Understanding Acids and Bases – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain the basic differences between acids and bases using taste and litmus observations. Give examples.
Answer:
Acids taste sour. Examples: lemon juice, vinegar.
Bases taste bitter and feel soapy. Examples: baking soda, soap.
Acids turn blue litmus to red. They keep red litmus unchanged.
Bases turn red litmus to blue. They keep blue litmus unchanged.
Never taste chemicals in a lab. Use indicators for safe testing.
These observable changes help us identify if a solution is acidic or basic.
Q2. What are indicators? Compare litmus, phenolphthalein, methyl orange, and turmeric with examples.
Answer:
Indicators are substances that change color in acids and bases.
Litmus: purple normally, red in acids, blue in bases.
Phenolphthalein: colorless in acids, pink in bases.
Methyl orange: red/pink in acids, yellow in bases.
Turmeric: yellow normally, turns reddish-brown in bases.
Example: NaOH gives pink with phenolphthalein and yellow with methyl orange. HCl stays colorless with phenolphthalein and turns red/pink with methyl orange.
Q3. How will you identify distilled water, vinegar, and baking soda solution using litmus paper?
Answer:
Use red and blue litmus papers for testing.
The acidic solution (like vinegar) turns blue litmus red. Red litmus shows no change.
The basic solution (like baking soda) turns red litmus blue. Blue litmus shows no change.
Distilled water is neutral. It shows no change with either litmus.
Test one drop at a time. Clean the dropper between tests.
Record observations clearly to match each solution to acid, base, or neutral.
Q4. Describe the lab test of acids with metals using zinc and explain the “pop test.”
Answer:
Add zinc granules to dilute acid like HCl or H₂SO₄.
You will see bubbles forming on zinc. This shows gas is produced.
The gas is hydrogen (H₂). Pass it through a soap solution to trap bubbles.
Bring a lighted splint near a bubble. It burns with a ‘pop’ sound.
This confirms the gas is hydrogen. The product in solution is a salt (e.g., zinc chloride with HCl).
The reaction is a sign that acids react with metals to form salt + hydrogen gas.
Q5. Explain what happens when sodium carbonate and sodium bicarbonate react with dilute HCl. How do you confirm the gas?
Answer:
Add dilute HCl to sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) in two test tubes.
In both cases, you see effervescence (bubbling). A gas is released.
The gas is carbon dioxide (CO₂).
Pass this gas through lime water. It turns milky.
This milky change confirms CO₂.
The reactions form salt + water + CO₂. Example: Na₂CO₃ + HCl → NaCl + H₂O + CO₂.
High Complexity (Analysis & Scenario-Based)
Q6. You are designing a science fair demo to teach acids and bases using indicators. What will you show and why?
Answer:
Start with litmus to show basic color changes: acids turn blue to red, bases turn red to blue.
Use phenolphthalein: show colorless in acids, pink in bases.
Use methyl orange: show red/pink in acids, yellow in bases.
Show turmeric on paper: apply a soap solution to turn it reddish-brown.
Explain with familiar items: lemon juice (acid), baking soda (base), distilled water (neutral).
Emphasize safety: small amounts, use droppers, wear gloves and goggles.
Q7. A clear solution turns blue litmus red, keeps red litmus unchanged, is colorless with phenolphthalein, and red with methyl orange. What is it? How will you confirm?
Answer:
The tests show it is an acidic solution.
Blue to red means acid. Red unchanged supports that.
Phenolphthalein colorless also signals acid.
Methyl orange red/pink confirms acid again.
It could be HCl, H₂SO₄, HNO₃, or acetic acid.
Confirm using smell (vinegar smell for acetic acid) or by testing reaction with metals (forms H₂ with active metals like zinc).
Q8. In a lab, a metal piece in an unknown solution produces bubbles. The gas burns with a pop. Analyze the solution and explain the reaction.
Answer:
Bubbles show a gas is forming.
The pop sound shows the gas is hydrogen (H₂).
Metals release H₂ when they react with acids.
So, the unknown solution is likely an acid like HCl or H₂SO₄.
The reaction forms salt + hydrogen gas.
Faster bubbling may mean a stronger acid or a more reactive metal.
Q9. A curry stain on cloth turns reddish-brown when soap is applied. Use the idea of indicators to explain and suggest a fix.
Answer:
Turmeric in curry acts like a natural indicator.
Soap is a base. It turns turmeric from yellow to reddish-brown.
So, the color change is due to the basic nature of soap.
Rinse the cloth well to remove the base.
Then dip the area in dilute acidic water (like water with a little vinegar or lemon juice).
The stain returns to yellow, as the solution becomes less basic.
Q10. You have two similar white powders: sodium carbonate and sodium bicarbonate. Both release CO₂ with HCl. How will you tell them apart using simple tests?
Answer:
Dissolve equal amounts in water and add phenolphthalein.
Sodium carbonate is more basic. It gives a deep pink color.
Sodium bicarbonate is milder. It gives light pink or almost no pink.
Add equal drops of HCl to equal masses. Carbonate shows stronger effervescence.
