Class 7 Science – Chapter 4: Acids, Bases and Salts (Maharashtra Board)

Maharashtra Board – Class 7 Science – Chapter 4: Acids, Bases and Salts

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20 Important Words (Meanings in Hindi)

Acid — खट्टा पदार्थ; जो नीले लिटमस को लाल करता है
Base — कड़वा/साबुन जैसा; जो लाल लिटमस को नीला करता है
Salt — अम्ल और क्षार की अभिक्रिया से बनने वाला यौगिक
Indicator — ऐसा पदार्थ जो अम्ल/क्षार में रंग बदलता है
Litmus — प्राकृतिक सूचक; अम्ल में लाल, क्षार में नीला
Turmeric — हल्दी; क्षारीय घोल में लाल-भूरा रंग दिखाती है
China Rose Indicator — गुड़हल सूचक; अम्ल में गुलाबी/मैजेंटा, क्षार में हरा
Phenolphthalein — सूचक; अम्ल में रंगहीन, क्षार में गुलाबी
Neutral — न अम्लीय न क्षारीय; लिटमस का रंग नहीं बदलता
Neutralisation — अम्ल-क्षार की अभिक्रिया; लवण + जल + उष्मा
Hydrochloric Acid (HCl) — अम्ल; पेट में प्राकृतिक रूप से पाया जाता है
Sodium Hydroxide (NaOH) — मजबूत क्षार; साबुन/क्लीनर में
Calcium Hydroxide — चूने का पानी; मिट्टी/अपशिष्ट उपचार में
Ammonium Hydroxide — क्षार; काँच-साफ़ करने के द्रव में
Acetic Acid — सिरके में पाया जाने वाला अम्ल
Formic Acid — चींटी के डंक में पाया जाने वाला अम्ल
Magnesium Hydroxide — मिल्क ऑफ मैग्नेशिया; एंटासिड
pH (idea) — अम्लीय/क्षारीयता का माप (कक्षा 7 में अवधारणा मात्र)
Acid Rain — वायु प्रदूषकों के कारण अम्लीय वर्षा
Corrosive — क्षरणकारी; त्वचा/धातु को हानि पहुँचाने वाला

Important Notes (Quick Revision)

Substances can be acidic, basic (alkaline) or neutral. Taste test is unsafe—use indicators.
Acids taste sour (e.g., lemon, vinegar); turn blue litmus → red.
Bases taste bitter/soapy (e.g., baking soda solution, soap); turn red litmus → blue.
Neutral solutions (e.g., sugar, salt in distilled water) do not change red/blue litmus.
Indicators: Litmus (red/blue), Turmeric (base → red-brown), China rose (acid → magenta, base → green), Phenolphthalein (acid → colourless, base → pink).
Neutralisation: Acid + Base → Salt + Water + Heat. Example:
$$\mathrm{HCl + NaOH \;\rightarrow\; NaCl + H_2O \;(+\,heat)}}$$
Everyday neutralisation: antacids for acidity; baking soda/calamine for ant bite; lime (CaO/Ca(OH)$_2$) to acidic soil; neutralise factory acidic wastes before disposal.
Safety: Lab acids/bases are corrosive—do not taste/touch; handle with care.
Acid rain: $CO_2$, $SO_2$, $NO_2$ dissolve in rain to form acids → damages monuments, plants, aquatic life.

