Acids, Bases & Salts
This page covers identifying acids and bases, indicators, the pH scale, the acids-bases-salts found in everyday life, and an industrial process like chlor-alkali, from the REET exam's point of view. REET asks direct questions from this topic both on acids found in natural sources and on chemical equations.
- 2 questions in REET 2024
- Class 6-8 level
- 11 min read
How this topic is asked in REET
REET 2024 asked two questions on this topic — one on identifying an acid found in a natural source (methanoic acid in the sting of a nettle/scorpion grass), and the other on identifying the correct chemical equation for the chlor-alkali process. This shows that REET asks this topic at two levels — acids and bases found in everyday sources (such as which fruit is known for which acid) and the correct equations for important industrial chemical reactions. Both kinds of preparation are needed, along with memorising the formulas of important salts and their everyday uses well.
Identifying Acids and Bases
Acids are substances that turn blue litmus red and release hydrogen ions (H⁺) when dissolved in water, which combine with water to form hydronium ions (H₃O⁺). Bases are substances that turn red litmus blue and release hydroxide ions (OH⁻) when dissolved in water — a water-soluble base is called an alkali, such as sodium hydroxide.
Both acids and bases can be strong or weak, depending on how many ions they release on dissolving in water — hydrochloric acid and sulphuric acid are strong acids, while the acetic acid in vinegar is a weak acid. Everyday life offers several practical examples of neutralisation reactions — when too much acid builds up in the stomach (indigestion/acidity), a mild antacid base like magnesium hydroxide (milk of magnesia) is taken, which neutralises the extra acid and gives relief. Similarly, baking soda (a mild base) is advised to relieve the irritation from the methanoic acid present in an ant's sting. Tooth decay also begins once the pH inside the mouth falls below 5.5, because the acid produced by bacteria acting on sugar damages the enamel layer of the tooth — this is why keeping the mouth clean after eating is advised.
Indicators
| Type | Example | Colour in Acid | Colour in Base |
|---|---|---|---|
| Natural indicator | Litmus (obtained from lichen) | Red | Blue |
| Natural indicator | Turmeric | Yellow (no change) | Reddish-brown |
| Olfactory indicator | Clove, vanilla, onion | Smell stays the same | Smell changes/disappears |
| Synthetic indicator | Methyl orange / phenolphthalein | Pink-orange / colourless | Yellow / pink |
Acids Found in Everyday Life
| Source | Acid |
|---|---|
| Vinegar | Acetic acid (ethanoic acid) |
| Citrus fruits (lemon, orange) | Citric acid |
| Curd/sour milk | Lactic acid |
| Tamarind | Tartaric acid |
| Ant and nettle (scorpion grass) sting | Methanoic acid (formic acid) |
| Gastric (stomach) juice | Hydrochloric acid |
The pH Scale
- Range of the pH scale
- 0 to 14 — indicates how acidic or basic a solution is
- pH < 7
- An acidic solution — the lower the pH, the greater the acidity
- pH = 7
- Neutral — such as pure distilled water
- pH > 7
- A basic solution — the higher the pH, the greater the basicity
- Importance for soil and crops
- The right soil pH is necessary for good plant growth; slaked lime is added to overly acidic soil and organic matter to overly basic soil to balance the pH
- Rainwater and pollution
- Dissolved atmospheric carbon dioxide naturally makes the pH of normal rainwater about 5.6 (mildly acidic); when air pollution pushes the pH even lower, it is called acid rain, which damages buildings and crops
Important Salts and Their Formulas
| Salt | Chemical Formula | Main Use |
|---|---|---|
| Common salt | NaCl | In food; the raw material for the chlor-alkali process |
| Baking soda | NaHCO₃ | Cooking, fire extinguishers |
| Washing soda | Na₂CO₃·10H₂O | Glass and soap industry, removing water hardness |
| Bleaching powder | CaOCl₂ | Bleaching cloth, water purification, disinfectant |
| Plaster of Paris | CaSO₄·½H₂O | Plastering a broken bone, making statues and moulds |
| Blue vitriol (copper sulphate) | CuSO₄·5H₂O | An example of water of crystallisation |
Chlor-Alkali Process
- Equation
- 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ (electrolysis of an aqueous solution/brine of sodium chloride)
- Products formed
- NaOH (base/sodium hydroxide), Cl₂ (chlorine gas), H₂ (hydrogen gas)
- Uses
- Cl₂ is used to make PVC, bleaching powder and disinfectants; H₂ is used in fuel and making vegetable ghee; NaOH is used in the soap-paper industry and other chemical processes
How to Teach This in Class
Activity
Test safe household items — lemon juice, vinegar, soap solution and baking soda solution — with litmus paper or homemade red cabbage juice (a natural indicator), and have students fill in a colour-change table.
