This MCQ module is based on: Neutralisation — Reactions of Acids and Bases
TOPIC 5 OF 46
Neutralisation — Reactions of Acids and Bases
🎓 Class 7
Science
CBSE
Theory
Ch 2 — Exploring Substances: Acidic, Basic and Neutral
⏱ ~14 min
🌐 Language: [gtranslate]
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2.2 What Happens When Acidic and Basic Substances Mix?
We have seen how indicators help us identify acids and bases. But what happens when we mix an acidic substance with a basic substance? Do they cancel each other out? Let us find out through an exciting activity.
Activity 2.7 — Observing the Effect of Mixing an Acid and a Base
L3 Apply
You know that lemon juice turns blue litmus red (because it is acidic). Now, if you add lime water (basic) to this red litmus paper, what do you think will happen to the colour?
- Take a test tube and pour a small amount of lemon juice into it.
- Dip a strip of blue litmus paper into the lemon juice. Observe: it turns red (confirming the solution is acidic).
- Now, slowly add lime water (a basic substance) to the same test tube, drop by drop, while stirring gently.
- Dip the red litmus paper (that was turned red by the acid) back into the mixture after adding the lime water.
- Observe the colour change carefully.
What happens:
• Initially, the blue litmus paper turned red in lemon juice (acidic).
• After adding lime water (base), the litmus paper changed from red back to blue!
• This indicates that the lime water (base) neutralised the effect of the lemon juice (acid).
• You may also notice the mixture becomes slightly warm — heat is produced during this reaction.
This process is called neutralisation.
• Initially, the blue litmus paper turned red in lemon juice (acidic).
• After adding lime water (base), the litmus paper changed from red back to blue!
• This indicates that the lime water (base) neutralised the effect of the lemon juice (acid).
• You may also notice the mixture becomes slightly warm — heat is produced during this reaction.
This process is called neutralisation.
The Neutralisation Reaction:
When an acid and a base are mixed together in the right proportions, they react to form a new substance called a salt and water. This reaction also releases heat.
Acid + Base → Salt + Water + Heat
This process is known as neutralisation. In this reaction, the properties of both the acid and the base are "cancelled out" or destroyed, producing a neutral substance.
When an acid and a base are mixed together in the right proportions, they react to form a new substance called a salt and water. This reaction also releases heat.
Acid + Base → Salt + Water + Heat
This process is known as neutralisation. In this reaction, the properties of both the acid and the base are "cancelled out" or destroyed, producing a neutral substance.
2.3 Neutralisation in Daily Life
Neutralisation is not just a laboratory concept — it is a process that occurs all around us, from nature to medicine to industry. Let us explore some real-life situations where neutralisation plays an important role.
Situation 1: Treating an Ant Bite
Have you ever been bitten by an ant? The stinging sensation you feel is caused by formic acid that the ant injects into your skin. To relieve the irritation, people often apply a paste of baking soda (a mild base) on the affected area. The baking soda neutralises the formic acid, reducing the pain and swelling.
Situation 2: Treating Acidic Soil
In some regions, the soil becomes too acidic for healthy plant growth. Farmers treat such soil by adding lime (calcium hydroxide) or wood ash, both of which are basic in nature. The base neutralises the excess acid in the soil, making it more suitable for growing crops.
Situation 3: Treating Factory Waste
Many factories produce acidic waste during manufacturing processes. If this acidic waste is released directly into rivers or lakes, it can harm aquatic life and pollute the water. Before discharging the waste, factories must treat it with basic substances to neutralise the acid, making the waste safer for the environment.
Environmental Connection: Untreated acidic factory waste can kill fish, destroy aquatic plants, and make water unfit for human use. Neutralisation is a critical step in waste water treatment to protect our water bodies.
Solving the Science Fair Mystery!
Remember the "magic writing" that appeared at the science fair entrance? Now we can explain it! The volunteers had written on white paper using a turmeric solution (a natural indicator). Since turmeric is pale yellow when dry, the writing was nearly invisible on white paper. When the volunteer sprayed baking soda solution (a base) onto the paper, the turmeric reacted with the base and turned reddish-brown, revealing the hidden message!
The Secret Behind the Invisible Writing:
• Ink used: Turmeric solution (natural indicator, nearly invisible when dry on white paper)
• Spray: Baking soda solution (base)
• Reaction: Turmeric + base → colour changes to reddish-brown
• Result: Hidden message appears!
• Ink used: Turmeric solution (natural indicator, nearly invisible when dry on white paper)
• Spray: Baking soda solution (base)
• Reaction: Turmeric + base → colour changes to reddish-brown
• Result: Hidden message appears!
In a Nutshell
Key Takeaways from Chapter 2:
• Substances can be classified as acidic, basic, or neutral based on their behaviour with indicators.
• Indicators are substances that show different colours (or smell changes) in acidic and basic solutions.
• Litmus (from lichens) is the most common indicator: acids turn blue litmus red; bases turn red litmus blue.
• Red rose extract turns green in bases, stays red in acids.
• Turmeric turns reddish-brown in bases; no change with acids or neutral substances.
• Olfactory indicators (like onion) change their smell in acidic or basic solutions.
• When an acid and a base are mixed, they undergo neutralisation: Acid + Base → Salt + Water + Heat.
• Neutralisation has many daily applications: treating ant bites, improving acidic soil, and treating factory waste.
• Substances can be classified as acidic, basic, or neutral based on their behaviour with indicators.
• Indicators are substances that show different colours (or smell changes) in acidic and basic solutions.
• Litmus (from lichens) is the most common indicator: acids turn blue litmus red; bases turn red litmus blue.
• Red rose extract turns green in bases, stays red in acids.
• Turmeric turns reddish-brown in bases; no change with acids or neutral substances.
• Olfactory indicators (like onion) change their smell in acidic or basic solutions.
• When an acid and a base are mixed, they undergo neutralisation: Acid + Base → Salt + Water + Heat.
• Neutralisation has many daily applications: treating ant bites, improving acidic soil, and treating factory waste.
Competency-Based Questions
A farmer noticed that plants in his field were not growing well. He tested the soil and found it was acidic. He consulted an agricultural scientist, who suggested adding a certain substance to the soil. After treatment, the crops grew much better.
Q1. L1 Remember What are the products of a neutralisation reaction?
Answer: C. A neutralisation reaction produces salt, water, and heat. The complete equation is: Acid + Base → Salt + Water + Heat.
Q2. L2 Understand Which substance would the scientist most likely have recommended the farmer to add to the acidic soil?
Answer: C. Lime (calcium hydroxide) is a base. Adding it to acidic soil causes neutralisation, reducing the acidity and making the soil suitable for crops. Vinegar and lemon juice are acidic (would worsen the problem), and common salt is neutral (would not help).
Q3. L3 Apply An ant injects formic acid when it bites. Suggest a household remedy and explain how it works using the concept of neutralisation. (Short Answer — 2 marks)
Answer: Apply a paste of baking soda (sodium bicarbonate) on the ant bite. Baking soda is a mild base. It reacts with the formic acid injected by the ant through a neutralisation reaction: Formic acid (acid) + Baking soda (base) → Salt + Water + Heat. This neutralises the acid, relieving the pain and irritation.
Q4. L4 Analyse A factory releases acidic waste into a nearby river. Explain why this is harmful and describe a chemical process the factory should use before releasing the waste. (Long Answer — 3 marks)
Answer: Acidic waste is harmful because it can (1) kill fish and other aquatic organisms, (2) destroy aquatic plants, and (3) make the water unfit for drinking or irrigation. Before releasing waste, the factory should treat it with a basic substance (such as lime or calcium hydroxide). This causes a neutralisation reaction — the base neutralises the excess acid, converting it into salt and water. The treated waste becomes much less harmful and safer to release into the river.
Q5. L5 Evaluate True or False: "Neutralisation reactions always produce common table salt (NaCl) as the product." Justify with an example. (3 marks)
Answer: False. Neutralisation reactions produce a salt, but not necessarily common table salt (NaCl). The type of salt formed depends on which acid and which base react. For example, when formic acid from an ant bite reacts with baking soda (sodium bicarbonate), the salt formed is sodium formate, not NaCl. Common table salt (NaCl) is only formed when hydrochloric acid reacts with sodium hydroxide specifically.
Assertion–Reason Questions
Assertion (A): When lemon juice and baking soda solution are mixed, the mixture becomes slightly warm.
Reason (R): Neutralisation is an exothermic reaction that releases heat energy.
Answer: A. Both are true, and the Reason correctly explains the Assertion. Lemon juice (acid) and baking soda (base) undergo neutralisation, which releases heat. This is why the mixture feels warm.
Assertion (A): Farmers add lime to acidic soil to improve crop growth.
Reason (R): Lime is acidic in nature and increases the soil's acidity.
Answer: C. The Assertion is true — farmers do add lime to acidic soil to improve crops. However, the Reason is false — lime is a base (not acidic). It works by neutralising the excess acid in the soil, not by increasing acidity.
Assertion (A): The neutralisation reaction produces water and salt.
Reason (R): In neutralisation, the acid and base cancel each other's properties.
Answer: A. Both are true. In neutralisation, an acid reacts with a base to form salt and water (Assertion). This happens because the acidic and basic properties cancel each other out (Reason), producing neutral products. The Reason correctly explains why salt and water are formed.
Frequently Asked Questions — Neutralisation — Reactions of Acids and Bases
What does the topic 'Neutralisation — Reactions of Acids and Bases' cover in Class 7 Science?
The topic 'Neutralisation — Reactions of Acids and Bases' is part of NCERT Class 7 Science Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. It covers the key ideas of neutralisation, acid-base reaction, salt formation, antacids, soil pH, bee sting, indigestion, explained through everyday examples, labelled diagrams and hands-on activities drawn from the NCERT Curiosity textbook. Students learn not just definitions but also the reasoning behind each concept so they can answer competency-based questions and assertion–reason items. The lesson helps Class 7 students build a strong base for higher classes by linking each idea to real observations at home, school and in nature, and by preparing them for CBSE school assessments and Olympiads.
Why is 'Neutralisation — Reactions of Acids and Bases' important for Class 7 NCERT Science?
'Neutralisation — Reactions of Acids and Bases' is important because it builds core scientific thinking that Class 7 students will use throughout middle and secondary school. NCERT Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — introduces neutralisation and related ideas that appear again in Class 8, 9 and 10 Science. Mastering this subtopic helps students read labels and safety signs, understand news about science and technology, and perform better in CBSE school exams. The chapter also encourages curiosity and evidence-based thinking — skills that support the National Education Policy (NEP) 2020 focus on conceptual understanding and competency-based learning.
What are the key concepts students should remember from Neutralisation — Reactions of Acids and Bases?
The key concepts in 'Neutralisation — Reactions of Acids and Bases' for Class 7 Science are: neutralisation, acid-base reaction, salt formation, antacids, soil pH, bee sting, indigestion. Students should be able to define each term in their own words, give at least one everyday example, and explain how the concept connects to other chapters in NCERT Class 7 Science. For example, linking the idea to daily life — in the kitchen, classroom or outdoors — makes revision easier. Writing short notes, drawing labelled diagrams and solving the NCERT in-text and exercise questions for Chapter 2 will help students retain these concepts for unit tests and the annual CBSE examination.
How is Neutralisation — Reactions of Acids and Bases taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Neutralisation — Reactions of Acids and Bases' using an inquiry-based approach with Predict–Observe–Explain activities. Students are asked to make a guess first, then perform a simple experiment with safe, easily available materials, and finally explain what they observed. This matches the NEP 2020 focus on learning by doing. For Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — the textbook includes hands-on tasks, labelled diagrams and questions that build Bloom's Taxonomy skills from Remember (L1) to Create (L6). Teachers use these activities, along with competency-based questions (CBQs) and assertion–reason items, to check real understanding rather than rote memorisation.
What real-life examples of neutralisation can Class 7 students observe at home?
Class 7 students can observe neutralisation at home in many simple ways linked to 'Neutralisation — Reactions of Acids and Bases'. Kitchens, school bags, playgrounds and the night sky are full of examples that connect to NCERT Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. For instance, students can check labels on food and cleaning products, watch changes while cooking, or observe the Sun and Moon across a week. Keeping a small science diary — noting the date, what was observed and a quick sketch — turns everyday life into a science lab. These real-life connections make concepts stick and prepare students well for competency-based questions in CBSE Class 7 Science.
How does 'Neutralisation — Reactions of Acids and Bases' connect to other chapters of Class 7 Science?
'Neutralisation — Reactions of Acids and Bases' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of neutralisation appear again when students study related topics like heat, light, changes, life processes and Earth-Sun-Moon. For example, understanding this subtopic helps in building mental models for later chapters and for Class 8, 9 and 10 Science. Teachers often use cross-chapter questions in CBSE examinations to test whether students can apply what they learned in Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — to new situations. This integrated approach matches the NEP 2020 and NCF 2023 focus on holistic, competency-based learning.
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