20 One-Word/One-Line Answer Type

Name the indicator extracted from lichens.
Litmus.
What is the taste of acids?
Sour (do not taste directly; infer via indicators).
Blue litmus turns ______ in acid.
Red.
Red litmus turns ______ in base.
Blue.
Turmeric shows what colour in base?
Reddish-brown.
China rose in acid gives which colour?
Dark pink/magenta.
China rose in base gives which colour?
Green.
Phenolphthalein is ______ in acid and ______ in base.
Colourless; Pink.
Product(s) of neutralisation.
Salt and water (with heat evolved).
Acid in vinegar.
Acetic acid.
Acid in ant sting.
Formic acid.
Largest use of milk of magnesia.
Antacid (neutralises excess stomach acid).
Base used to treat acidic soil.
Quick lime (CaO) or slaked lime (Ca(OH)$_2$).
Nature of baking soda solution.
Basic (alkaline).
Nature of common salt solution.
Neutral (approximately).
Neutral solutions affect litmus?
No change in red/blue.
Equation: $\mathrm{HCl}$ + $\mathrm{NaOH}$ → ?
$\mathrm{NaCl + H_2O}$.
Which turns soapy on touch: acid or base?
Base (solution).
Rain with excess acids is called?
Acid rain.
A solution that does not change either red/blue litmus is?
Neutral.

20 Very Short Answer Type (1–2 lines)

Define an acid with one indicator test.
A substance turning blue litmus red is acidic.
Define a base with one indicator test.
A substance turning red litmus blue is basic.
Give two natural indicators.
Litmus, Turmeric, China rose (any two).
What is neutralisation?
Reaction of acid and base to form salt + water with heat.
Write a neutralisation example.
$\\mathrm{HCl + NaOH \\to NaCl + H_2O}$.
Why tasting unknown substances is unsafe?
Lab acids/bases are corrosive and harmful.
State the colour change of turmeric in base.
Yellow → Reddish-brown.
How to treat ant bite?
Apply moist baking soda or calamine to neutralise acid.
What is the nature of distilled water?
Neutral (no colour change with litmus).
Give one use of lime water in daily life of soil.
Treat acidic soil to improve plant growth.
Name one weak organic acid from kitchen.
Acetic acid (vinegar) or citric acid (lemon).
Litmus shows no change in a solution. What is inferred?
Solution is likely neutral.
China rose in base gives which colour?
Green.
Why is heat produced in neutralisation?
It is an exothermic process.
Name any two bases used at home.
Baking soda solution, lime water, soap solution (any two).
Which gas dissolved in rain makes it slightly acidic?
Carbon dioxide (forms carbonic acid).
Which litmus paper turns red in acid?
Blue litmus.
Which litmus paper turns blue in base?
Red litmus.
Give one neutral example from kitchen.
Sugar solution/common salt solution (approx. neutral).
State one precaution while using indicators.
Use clean droppers; avoid contamination and contact with eyes/skin.

20 Short Answer Type (2–3 lines)

How does turmeric act as an indicator?
Turmeric remains yellow in acids/neutral, but turns reddish-brown in bases; hence it detects basic solutions.
Differentiate between acids and bases (any two points).
Acids: sour, turn blue litmus red. Bases: bitter/soapy, turn red litmus blue.
What happens when acid reacts with base?
Neutralisation producing salt + water with evolution of heat, e.g., $\\mathrm{HCl + NaOH \\to NaCl + H_2O}$.
Explain neutralisation in indigestion.
Excess stomach acid is neutralised by antacids like magnesium hydroxide (milk of magnesia).
Why apply calamine on ant bite?
Ant sting injects formic acid; calamine (zinc carbonate) or baking soda neutralises the acid.
How are acidic soils treated?
By adding quick lime (CaO) or slaked lime (Ca(OH)$_2$) to neutralise acidity.
What is acid rain and its impact?
Rain with dissolved acids ($\\mathrm{H_2CO_3, H_2SO_4, HNO_3}$) damages buildings, plants, and aquatic life.
State litmus colours in acid and base.
Acid: blue → red; Base: red → blue.
How does China rose indicator behave?
Acidic solution → magenta; Basic solution → green; Neutral → little/no change.
Why are lab acids/bases handled carefully?
They are corrosive/irritant and can harm skin/eyes/cloth.
Name two natural acids with sources.
Acetic acid (vinegar), Citric acid (lemon/orange), Lactic acid (curd).
Name two household bases.
Baking soda solution (NaHCO$_3$), Lime water (Ca(OH)$_2$).
What is the nature of soap solution? Prove using indicator.
Basic; turns red litmus blue and turmeric reddish-brown.
Why must factory acidic wastes be treated?
To neutralise acids before disposal, preventing harm to aquatic life and pipes.
What is meant by neutral solution?
Solution with neither acidic nor basic nature; no colour change with red/blue litmus.
How can you show neutralisation is exothermic?
Temperature of mixture rises on mixing acid and base—test tube feels warm.
Write ionic form of neutralisation.
$\\mathrm{H^+ + OH^- \\to H_2O}$ (heat released).
What will turmeric paper show for lemon juice? for baking soda?
Lemon: no change (yellow). Baking soda: reddish-brown (base).
Why does coffee’s taste not prove acidity/basicity?
Taste can mislead; use indicators—never identify only by taste.
Mention one neutral salt from everyday life.
Sodium chloride (table salt) solution is approximately neutral.