Teaching aid
Put up a colourful pH scale chart (0-14) in class and have students mark everyday substances (lemon, milk, soap, baking soda) at their approximate pH position.
Safety and common misconception
Make it clear to students that concentrated laboratory acids and bases are never tasted or touched — this is only safe with dilute household substances. Also clear up the misconception that all sour substances are dangerous acids — food acids are dilute and present in safe amounts.
Assessment
Ask students whether an unknown solution with a pH meter reading of 4 is acidic or basic, and have them answer with reasoning; also ask them to give an example of neutralisation from an everyday situation like an ant sting or acidity, to check understanding, and have them recite the formulas of important salts orally.
Memory trick / exam tip
Plaster of Paris (CaSO₄·½H₂O) is always kept in a moisture-proof (airtight) container, because it absorbs moisture from the air and turns into gypsum (CaSO₄·2H₂O), becoming hard and useless — this fact is asked in some form in REET-level exams almost every year, so remember both formulas and this change. Similarly, remember that all the 'soda' compounds are different from each other — practise clearly distinguishing baking soda, washing soda and caustic soda (NaOH) by their formulas.
Questions Asked in REET 2024 (With Solutions)
Question 137
The acid found in nettle’s sting is
- Aलैक्टिक अम्ल
- Bसिट्रिक अम्ल
- Cमेथेनॉइक अम्ल
- Dटार्टरिक अम्ल
Explanation
नेटल (बिच्छू घास) की पत्तियों के डंकनुमा रोमों में मेथेनॉइक अम्ल (methanoic acid, फॉर्मिक अम्ल) होता है, जिससे जलन होती है। लैक्टिक अम्ल दही में, सिट्रिक अम्ल नींबू-संतरे में और टार्टरिक अम्ल इमली में पाया जाता है।
Question 125
Chemical equation of ‘Chlor-alkali’ process is
- ACa(OH)₂ + Cl₂ → CaOCl₂ + H₂O
- BNaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
- CNa₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
- D2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Explanation
क्लोर-क्षार प्रक्रिया में सोडियम क्लोराइड के जलीय विलयन (ब्राइन) का विद्युत अपघटन किया जाता है, जिससे NaOH (क्षार), Cl₂ (क्लोर) और H₂ बनते हैं: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂। विकल्प (A) विरंजक चूर्ण बनने की तथा (B) बेकिंग सोडा (सॉल्वे प्रक्रम) बनने की अभिक्रिया है।
Practice Questions
Question 1
What is the pH value of pure distilled water?
- A0
- B14
- C10
- D7
Explanation
Pure water is neutral, so its pH is exactly 7 — neither acidic (below 7) nor basic (above 7).
Question 2
The reaction in which an acid and a base react to form a salt and water is called:
- Aऑक्सीकरण
- Bउदासीनीकरण
- Cअपघटन
- Dसंक्षारण
Explanation
When an acid and a base react to form a salt and water, and both lose their properties, it is called neutralisation — such as HCl + NaOH → NaCl + H₂O.
Question 3
The chemical formula of baking soda is:
- ANaHCO₃
- BNa₂CO₃
- CNaOH
- DCaCO₃
Explanation
The chemical name for baking soda is sodium hydrogen carbonate, with the formula NaHCO₃ — it is used in cooking and fire extinguishers.
Question 4
Soap solution is generally of what nature, which is why it may cause a mild irritation on skin?
