What Happens When Acids or Bases Meet Water?
Exploring Temperature Changes During Dilution
When you add an acid or a base to water, do you think the solution will get hotter or colder? This activity delves into the fascinating energy changes that occur during the dilution of acids and bases, revealing whether the temperature of the solution increases or decreases.
Aim of the Activity
To observe and analyze the temperature change that occurs when concentrated sulphuric acid and sodium hydroxide pellets are added to water.
Materials & Safety Gear
- Approximately 10 mL of water
- Beakers (at least two)
- Concentrated Sulphuric Acid (H₂SO₄)
- Sodium Hydroxide (NaOH) pellets
- Glass rod (for stirring)
- Safety Gloves and Goggles (Mandatory)
Procedure
-
Part 1: Acid Dilution
- Take about 10 mL of water in a clean beaker.
- Carefully add a few drops of concentrated Sulphuric Acid (H₂SO₄) to the water.
- Slowly swirl the beaker.
- Immediately and carefully touch the base of the beaker to observe any change in temperature.
-
Part 2: Base Dissolution
- In a separate clean beaker, take another 10 mL of water.
- Add a few sodium hydroxide (NaOH) pellets to the water.
- Stir gently with a glass rod until the pellets start to dissolve.
- Carefully touch the base of this beaker to observe the temperature change.
Crucial Safety Note for Diluting Acids:
Always add acid to water, slowly and with continuous stirring. NEVER add water to concentrated acid. Adding water to concentrated acid can generate a large amount of heat very rapidly, causing the solution to boil violently and splash, leading to severe burns.
Observation
- When concentrated Sulphuric Acid is added to water, the temperature of the solution significantly increases. The beaker feels warm or hot to the touch.
- Similarly, when sodium hydroxide pellets are added to water, the temperature of the solution also increases noticeably. The beaker feels warm or hot.
In both cases, the temperature of the beaker rises, indicating that heat is released during the process.
Explanation
The increase in temperature observed in both instances is due to the process being exothermic, meaning heat is released into the surroundings.
- For Acids (e.g., H₂SO₄): When an acid like sulphuric acid is added to water, it undergoes ionization (dissociation) into its respective ions (e.g., H⁺ and SO₄²⁻). The hydrogen ions (H⁺) then combine with water molecules (H₂O) to form hydronium ions (H₃O⁺):
H⁺(aq) + H₂O(l) → H₃O⁺(aq)
The formation of these new, stronger bonds between the ions and water molecules releases a significant amount of energy as heat. This process is highly exothermic.
- For Bases (e.g., NaOH): Similarly, when a base like sodium hydroxide dissolves in water, it dissociates into its ions (Na⁺ and OH⁻). The strong attraction between these ions and the water molecules (hydration) also releases a considerable amount of heat.
Conclusion
Through this activity, we can conclude that the dilution of concentrated acids (like H₂SO₄) and the dissolution of strong bases (like NaOH pellets) in water are both exothermic processes. This means that a considerable amount of heat is released, causing the temperature of the solution to rise.
Multiple Choice Questions
1. What happens when concentrated sulphuric acid is added to water?
- a) Temperature increases
- b) Temperature decreases
- c) No change
- d) None of the above
2. What type of chemical reaction occurs when H₂SO₄ is added to water?
- a) Endothermic
- b) Exothermic
- c) Neutral
- d) No reaction
3. While diluting concentrated acid, we should always:
- a) Always add water to acid
- b) Always add acid to water
- c) Mix both at the same time
- d) All of the above
4. What change in the temperature is observed when sodium hydroxide pellets are added to water?
- a) Decreases
- b) Increases
- c) Remains the same
- d) First increases and then decreases
5. The chemical reaction in which heat is released is called:
- a) Exothermic reaction
- b) Endothermic reaction
- c) Neutralization reaction
- d) None of the above
Important Precaution
Concentrated acids and strong bases are corrosive. Always wear safety goggles and gloves. This experiment must be performed under the strict supervision of a qualified teacher or adult. Pay close attention to the dilution procedure for acids to ensure safety.