Textbook Exercise – Perfect Answers (Chapter 4)

1. State differences between acids and bases.
Acids: Sour; turn blue litmus red; many contain $H^+$ in water (e.g., HCl, acetic acid). Bases: Bitter/soapy; turn red litmus blue; many contain $OH^-$ in water (e.g., NaOH, Ca(OH)$_2$).
2. Ammonia in window cleaners turns red litmus blue. What is its nature?
Basic (alkaline).
3. Source of litmus and its use?
Litmus is obtained from lichens; used as an indicator: acid → red, base → blue.
4. Is distilled water acidic/basic/neutral? How to verify?
Neutral. Test with red and blue litmus—no colour change with either.
5. Describe neutralisation with an example.
When acid reacts with base, salt + water form and heat is evolved. Example: $\\mathrm{HCl + NaOH \\rightarrow NaCl + H_2O}$.
6. Mark T/F.
(i) Nitric acid turn red litmus blue — F (acids turn blue → red).
(ii) Sodium hydroxide turns blue litmus red — F (bases turn red → blue).
(iii) NaOH and HCl neutralise each other to form salt and water — T.
(iv) Indicator shows different colours in acidic and basic solutions — T.
(v) Tooth decay is caused by the presence of a base — F (acids from bacteria cause decay).
7. Dorji must serve one acidic, one basic, one neutral soft drink. How to decide?
Test small samples with red/blue litmus: Acidic turns blue → red; Basic turns red → blue; Neutral shows no change with either. (Use clean droppers and labels.)
8. Explain why:
(a) Antacid tablet is taken in acidity — neutralises excess stomach acid.
(b) Calamine is applied on ant bite — neutralises formic acid injected by ant.
(c) Factory waste is neutralised — to avoid killing aquatic life and corrosion of drains.
9. Three liquids: HCl, NaOH, sugar solution. Identify using only turmeric indicator.
NaOH turns turmeric paper reddish-brown → identify the base. HCl and sugar cause no change with turmeric (both keep yellow). With turmeric alone you cannot distinguish acid from neutral; use another indicator (e.g., litmus) to tell HCl (acid) from sugar (neutral).
10. Blue litmus remains blue in a solution. Nature?
Either basic or neutral. To confirm, use red litmus: if red → blue, it’s basic; if no change with either, it’s neutral.
11. Consider statements (a–d). Which are correct?
(a) Both acids and bases change colour of all indicators — Incorrect.
(b) If an indicator changes with an acid, it does not with a base — Not generally true.
(c) If an indicator changes with a base, it does not with an acid — Not generally true.
(d) Colour change depends on the type of indicator — Correct.
Answer: (iv) only d.

Important Equations & Numbers (MathJax)

Neutralisation (general): $$\mathrm{Acid + Base \;\rightarrow\; Salt + H_2O \;(+\,heat)}$$

Example: $$\mathrm{HCl + NaOH \;\rightarrow\; NaCl + H_2O}$$

Ionic view: $$\mathrm{H^+ (aq) + OH^- (aq) \;\rightarrow\; H_2O (l)}$$

Acid rain formation (concept): $$\mathrm{SO_2 + H_2O \rightarrow H_2SO_3;\;\; 2NO_2 + H_2O \rightarrow HNO_2 + HNO_3}$$