Chapter 2: Exploring Substances — Acidic, Basic & Neutral
Indicators • Neutralisation • Daily-life links — learn through colourful chemistry!
🧪 Observe • 🎨 Indicate • ⚖️ Neutralise • 🌱 Apply
🗝️Section 1 — 20 Most-Important Words (Meanings in Simple Hindi)
Two-column tabular layout: 10 words in each column. Words in English • Meanings in Hindi
| Word (A) | Meaning (Hindi) | Word (B) | Meaning (Hindi) |
|---|---|---|---|
| Acid | खट्टा स्वभाव; नीला लिटमस लाल कर देता है। | Base (Alkali) | कड़वा/फिसलन; लाल लिटमस नीला कर देता है। |
| Neutral | न अम्ल न क्षार; लिटमस का रंग नहीं बदलता। | Indicator | घोल का अम्ल/क्षार स्वभाव रंग बदलकर बताता है। |
| Litmus | लाईकेन से प्राप्त प्राकृतिक सूचक (नीला/लाल कागज़)। | Red litmus | क्षारीय घोल में नीला हो जाता है। |
| Blue litmus | अम्लीय घोल में लाल हो जाता है। | Rose extract | अम्ल में लाल, क्षार में हरा रंग देता है। |
| Turmeric | क्षार के सम्पर्क में लाल/गुलाबी हो जाती है। | Turmeric paper | हल्दी-रंगा कागज़; क्षार पहचानता है। |
| Olfactory indicator | गन्ध बदलकर सूचित करता है (जैसे प्याज़ कागज़)। | Baking soda | क्षारीय पदार्थ; चींटी के काटने पर राहत देता है। |
| Lime water | कैल्शियम हाइड्रॉक्साइड का क्षारीय घोल। | Vinegar | खट्टा तरल; इसमें एसीटिक अम्ल होता है। |
| Neutralisation | अम्ल + क्षार → नमक + जल + ऊष्मा। | Salt | तटस्थीकरण में बनने वाला ठोस पदार्थ। |
| Heat | तटस्थीकरण में निकलने वाली ऊर्जा/गर्मी। | Lichen | कवक+शैवाल का सहजीवी पौधा; लिटमस का स्रोत। |
| Hydrangea | अम्लीय मिट्टी में नीले, क्षारीय में गुलाबी/लाल फूल। | Formic acid | लाल चींटी का अम्ल; त्वचा में जलन करता है। |
📌Section 2 — Important Notes
- Substances in our surroundings can be acidic, basic, or neutral.
- Litmus is a natural indicator obtained from lichens. It comes as blue/red paper or solution.
- Colour rules: Acids turn blue litmus to red; bases turn red litmus to blue.
- Grouping by litmus (Table idea): Group A (blue→red) = acidic; Group B (red→blue) = basic; Group C (no change) = neutral.
- Taste & touch (with caution): Sour-tasting edible items (lemon, amla, tamarind, vinegar) are usually acidic. Bases often feel soapy/slippery. Not every bitter thing is a base (e.g., bitter gourd isn’t basic).
- Preparing lime water: Mix lime (chuna, calcium oxide) in water, let it settle ~1 hour, filter the clear liquid — a basic solution of calcium hydroxide.
- Natural indicators: red rose, red hibiscus, purple cabbage, beetroot, Indian blackberry (jamun), turmeric, etc.
- Red rose extract: appears red in acidic solutions and green in basic solutions; minimal change in neutral solutions.
- Turmeric/turmeric paper: turns red/pink in basic solutions; shows no change in acidic and neutral — so it detects bases but cannot distinguish acid vs neutral.
- Olfactory (smell) indicators: onion-infused strips can show odour changes in acidic/basic media.
- Neutralisation reaction: When an acid and a base are mixed in suitable amounts, the effect of each is cancelled: Acid + Base → Salt + Water + Heat.
- Daily-life neutralisation:
- Ant bite: formic acid is neutralised using moist baking soda (a base).
- Acidic soil: treated with lime (base). Basic soil: treated with organic matter/compost that releases mild acids.
- Industrial wastewater: if acidic, it should be neutralised with a base before release to protect aquatic life.
- Secret-message idea: On turmeric-coated paper, writing with soap/baking-soda solution appears red/pink when lightly sprayed with turmeric solution/water.
- Safety: Never taste unknown substances. Handle hot water/chemicals with adult supervision. Keep indicators and reagents away from eyes/skin.
- Scientist to know: Acharya Prafulla Chandra Ray — “Father of Modern Indian Chemistry”; founded India’s first pharmaceutical company in 1901; highlighted India’s chemistry heritage.
- Hydrangea fact: flower colour depends on soil nature — blue in acidic soils; pink/red in basic soils (gardeners can tweak soil to change colours).
- Experiment link (lemon + lime water): lemon solution turns red with blue litmus (acidic). Adding lime water gradually shifts it to blue (basic) — the mixture gets neutralised. Adding lemon again reverses the colour.
- Reminder: “Lime water” is not the citrus fruit “lime”; it’s a calcium hydroxide solution.
🎯Section 3 — One-Word Answers (10)
1) The property that turns blue litmus red:
Acidic
2) The property that turns red litmus blue:
Basic
3) Neither acidic nor basic:
Neutral
4) Natural source of litmus:
Lichen
5) Indicator that turns pink/red with bases only:
Turmeric
6) Acid injected by red ants:
Formic acid
7) Common name of calcium hydroxide solution:
Lime water
8) Acid + Base → ______ + Water + Heat
Salt
9) Rose extract shows this colour in a basic solution:
Green
10) Process cancelling acid and base effects:
Neutralisation
✍️Section 4 — Very Short Answers (10) • 2–3 lines each
1) What is litmus and how does it help?
Litmus is a natural indicator from lichens. In acids, blue litmus turns red; in bases, red litmus turns blue. This colour change helps identify the nature of a solution.
2) How do you prepare lime water?
Mix lime (chuna, CaO) in water and leave it undisturbed for about an hour. Filter the clear liquid — the filtrate is calcium hydroxide solution, called lime water (basic).
3) Are all bitter substances basic?
No. Bases are often bitter and slippery, but bitterness alone isn’t proof. Bitter gourd tastes bitter yet isn’t basic. Always use indicators to test.
4) Why does a turmeric stain change colour with soap?
Soap solution is basic. Turmeric turns red/pink in bases, so the stain shows a colour change — it’s turmeric acting as a natural indicator.
5) How does red rose extract behave in acids and bases?
In acidic solutions it appears red, while in basic solutions it turns green. In neutral solutions, there’s little to no colour change.
6) What are olfactory indicators?
They are smell-based indicators. Onion-infused cloth/paper can show odour changes when contacted with acidic or basic substances, helping identify the solution’s nature.
7) What is formed during neutralisation?
When an acid and a base react in suitable amounts, their effects cancel, forming salt and water with the evolution of heat.
8) How can we treat acidic and basic soils?
Acidic soil is treated with lime (a base). Basic soil is treated with organic matter/compost, which gradually releases mild acids to balance the soil.
9) Why is baking soda used for ant bites?
Ants inject formic acid. Baking soda is basic and neutralises the acid, reducing pain and irritation at the bite site.
10) How was the “secret welcome message” likely revealed at the fair?
Words were probably written using a basic solution (soap/baking soda) on a turmeric-coated sheet. Spraying turmeric solution made the writing appear red/pink.
📝Section 5 — Long Answer Type (10) • 3–4 lines each
1) Distinguish between acidic, basic, and neutral solutions with examples.
Acids taste sour and turn blue litmus red (e.g., lemon juice, vinegar). Bases feel slippery and turn red litmus blue (soap solution, lime water). Neutral solutions show no change with red/blue litmus (tap water, sugar solution).
2) Compare three indicators: litmus, rose extract, and turmeric.
Litmus: blue→red in acids; red→blue in bases. Rose extract: red in acids, green in bases. Turmeric: turns red/pink only in bases and remains unchanged in acids/neutral — so it detects bases but can’t separate acid vs neutral.
3) Explain neutralisation and list three everyday applications.
Neutralisation cancels acidic/basic effects forming salt + water + heat. Uses: (i) baking soda on ant bites, (ii) treating acidic soil with lime or basic soil with compost, (iii) neutralising acidic industrial effluents before release.
4) Describe the “secret message” activity and its principle.
Coat paper with turmeric and let it dry. Write using a basic solution (soap/baking soda). On spraying with turmeric solution/water, basic areas turn red/pink, revealing the message — an indicator-based colour change.
5) Why do hydrangea flowers change colour with soil nature?
Pigments in hydrangea respond to soil conditions: acidic soils produce blue flowers; basic soils yield pink/red. Gardeners can adjust soil nature to influence flower colour.
6) In the lemon–lime-water experiment, why do colours reverse when reagents are added alternately?
Lemon solution is acidic, so litmus indicates red. Adding lime water (basic) neutralises the acid, shifting to blue. Adding lemon again makes the mixture acidic, reversing the colour — a see-saw around neutrality.
7) Explain the A–B–C grouping of samples using litmus tests and place examples.
Group A: blue→red (acidic) — lemon, tamarind, amla, vinegar. Group B: red→blue (basic) — soap solution, baking soda, lime water, washing powder. Group C: no change (neutral) — tap water, sugar solution, salt solution.
8) How do you prepare and use an onion olfactory indicator?
Keep clean cotton/filter strips with finely chopped onion in a closed container overnight. Then test strips with acidic/basic drops and observe odour changes. It helps identify the nature of unknown solutions.
9) List key safety rules for indicator experiments in school.
Never taste unknown substances. Work with hot water/chemicals only under adult supervision. Keep reagents away from eyes/skin; clean spills promptly and wash hands after activities.
10) What does the chapter’s opening science-fair scene teach about indicators?
It shows how everyday materials (turmeric and soap solution) can act as simple indicators to create visible colour changes. Indicators help detect acidic/basic nature in a fun, meaningful way.