Exploring Substances: Acidic, Basic, and Neutral – Class 7 Science Chapter 2 Notes & NCERT PDF

Class 7ScienceChapter 2NCERT book: Curiosity - Science Class 7

Notes and a simple summary of Chapter 2 of the NCERT Class 7 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

🎧 Listen to the content

Chapter 2: Exploring Substances: Acidic, Basic, and Neutral

Introduction

On 28 February (National Science Day) siblings Ashwin and Keerthi visit their school science fair. At the gate they get a blank white sheet; a volunteer sprays a liquid on it and the words "Welcome to the Wonderful World of Science" suddenly appear! At the 'Colourful World of Substances' stall they see colour changes and set off to find out why.

Key Concepts

2.1 Nature — our science laboratory

Litmus as an indicator (Activity 2.1)

Drops of lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution and salt solution are put on pieces of blue and red litmus paper. The samples fall into three groups:

Group A – turn blue litmus red → acidic: lemon juice, amla juice, tamarind water, vinegar

Group B – turn red litmus blue → basic: soap solution, baking soda solution, lime water, washing powder solution

Group C – no change in either → neutral: tap water, sugar solution, salt solution

Litmus is a natural substance obtained from lichens. Because it shows different colours in acidic and basic solutions, it is called an acid-base indicator.

Lichens are formed by the association of a fungus and an alga; they grow on rocks and trees where there is plenty of rain and clean air.

Lime water is a solution of calcium hydroxide, made by mixing lime (chuna, calcium oxide) in water, letting it settle and filtering it. Don't confuse it with the fruit lime!

Taste and touch clues

Acidic substances taste sour. Common acids: citric acid (lemon), ascorbic acid or Vitamin C (amla), tartaric acid (tamarind); curd and vinegar contain lactic and acetic acid.

Basic substances feel soapy/slippery (rub baking soda solution between fingers) and generally taste bitter — but not everything bitter is basic (karela is bitter but not basic).

Caution: never taste an unknown substance.

Red rose as an indicator (Activities 2.3–2.4)

Crush fallen red rose petals, soak in hot water, filter — the filtrate is the indicator. It turns red in acidic solutions and green in basic solutions. Beetroot, purple cabbage, turmeric, jamun and red hibiscus (gudhal) can also act as indicators.

Turmeric as an indicator (Activity 2.5)

Filter paper dipped in turmeric paste and dried = turmeric paper. Its yellow colour turns red in basic solutions and stays yellow in acidic and neutral ones. So turmeric can test bases but cannot tell acidic from neutral. That is why soap turns a curry stain red!

Olfactory indicators (Activity 2.6)

Cloth strips kept overnight with chopped onion change odour when acids or bases are added. Indicators whose smell changes are olfactory indicators — useful for someone who cannot see.

Fascinating fact: Hydrangea flowers (Himalayas and North-east) are blue in acidic soil and pink or red in basic soil.

2.2 Mixing acids and bases (Activity 2.7)

Lemon juice + water + blue litmus solution → red. Adding lime water drop by drop turns it blue — the acid's effect has been cancelled.

Neutralisation: Acid + Base → Salt + Water + Heat

2.3 Neutralisation in daily life

Red ant bite: ants inject formic acid; rubbing moist baking soda (a base) relieves it.

Acidic soil (from too much chemical fertiliser) is treated with lime (a base); basic soil is treated with manure/compost, which release acids.

Acidic factory waste should be neutralised with basic substances before it reaches lakes, to save fish.

Let us wrap up

The welcome message: the words were probably written in soap solution (basic) and sprayed with turmeric solution — the letters turned red!

Key Terms

Acidic — turns blue litmus red; tastes sour (अम्लीय)

Basic — turns red litmus blue; feels soapy (क्षारीय)

Neutral — neither acidic nor basic (उदासीन)

Indicator — shows different colours in acids and bases (सूचक)

Litmus — natural indicator from lichens (लिटमस)

Lichen — fungus + alga living together (लाइकेन)

Olfactory indicator — indicator whose smell changes (गंधीय सूचक)

Neutralisation — acid + base → salt + water + heat (उदासीनीकरण)

Filtrate — liquid obtained after filtering (निस्यंद)

Formic acid — acid injected by ant bites (फॉर्मिक अम्ल)

Know a Scientist

Acharya Prafulla Chandra Ray — the Father of Modern Indian Chemistry. He set up India's first pharmaceutical company in 1901, wrote on the history of chemistry in India, and supported teaching in the mother tongue.

Common Misconceptions

"All bitter things are bases." — Karela is bitter but not basic.

"Turmeric can identify acids." — It only changes colour with bases.

"Neutral means no chemicals." — Salt solution and sugar solution are neutral substances.

Confusing lime water (calcium hydroxide solution) with lime juice.

Keep the Curiosity Alive (exercise hints)

A solution that turns red litmus blue is basic; adding excess vinegar (acidic) reverses the change.

A basic solution turns red litmus blue, turmeric red and rose extract green.

A blindfolded person should use an olfactory indicator (onion strips).

To reveal a message written in orange juice (acidic), use an indicator such as blue litmus or rose extract — not turmeric.

Vinegar, baking soda solution and sugar solution: turmeric identifies only the base; you need another test to separate the other two.

💡 Key Learning Points

✓Substances can be classified as acidic, basic or neutral using acid-base indicators.
✓Acids turn blue litmus red; bases turn red litmus blue; neutral substances change neither.
✓Natural indicators: litmus (lichens), red rose (red in acid, green in base), turmeric (red only in base), onion (olfactory).
✓Acid + Base → Salt + Water + Heat is called neutralisation.
✓Neutralisation solves daily problems: ant bites, acidic/basic soil and acidic factory waste.

👨‍🏫 Teaching Tips

→Recreate the science-fair 'magic message': write with soap solution on turmeric paper and let students guess the trick.
→Do Activity 2.1 as a group sort — each group records colours and builds Groups A, B and C on the board.
→Collect fallen rose petals, hibiscus or purple cabbage and let groups prepare their own indicators.
→Stress safety: never taste unknown substances; hot water steps only with an adult.
→Show neutralisation with blue litmus + lemon juice and lime water drop by drop; ask students to predict each colour.
→Discuss the ant bite and soil examples; invite students to share local remedies.
→Introduce P.C. Ray and discuss why learning in the mother tongue matters.

📋 Assessment Questions

Can the student classify a set of household substances as acidic, basic or neutral using litmus?
Can the student explain why turmeric cannot separate acidic and neutral substances?
Can the student write the word equation for neutralisation and give a daily-life example?
Can the student choose a suitable indicator for a given situation (e.g. a blindfolded tester)?
Can the student explain the 'Welcome' message trick at the science fair?

Ask the AI tutor about Class 7 Science Chapter 2

Get question answers, important questions, a quick MCQ quiz or a simple explanation of “Exploring Substances: Acidic, Basic, and Neutral”. Free — sign in with Google.

How the AI tutor works →

👩‍🏫 Class 7 Science classes near you — online or at home

Prefer a real teacher? TalentJR teachers take live online classes anywhere in India and visit homes in selected cities. Tell us what you need and our team will call you with timings and fees.

Live online classesSmall groups or 1:1 with a teacher, from home, any city.
Teacher at homeA TalentJR teacher visits your home (selected cities).
At a centreClasses at a TalentJR partner centre near you, where available.

Keep learning

← Previous: Chapter 1The Ever-Evolving World of Science Next: Chapter 3 →Electricity: Circuits and their Components