Acids, Bases and Salts – Class 7 Science [Full Chapter Notes + Practice Questions]

Key Learning Outcomes

The chapter “Acids, Bases and Salts” from Class 7 Science helps us understand the nature of substances we use daily like lemon juice, soap, baking soda, vinegar, etc. Are they acidic or basic? What happens when acids and bases mix? Let’s explore!

• What are acids, bases, and salts?
• How can we test them?
• What are indicators?
• What is a neutralization reaction?
• Importance and uses in daily life.

Introduction

Everyday, we come across various substances – lemon, vinegar, soap, toothpaste, salt, etc. Some of them taste sour, some bitter, some feel soapy, and others are neutral. These differences are due to their chemical nature – whether they are acidic, basic, or neutral.

What Are Acids?

Acids are substances that are sour in taste and turn blue litmus paper red. They contain hydrogen ions (H⁺).

Examples of Common Acids:

Acid Name	Found In	Nature
Citric acid	Lemon, orange	Weak natural acid
Acetic acid	Vinegar	Used in cooking
Tartaric acid	Tamarind, grapes	Used in baking
Lactic acid	Curd	Causes sourness
Hydrochloric acid (HCl)	Stomach	Strong acid (helps in digestion)

Note:

Strong acids like HCl, H₂SO₄ are corrosive and should be handled carefully.

Acid wale substances khatte taste ke hote hain aur blue litmus ko red bana dete hain.

What Are Bases?

Bases are substances that are bitter in taste and feel soapy to touch. They turn red litmus paper blue. Bases contain hydroxide ions (OH⁻).

Examples of Common Bases:

Base Name	Found In	Nature
Sodium hydroxide (NaOH)	Soaps	Strong base
Calcium hydroxide	Lime water	Mild base
Ammonium hydroxide	Used in cleaning	Mild base
Magnesium hydroxide	Milk of magnesia	Antacid

Bases kadwe taste ke hote hain, sabun jaise feel hote hain, aur red litmus ko blue bana dete hain.

What Are Salts?

Salts are substances formed when an acid reacts with a base. This reaction is called a neutralization reaction.

Acid + Base → Salt + Water

Example:

• HCl + NaOH → NaCl + H₂O (Hydrochloric acid + Sodium hydroxide → Common salt + Water)
• H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O (Sulphuric acid + Calcium hydroxide → Calcium sulfate + Water)

Jab acid aur base react karte hain to salt aur water banta hai. Is reaction ko neutralization kehte hain.

Indicators – How to Test Acids and Bases?

Indicators are substances that change colour when added to acidic or basic solutions. They help us identify the nature of a substance.

Natural Indicators:

Indicator	In Acid	In Base
Litmus (from lichen)	Blue → Red	Red → Blue
Turmeric	No change	Yellow → Reddish-brown
China Rose (Hibiscus)	Light pink	Green

Activity Tip:

• Put some turmeric on a white paper. Drop soap solution → turns reddish-brown.
• Drop lemon juice on the same → no change.

Neutralization Reaction – Very Important

When an acid reacts with a base, it forms salt and water. This process is called neutralization.

Acid + Base → Salt + Water

Examples:

1. Antacid and stomach acid

HCl (acid in stomach) + Mg(OH)₂ (milk of magnesia) → MgCl₂ (salt) + H₂O

2. Soil treatment

Acidic soil + Slaked lime (base) → Neutral soil

3. Bee sting and baking soda

Formic acid from sting + Sodium bicarbonate → Neutralization → Relief

Neutralization mein acid aur base milke ek harmless salt aur water banate hain – daily life mein yeh process bahut useful hai.

Importance and Uses in Daily Life

Situation	Explanation
Indigestion	Excess acid in stomach is neutralized by antacid (milk of magnesia – a base)
Tooth decay	Bacteria produce acids → Toothpaste (basic) neutralizes it
Soil treatment	Acidic soil is treated with quicklime (a base)
Bee sting	Contains formic acid → Apply baking soda (a base) to get relief
Wasps sting (alkaline)	Apply vinegar (acid) to neutralize

What is the pH Scale?

The pH scale is a special scale that measures how acidic or basic a substance is. The full form of pH is “potential of Hydrogen” or “power of Hydrogen ions”.

It tells us the concentration of hydrogen ions (H⁺) in a solution.

pH Scale Range

The pH scale ranges from 0 to 14:

pH Value	Nature of Solution	Example
0 – 6.9	Acidic	Lemon juice, Vinegar
7	Neutral	Pure Water
7.1 – 14	Basic (Alkaline)	Soap, Baking soda, Bleach

Color Indicators for pH

We use special chemicals called indicators to detect pH values:

Universal Indicator

A universal indicator is a mixture of indicators that changes color depending on the pH.

pH Value	Color	Type
1	Red	Strong Acid
3	Orange	Weak Acid
7	Green	Neutral
9	Blue	Weak Base
14	Violet/Purple	Strong Base

Importance of pH in Everyday Life

1. In the Human Body

• Our body works best at pH 7.4 (slightly basic).
• Changes in pH can make us feel sick.

2. In Soil

• Farmers test soil pH to decide the best crops.
• Some plants grow well in acidic soil, while others prefer basic.

3. In the Mouth

• Bacteria create acids that lower the pH in the mouth and cause tooth decay.
• Toothpaste is basic and helps neutralize these acids.

4. In Aquatic Life

• Fish and aquatic animals can only survive in water with pH close to 7.

5. During Acid Rain

• Rainwater normally has pH ~5.6 (slightly acidic).
• Acid rain (with lower pH) harms plants, animals, and buildings.

Summary Table

Property	Acids	Bases	Salts
Taste	Sour	Bitter	No fixed taste
Feel	Corrosive	Soapy	Neutral
Litmus Test	Blue → Red	Red → Blue	No effect
Examples	Lemon juice, vinegar	Soap, milk of magnesia	Sodium chloride, baking soda

Interesting Facts About Integers

1. The word “acid” comes from the Latin word “acidus”, which means sour.
2. Citric acid is found in citrus fruits like lemons, oranges, and amla.
3. Acetic acid is the main component of vinegar.
4. Lactic acid is produced in curd and also in muscles during heavy exercise.
5. Strong acids can burn skin and materials, so they should never be tasted or touched.
6. Bases are soapy to touch and bitter in taste.
7. Sodium hydroxide (NaOH) is called caustic soda and is used in making soap.
8. Magnesium hydroxide is a mild base, also known as milk of magnesia, and is used as an antacid.
9. Bases turn red litmus blue, opposite to acids.
10. Some bases are water-soluble (alkalis), while others are not.
11. Salt is the product of neutralization between an acid and a base.
12. Common salt (NaCl) is essential in our food and body functions.
13. Different combinations of acids and bases form different types of salts.
14. Salts can be neutral, acidic, or basic in nature depending on their components.
15. Litmus is a natural indicator obtained from a plant called lichen.
16. Turmeric, China rose, and red cabbage are natural indicators.
17. A neutralization reaction always produces salt and water.
18. This reaction is used in antacid medicines, soil treatment, and sting relief.
19. Bee stings are acidic in nature; applying baking soda (a base) gives relief.
20. Wasp stings are alkaline; applying lemon juice or vinegar (acids) helps neutralize them.
21. The pH scale (not in Class 7 but interesting) measures how acidic or basic a substance is, from 0 (strong acid) to 14 (strong base).
22. Litmus paper can only tell whether a solution is acidic or basic. But a pH scale can tell how much acidic or basic a solution is. That’s why scientists prefer using the pH scale for accuracy.

Activity (Exercise):

A. Fill in the Blanks:

1. Acids turn __________ litmus __________.
2. Bases are __________ to taste and __________ to touch.
3. The reaction between an acid and a base is called __________.
4. __________ is used to treat indigestion.
5. Litmus is a natural indicator obtained from __________.

Answers

1. blue, red
2. bitter, soapy
3. neutralization
4. Milk of magnesia
5. lichen

B. Match the Following:

Column A	Column B
1. Turmeric	a. Blue → Red
2. Litmus (acid test)	b. Bitter and soapy
3. Vinegar	c. Yellow → Reddish-brown
4. Baking soda	d. Contains acetic acid
5. Base	e. Sodium bicarbonate

Answer

1 → c
2 → a
3 → d
4 → e
5 → b

C. Very Short Answer Questions:

1. What is the taste of acids?
2. Which acid is found in curd?
3. What happens when blue litmus is added to an acid?
4. Name one natural indicator.
5. What is formed in a neutralization reaction?
6. What is the full form of pH?
7. What is the pH of pure water?

Answers

1. Acids are sour in taste.
2. Lactic acid is found in curd.
3. Blue litmus turns red in the presence of an acid.
4. Litmus is a natural indicator obtained from lichen.
5. A salt and water are formed in a neutralization reaction.

6. The full form of pH is “potential of Hydrogen”.

7. The pH of pure water is 7, which is neutral.

D. Short Answer Questions:

1. Define base and give two examples.
2. Explain the role of antacids in our body.
3. What is the use of turmeric as an indicator?
4. Why do we apply baking soda on bee stings?
5. Write two examples of natural acids.
6. Why is pH important for farmers?
7. Which indicator is used to determine the exact pH value?

Answers

1. A base is a substance that is bitter in taste, feels soapy to touch, and turns red litmus blue. Examples: Sodium hydroxide (NaOH) and Magnesium hydroxide (Mg(OH)₂)
2. Our stomach produces hydrochloric acid for digestion. Sometimes, excess acid causes indigestion. Antacids like milk of magnesia (a mild base) neutralize the excess acid and provide relief from indigestion.
3. Turmeric is a natural indicator. It remains yellow in acidic or neutral solutions but turns reddish-brown in basic solutions. It helps in identifying bases.
4. A bee sting injects formic acid into the skin. Baking soda (a base) neutralizes the acid, reducing pain and irritation.
5. Two natural acids are: (I) Citric acid (found in lemon and orange) (II) Acetic acid (found in vinegar)
6. pH is important for farmers because different crops grow best in different pH ranges of soil. By testing the pH of soil, farmers can decide which crop to grow or whether the soil needs to be treated to improve its condition.
7. A universal indicator is used to determine the exact pH value of a solution. It shows different colours for different pH levels from 0 to 14.

E. Long Answer Questions:

1. Differentiate between acids and bases on the basis of taste, touch, litmus test, and examples.
2. Describe the process of neutralization with a labelled chemical equation and real-life examples.
3. Explain the importance of natural indicators. Name any three.
4. How are salts formed? Mention one activity to show this formation.
5. What happens when an acid reacts with a base? Give examples of real-life applications of this reaction.
6. Explain the use of pH scale in daily life with examples.
7. What happens when the pH of rainwater becomes less than 5.6?

Answers

1.

Property	Acids	Bases
Taste	Sour	Bitter
Touch	Corrosive	Soapy
Litmus Test	Turns blue litmus red	Turns red litmus blue
Examples	Lemon juice (citric acid), vinegar (acetic acid)	Soap (sodium hydroxide), milk of magnesia (magnesium hydroxide)

2. When an acid reacts with a base, it forms a salt and water. This reaction is called neutralization.

General Equation:

Acid + Base → Salt + Water

Example:

HCl + NaOH → NaCl + H₂O

Real-life examples:

a) Wasp stings (alkaline) are treated with vinegar (acidic).

b) Antacids neutralize stomach acid.

c) Bee stings (acidic) are treated with baking soda (basic).

3. Natural indicators help us identify whether a substance is acidic or basic by changing color in different media. They are useful for performing safe and easy tests without lab chemicals.

Three natural indicators:

I) China Rose (Hibiscus) – Light pink in acid, Green in base

II) Litmus – Blue in base, Red in acid

III) Turmeric – Yellow in acid, Reddish-brown in base

4. Salts are formed when an acid reacts with a base in a neutralization reaction.

Example Equations

HCl + NaOH → NaCl + H₂O

Activity:

a) Evaporate the water → Salt (NaCl) remains.

b) Take dilute HCl in a beaker.

c) Add dilute NaOH slowly while stirring.

d) Check with litmus – solution becomes neutral.

5. An acid reacts with a base to form a salt and water. This is a neutralization reaction.

Reaction:

Acid + Base → Salt + Water

Applications:

I) Quicklime/slaked lime neutralizes acidic soil in agriculture.

II) Antacids treat indigestion by neutralizing stomach acid.

III) Baking soda neutralizes bee sting acid.

6. The pH scale has many important uses in our daily life:

Acid rain: When pH of rainwater drops below 5.6, it becomes acid rain and can damage plants, buildings, and aquatic life.

Human body: Our body functions properly at pH around 7.4. A change in pH can make us sick.

Mouth health: Bacteria in our mouth produce acids that lower pH and cause tooth decay. Toothpaste is basic and helps to neutralize the acid.

Soil: Farmers test the pH of soil before planting crops. Some crops grow well in acidic soil, while others need basic soil.

Water for aquatic animals: Fishes and other aquatic animals live in water that has a pH close to 7.

7. When the pH of rainwater becomes less than 5.6, it is called acid rain. Acid rain is harmful because:

• It damages leaves and stems of plants.
• It pollutes rivers and lakes, harming aquatic animals.
• It erodes buildings, especially those made of marble (like the Taj Mahal).
• It changes the pH of soil, making it less suitable for farming.

Quiz:

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