Exploring Reactions: Metal Carbonates, Bicarbonates, and Acids
Have you ever seen baking soda fizz when mixed with vinegar? That's a classic example of a reaction between a carbonate (or bicarbonate) and an acid! This activity, a fundamental concept from the "Acids, Bases, and Salts" chapter, delves into how metal carbonates and metal hydrogen carbonates react with acids, and what gas they produce.
Aim of the Activity
To observe and understand the chemical reactions that occur when metal carbonates and metal hydrogen carbonates interact with acids, and to identify the gas evolved.
Materials & Apparatus
- Sodium Carbonate (Na₂CO₃) - solid
- Sodium Hydrogen Carbonate (NaHCO₃) - solid (also known as baking soda)
- Dilute Hydrochloric Acid (HCl)
- Lime water (Calcium Hydroxide solution, Ca(OH)₂)
- Test tubes (at least two)
- Dropper
- Delivery tubes
- Rubber corks (to fit test tubes and delivery tubes)
- Test tube stand
Procedure
- Prepare your test tubes: Take two clean test tubes and label them clearly as 'A' and 'B'.
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Add solids:
- In test tube A, add approximately 0.5 g of Sodium Carbonate (Na₂CO₃).
- In test tube B, add approximately 0.5 g of Sodium Hydrogen Carbonate (NaHCO₃).
- Add acid: Using a dropper, carefully add about 2 mL of dilute Hydrochloric Acid (HCl) to both test tube A and test tube B.
- Observe initial changes: Immediately observe the changes occurring in both test tubes. Note any fizzing or gas evolution.
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Set up for gas test: Quickly fit the test tubes with corks and delivery tubes. Lead the delivery tube from each test tube into a separate test tube containing a small amount of lime water.
(Ensure the end of the delivery tube is submerged in the lime water.)
- Observe lime water: Observe the changes in the lime water in both setups as the gas bubbles through it.
Our Observations
Here's what we observed in each test tube:
| Test Tube | Substance Used | After Adding HCl | Effect on Lime Water |
|---|---|---|---|
| A | Na₂CO₃ (Sodium Carbonate) | Vigorous effervescence (rapid bubbling) | Lime water turns milky |
| B | NaHCO₃ (Sodium Hydrogen Carbonate) | Less vigorous effervescence (steady bubbling) | Lime water turns milky |
Explanation of the Reactions
1. In both reactions, the effervescence indicates the production of a gas. This gas is carbon dioxide (CO₂).
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
Sodium Carbonate + Hydrochloric Acid → Sodium Chloride + Water + Carbon Dioxide
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
Sodium Hydrogen Carbonate + Hydrochloric Acid → Sodium Chloride + Water + Carbon Dioxide
2. The milky appearance of lime water is a classic test for carbon dioxide gas. When CO₂ is passed through lime water (calcium hydroxide solution), it reacts to form an insoluble white precipitate of calcium carbonate (CaCO₃), which makes the solution milky.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ + H₂O(l)
Calcium Hydroxide (Lime Water) + Carbon Dioxide → Calcium Carbonate (insoluble) + Water
3. If carbon dioxide gas is passed in excess through the milky lime water, the insoluble calcium carbonate further reacts with CO₂ and water to form soluble calcium hydrogen carbonate (Ca(HCO₃)₂). This causes the milky appearance to disappear.
CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)
Calcium Carbonate + Carbon Dioxide + Water → Calcium Hydrogen Carbonate (soluble)
Conclusion
This activity clearly demonstrates that all acids react with metal carbonates and metal hydrogen carbonates to produce a corresponding salt, water, and carbon dioxide gas. The formation of carbon dioxide is confirmed by the lime water test, which turns milky due to the formation of insoluble calcium carbonate.
Frequently Asked Questions (FAQs)
1. Why does lime water turn milky in this activity?
Lime water turns milky because the carbon dioxide (CO₂) gas produced reacts with calcium hydroxide in lime water to form an insoluble precipitate of calcium carbonate (CaCO₃).
2. In which test tube did a more vigorous reaction take place?
The reaction in test tube A (with sodium carbonate, Na₂CO₃) was more vigorous compared to test tube B (with sodium hydrogen carbonate, NaHCO₃).
3. What is the chemical formula of lime water?
The chemical formula of lime water (calcium hydroxide solution) is Ca(OH)₂.
Important Precaution
Always handle acids with care. This experiment should be performed under the supervision and guidance of a qualified teacher or adult.