Do Acids Produce Ions in Aqueous Solutions?
Exploring the Role of Water in Acidic Properties
Acids are known for their characteristic properties: a sour taste, their ability to conduct electricity, and turning blue litmus paper red. These properties are all attributed to the release of hydrogen ions (H⁺) by acids. But does this happen under all conditions? This activity investigates whether acids produce these crucial ions only when dissolved in water.
Aim of the Activity
To determine if hydrochloric acid (HCl) exhibits its acidic properties, specifically the production of H⁺ ions, only in the presence of an aqueous solution (water).
Materials & Apparatus
- Sodium Chloride (NaCl) - solid
- Concentrated Sulphuric Acid (Conc. H₂SO₄)
- Clean and dry Test tube
- Delivery tube
- Dry blue litmus paper
- Wet (moist) blue litmus paper
- Test tube clamp / Tongs
- Calcium Chloride (CaCl₂) - anhydrous (optional, for drying tube)
Procedure
- Setup: Take about 1 gram of solid Sodium Chloride (NaCl) in a clean and dry test tube. Set up the apparatus to collect any gas produced (e.g., using a delivery tube to direct gas towards litmus paper).
- Add acid: Carefully add some concentrated sulphuric acid to the test tube containing NaCl.
- Observe gas production: Observe the production of a gas from the reaction.
- Test with dry litmus: Bring a piece of dry blue litmus paper into contact with the gas evolved from the delivery tube. Observe any change in color.
- Test with wet litmus: Immediately after, bring a piece of wet (moist) blue litmus paper into contact with the same gas. Observe and note the change in color.
Note to Teachers:
If the climate is very humid, it is recommended to pass the gas produced through a guard tube (drying tube) containing anhydrous calcium chloride (CaCl₂) before testing with litmus paper. This ensures the gas is completely dry for accurate observation with dry litmus paper.
Observation
- A colorless gas is evolved from the reaction mixture.
- When the gas comes into contact with dry blue litmus paper, there is no change in its color.
- However, when the gas comes into contact with wet blue litmus paper, it immediately turns red.
Explanation
When solid sodium chloride (NaCl) reacts with concentrated sulphuric acid (H₂SO₄), hydrogen chloride gas (HCl) is produced:
NaCl(s) + H₂SO₄(conc.) → NaHSO₄(aq) + HCl(g)
The HCl gas produced in this reaction is initially dry. Dry HCl gas is a covalent compound and does not ionize (release H⁺ ions) on its own. Therefore, when it comes into contact with dry blue litmus paper, there is no acidic effect, and thus no color change.
However, when HCl gas comes into contact with wet or moist blue litmus paper, the water present on the litmus paper allows the HCl gas to ionize. In the presence of water, HCl dissociates to produce hydronium ions (H₃O⁺) and chloride ions (Cl⁻):
HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
It is these hydronium ions (H₃O⁺) that are responsible for the acidic properties, causing the wet blue litmus paper to turn red.
Conclusion
This activity conclusively demonstrates that:
- Dry HCl gas does not release H⁺ ions and therefore does not show acidic properties.
- Hydrochloric acid (and by extension, other acids) exhibits its acidic characteristics, such as turning blue litmus red, only in the presence of water (aqueous solutions), where it can ionize and produce H⁺ (or H₃O⁺) ions.
Multiple Choice Questions
1. Which gas is evolved when NaCl reacts with H₂SO₄?
- a) Chlorine gas
- b) Hydrogen gas
- c) Hydrogen chloride gas
- d) Sulphur dioxide gas
2. What happens when dry HCl gas comes in contact with dry blue litmus paper?
- a) It turns red
- b) It turns black
- c) No change
- d) It turns yellow
3. What does this activity 2.9 prove?
- a) Acids are liquid
- b) Acids produce hydrogen ions in water
- c) Acids are sour
- d) All of the above
4. What happens when HCl gas comes in contact with wet blue litmus paper?
- a) Turns red
- b) Turns yellow
- c) No change
- d) Turns green
5. Why do we use CaCl₂ in this activity (in humid conditions)?
- a) It is an activator
- b) It is a drying agent
- c) It is a catalyst
- d) None of the above
Important Precaution
Concentrated sulphuric acid is highly corrosive. Always handle it with extreme care and wear appropriate safety goggles and gloves. This experiment must be performed under the strict supervision and guidance of a qualified teacher or adult. Ensure proper ventilation.