications.

Antacids

The stomach naturally contains hydrochloric acid, which helps digestion.

If excess acid causes indigestion or heartburn, some antacid products contain carbonate or hydrogen carbonate compounds that can neutralize some of the acid.

For example, calcium carbonate can react with hydrochloric acid:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Calcium carbonate is used as an active ingredient in some antacid products.

Because carbon dioxide is produced, carbonate-containing antacids can sometimes cause belching as the gas escapes.

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Carbonates and Acidic Soil

Plants generally grow best within particular soil pH ranges.

If soil becomes too acidic, farmers and gardeners may add materials containing calcium compounds to reduce the acidity.

Ground limestone contains large amounts of calcium carbonate.

The carbonate reacts with acids in the soil, reducing the concentration of H⁺ ions.

This process is commonly called liming.

The same chemical principle is being used:

carbonate + acid → neutralization

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Limestone and Acid Rain

Acid-carbonate reactions can also cause problems.

Limestone and marble contain large amounts of calcium carbonate.

Acids can react with calcium carbonate, gradually wearing away the material. This means acidic rainwater can contribute to the weathering of limestone and marble structures.

The reaction produces:

• a calcium salt
• water
• carbon dioxide

Over long periods, repeated reactions can damage stone surfaces.

This is particularly important for:

• limestone buildings
• marble statues
• monuments
• gravestones

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Carbonates in Everyday Life

Carbonate chemistry occurs outside the laboratory as well.

One familiar example involves sodium hydrogen carbonate, NaHCO₃, commonly called baking soda.

When a suitable acid reacts with sodium hydrogen carbonate, carbon dioxide is produced.

For example:

acid + sodium hydrogen carbonate → salt + water + carbon dioxide

The CO₂ bubbles can become trapped in dough or batter, helping it expand. This principle is used in baking.

So the same basic chemistry responsible for fizzing in a laboratory reaction can also help make cakes and breads rise.

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Comparing Common Acid Reactions

It is useful to learn the major reaction patterns together.

Look for the gas

If a carbonate is involved, remember:

CO₂ is produced.

If a suitable metal reacts with a common dilute non-oxidizing acid, the gas is usually:

H₂

So the gases are different.

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Worked Example: Identifying a Carbonate

A student is given an unknown white solid.

The student adds dilute hydrochloric acid.

The solid begins to fizz.

The gas produced is passed through limewater, and the limewater becomes cloudy.

What can the student conclude?

Step 1

Fizzing shows that a gas has been produced.

Step 2

The gas makes limewater cloudy.

Step 3

The limewater test identifies the gas as carbon dioxide.

Step 4

Producing carbon dioxide after adding dilute acid provides evidence that the original substance contained carbonate or hydrogen carbonate ions.

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Reaction Rate and Carbonates

The rate of an acid-carbonate reaction can vary.

For example, compare:

• one large piece of calcium carbonate
• powdered calcium carbonate

The powder usually reacts more quickly because it has a larger surface area exposed to the acid.

More carbonate particles are available for collisions with acid particles at the same time.

Similarly, increasing acid concentration or temperature can generally increase the rate of reaction.

This makes acid-carbonate reactions useful for investigating rates of reaction because CO₂ production can be measured over time.

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Measuring Carbon Dioxide Production

Instead of simply observing bubbles, scientists can measure the amount of carbon dioxide produced.

For example, a reaction flask can be connected to a gas syringe.

As CO₂ forms, it enters the syringe and pushes the plunger outward.

The gas volume can then be recorded.

Measurements might be taken every 10 seconds.

The results could then be plotted as:

volume of CO₂ produced vs time

This allows scientists to compare reaction rates quantitatively rather than relying only on visual observations.

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Common Mistakes

Forgetting carbon dioxide

Acid-carbonate reactions produce three products:

salt + water + carbon dioxide

Do not write only salt and water.

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Writing hydrogen as the gas

Hydrogen is associated with reactions between acids and suitable metals.

Carbonates produce:

carbon dioxide, CO₂

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Writing carbon monoxide

Carbon monoxide is CO.

The gas produced in the usual acid-carbonate reaction is:

CO₂

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Assuming bubbles prove the gas is CO₂

Fizzing shows that a gas is being produced, but it does not identify the gas.

