Equipment:

• 2x 100 ml beaker
• 50 ml of water in each one
• 3 drops of BTB in each one
• 1x straw
• 1 bottle of [[$\text{HCl(aq)}$]]
• 1 bottle of [[$\text{NaOH(aq)}$]]
• 10 slips of pH paper
• 1 tissue

Instructions:

1. Pick either beaker.
2. Blow air into the liquid with a straw.
• Caution! Do not inhale, just exhale.
• Caution! Blow gently to prevent spills.
3. Observe the change.
4. Write the reaction as an equation.
   • What is the substance you exhale?
   • What is the substance you exhale into?
   • What is the substance the reaction yields?
5. Switch to the unused beaker
6. Add 15 drops of [[$\text{HCl}$]] into the solution, one drop at a time.
7. Measure the pH of the solution with a slip of pH paper.
8. Place the slip on the tissue.
9. Add 3 drops of [[$\text{NaOH}$]] into the solution.
10. Repeat from step 7 onwards until the pH slips are exhausted.
11. Plot the pH changes as a diagram.
    • horizontal: drops of [[$\text{NaOH}$]]
    • vertical: pH of solution
    • With a trendline, find the amount of [[$\text{NaOH}$]] drops needed for pH 7.