Neutralization Reaction Experiment: Acids, Bases, and Indicators
Have you ever wondered what happens when an acid meets a base? This activity explores one of the most fundamental reactions in chemistry: the neutralization reaction. We'll use a special indicator to observe this process firsthand.
Aim of the Activity
To observe the reaction between an acid and a base, and to understand the concept of neutralization using a pH indicator.
Materials & Apparatus
- Dilute Hydrochloric Acid (HCl)
- Dilute Sodium Hydroxide (NaOH) solution
- Phenolphthalein solution (indicator)
- Test tube
- Dropper
- Test tube stand (optional, for convenience)
Procedure
- Prepare the base solution: Take a clean test tube and pour approximately 2 mL of dilute Sodium Hydroxide (NaOH) solution into it.
- Add indicator: Add two drops of phenolphthalein solution to the NaOH solution in the test tube. Gently swirl the test tube and observe any color change.
- Add acid drop by drop: Now, using a dropper, carefully add dilute Hydrochloric Acid (HCl) solution to the mixture in the same test tube, drop by drop. After each drop, gently swirl the test tube and observe the solution's color. Continue adding HCl until a significant color change occurs.
- Re-add base: After observing the color change from adding HCl, add a few drops of NaOH solution back into the test tube. Observe the color change again.
Observation
Upon adding phenolphthalein to the Sodium Hydroxide (NaOH) solution, the solution immediately turns a vibrant pink color.
As dilute Hydrochloric Acid (HCl) is added drop by drop, the pink color gradually fades and eventually disappears completely, leaving a colorless solution.
When a few more drops of NaOH solution are added back to this colorless mixture, the pink color reappears.
Explanation
Phenolphthalein is a common pH indicator. It is colorless in acidic and neutral solutions but turns pink in basic (alkaline) solutions.
- When phenolphthalein is added to NaOH solution (which is a strong base), the solution turns pink, indicating its alkaline nature.
- As HCl (a strong acid) is added, it reacts with the NaOH. This reaction is called a neutralization reaction. The acid neutralizes the base, and as the solution becomes less alkaline and then acidic, the phenolphthalein loses its pink color and becomes colorless.
- When NaOH is added again, the solution becomes alkaline once more, and phenolphthalein turns pink again.
Conclusion
This experiment clearly demonstrates the concept of neutralization. The addition of HCl solution effectively cancels out the alkaline nature of NaOH, causing the indicator to change color. Conversely, adding NaOH solution cancels out the acidic nature of HCl. In essence, acids and bases appear to cancel out each other's effects, forming a neutral solution (or close to it) and a salt and water.
Multiple Choice Questions
1. What is the initial color of the solution when phenolphthalein is added to sodium hydroxide?
- a) White
- b) Red
- c) Pink
- d) Yellow
2. What happens when dilute hydrochloric acid is added to the sodium hydroxide and phenolphthalein solution?
- a) The solution turns red
- b) The solution turns black
- c) The pink color becomes darker
- d) The pink color disappears
3. Which chemical reaction takes place when HCl is added to sodium hydroxide solution?
- a) Combination reaction
- b) Displacement reaction
- c) Neutralization reaction
- d) Decomposition reaction
Answer the Following Questions
Q1. What is a neutralization reaction?
Ans. A neutralization reaction is a chemical reaction between an acid and a base that produces a salt and water.
Q2. Write one chemical equation for the above reaction.
Ans.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Q3. What will happen if we add phenolphthalein to hydrochloric acid?
Ans. When phenolphthalein is added to hydrochloric acid (an acid), it remains colorless.
Q4. What happens when you add phenolphthalein to sodium hydroxide?
Ans. When phenolphthalein is added to sodium hydroxide (a base), it changes to a pink color.
Q5. Which reaction takes place between a base and an acid?
Ans. A neutralization reaction takes place between a base and an acid.