To identify carbon dioxide:

bubble the gas through limewater

A positive result is:

limewater turns cloudy/milky

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Forgetting that the acid determines the salt

Remember:

hydrochloric acid → chloride

sulfuric acid → sulfate

nitric acid → nitrate

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Confusing carbonate with carbon dioxide

They are different substances.

Carbonate ion = CO₃²⁻

Carbon dioxide = CO₂

The carbonate ion is part of the reactant.

Carbon dioxide is one of the products.

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Key Terms

Acid — A substance that produces H⁺ ions in aqueous solution.

Carbonate — A compound containing the carbonate ion.

Carbonate ion — CO₃²⁻; an ion containing one carbon atom and three oxygen atoms with an overall 2− charge.

Hydrogen carbonate — An ion, HCO₃⁻, that also reacts with acids to produce carbon dioxide and water.

Calcium carbonate — CaCO₃; a common carbonate found in limestone, chalk and marble.

Neutralization — A reaction in which an acid reacts with a base and its acidic properties are reduced.

Salt — An ionic compound formed in many acid reactions.

Carbon dioxide — CO₂; the colourless gas produced during acid-carbonate reactions.

Effervescence — Fizzing or bubbling caused by gas escaping from a liquid.

Limewater — A calcium hydroxide solution used to test for carbon dioxide.

Precipitate — An insoluble solid that forms within a solution.

Chloride — A salt associated with hydrochloric acid.

Sulfate — A salt associated with sulfuric acid.

Nitrate — A salt associated with nitric acid.

Antacid — A substance used to neutralize excess stomach acid.

Liming — The addition of suitable calcium-containing materials to acidic soil to reduce its acidity.

Surface area — The total area of a substance exposed to its surroundings; increasing it can increase reaction rate.

Gas syringe — Laboratory equipment used to collect and measure the volume of gas produced during a reaction.

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Key Takeaways

• Carbonates contain the carbonate ion, CO₃²⁻.
• Carbonates react with acids.
• The general reaction is:

acid + carbonate → salt + water + carbon dioxide

• Acid-carbonate reactions produce three products.
• Carbon dioxide escaping from the mixture causes fizzing or effervescence.
• Carbon dioxide can be identified by passing it through limewater.
• A positive CO₂ test causes limewater to become cloudy or milky.
• Hydrochloric acid forms chloride salts.
• Sulfuric acid forms sulfate salts.
• Nitric acid forms nitrate salts.
• The carbonate provides the metal ion for a metal salt.
• At the particle level:

CO₃²⁻ + 2H⁺ → CO₂ + H₂O

• Carbonates can neutralize acids because they react with H⁺ ions.
• Calcium carbonate can be used in some antacids.
• Carbonate-containing materials can help neutralize acidic soils.
• Acids can also react with calcium carbonate in limestone and marble, contributing to weathering.
• Hydrogen carbonates such as baking soda also produce CO₂ when they react with acids.
• Acid-carbonate reactions can be used to investigate reaction rates by measuring CO₂ production.
• Remember the major distinction:

acid + metal oxide → salt + water

but

acid + carbonate → salt + water + carbon dioxide

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Check Your Understanding

1. What ion is found in all carbonates?

2. Write the general word equation for an acid-carbonate reaction.

3. Name the three products formed when an acid reacts with a carbonate.

4. What causes the fizzing observed during an acid-carbonate reaction?

5. How can carbon dioxide gas be identified experimentally?

6. What observation indicates a positive limewater test?

7. Complete:

calcium carbonate + hydrochloric acid → ______ + ______ + ______

8. What salt forms when magnesium carbonate reacts with sulfuric acid?

9. What salt forms when calcium carbonate reacts with nitric acid?

10. Explain why carbonates can be used to neutralize acids.

11. Explain how calcium carbonate can reduce excess stomach acidity.

12. Why can acidic rainwater damage limestone and marble?

13. A student adds hydrochloric acid to an unknown solid. The mixture fizzes. Explain why this observation alone is not enough to prove that the solid is a carbonate.

14. Describe an experiment that could provide stronger evidence that the unknown solid contains carbonate ions.

15. Explain why powdered calcium carbonate usually reacts with acid faster than a large piece of calcium carbonate of the same mass.

16. Challenge: At the ion level, explain why adding an acid to a carbonate produces carbon dioxide and water.