- Aप्रबल अम्लीय (pH लगभग 2-3)
- Bउदासीन (pH ठीक 7)
- Cहल्का क्षारीय (pH लगभग 9-10)
- Dप्रबल क्षारीय (pH 14)
Explanation
Soap is the sodium/potassium salt of high fatty acids, and its water solution is usually mildly basic (pH about 9-10) — this is why prolonged use can cause mild irritation on the skin.
Question 5
Which acid is mainly found in tamarind?
- Aसिट्रिक अम्ल
- Bलैक्टिक अम्ल
- Cटार्टरिक अम्ल
- Dमेथेनॉइक अम्ल
Explanation
Tamarind mainly contains tartaric acid, which gives it its distinctive sour taste. Citric acid is found in citrus fruits, lactic acid in curd, and methanoic acid in ant/nettle stings.
Question 6
How many molecules of water of crystallisation are present in washing soda (Na2CO3) crystals?
- A2
- B5
- C7
- D10
Explanation
The full chemical formula of washing soda is Na₂CO₃·10H₂O, meaning each molecule contains 10 molecules of water of crystallisation.
Question 7
When water is added to Plaster of Paris, it hardens and turns into:
- Aबेकिंग सोडा
- Bजिप्सम
- Cविरंजक चूर्ण
- Dधोवन सोडा
Explanation
When water is added to Plaster of Paris (CaSO₄·½H₂O), it absorbs the water, hardens, and turns into gypsum (CaSO₄·2H₂O) — this property is why it is useful for plastering a broken bone and making moulds.
Question 8
The main reason for milk turning sour is the formation of which acid?
- Aसिट्रिक अम्ल
- Bऐसिटिक अम्ल
- Cलैक्टिक अम्ल
- Dटार्टरिक अम्ल
Explanation
Bacteria present in milk ferment the sugar (lactose) into lactic acid, which is why milk turns sour and can also curdle.
Question 9
The most suitable and safe activity for a teacher to teach class 7 students how to identify acids and bases is:
- Aप्रयोगशाला के सांद्र अम्ल चखवाना
- Bघरेलू पदार्थों व लिटमस/पत्तागोभी सूचक से सुरक्षित परीक्षण करवाना
- Cकेवल पाठ्यपुस्तक की परिभाषा पढ़ाना
- Dबिना समझाए सीधे pH मीटर का मान बताना
Explanation
At the class 6-8 level, letting students do hands-on tests on safe household substances (lemon, soap, baking soda) using litmus or a homemade natural indicator (such as red cabbage juice) is the safest and most effective approach. Concentrated laboratory chemicals are never tasted or used in unsafe ways.
Question 10
The chemical formula of bleaching powder is:
- ACaOCl₂
- BCaCO₃
- CCaSO₄
- DCa(OH)₂
Explanation
Bleaching powder is made by passing chlorine gas over slaked lime [Ca(OH)₂], and its chemical formula is CaOCl₂ — it is used for bleaching cloth, water purification and as a disinfectant.
Frequently Asked Questions
What is pH?+
pH is a measure (on a scale of 0 to 14) that shows how acidic or basic a solution is — a pH below 7 is considered acidic, 7 is neutral, and above 7 is basic.
What is a neutralisation reaction, and how does an antacid relieve indigestion?+
When an acid and a base react to form a salt and water, it is called a neutralisation reaction. On this same principle, when excess acid builds up in the stomach (indigestion/acidity), a mildly basic antacid such as milk of magnesia neutralises the extra acid and gives relief.
What products are formed in the chlor-alkali process?+
Electrolysis of an aqueous solution of sodium chloride produces sodium hydroxide (NaOH), chlorine gas (Cl₂) and hydrogen gas (H₂).
Which acid is found in the sting of a nettle (scorpion grass)?+
Methanoic acid (formic acid) is found in the sting-like hairs of the nettle plant, which causes irritation on the skin.
What is the difference between washing soda and baking soda?+
Washing soda (Na₂CO₃·10H₂O) has 10 molecules of water of crystallisation and is mainly used in cleaning and the glass-soap industry, while baking soda (NaHCO₃) is mainly used in cooking and fire extinguishers.
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Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked