⚗️ Grade 9 Science · Chemistry

Samacheer Class 9 Science - Acids, Bases and Salts

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Complete Grade 9 Science questions and answers for Acids, Bases and Salts. Click any question to expand the answer.

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Acids, Bases and Salts — key concepts & quick answers

What is the difference between an acid and a base?
An acid releases hydrogen ions (H⁺) in water and turns blue litmus red; a base releases hydroxide ions (OH⁻) in water and turns red litmus blue.
What is the pH scale?
The pH scale measures how acidic or basic a solution is, ranging from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic.
What is a neutralisation reaction?
A neutralisation reaction is the reaction between an acid and a base to form a salt and water. Example: HCl + NaOH → NaCl + H₂O.
What are indicators? Give examples.
Indicators are substances that reveal whether a solution is acidic or basic by changing colour — for example litmus, methyl orange and phenolphthalein.
What is a salt?
A salt is a compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion, usually produced in a neutralisation reaction (e.g. sodium chloride, NaCl).
📋 Sections in this chapter
I. Choose the Correct AnswerII. Answer BrieflyIII. Answer in DetailIntext ActivitiesActivity – 2Activity – 3Activity – 4Activity – 5
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1I. Choose the Correct Answer6 questions

Q.1Zn + 2 HCl → ZnCl2 + …↑(H2, O2, CO2)
✓ Answer
$$Zn + 2HCl \rightarrow ZnCl_2 + ___\uparrow$$

Options:

  • (H_2)
  • (O_2)
  • (CO_2)

Answer:

$$H_2$$

Q.2Apple contains malic acid. Orange contains
✓ Answer
  • citric acid
  • ascorbic acid

Answer:
Citric acid

Orange also contains ascorbic acid, but the characteristic organic acid in orange is citric acid.

Q.3Acids present in plants and animals are organic acids, whereas acids in rocks and minerals are ______.

Answer: Inorganic acids

Q.4Acids turn blue litmus paper to
✓ Answer
  • green
  • red
  • orange

Answer:
Red


Q.5Metal carbonates and bicarbonates react with acids to liberate
✓ Answer
  • (NO_2)
  • (SO_2)
  • (CO_2)

Answer:

$$CO_2$$

Q.6Hydrated copper sulphate is ______.

Answer: Blue


2II. Answer Briefly9 questions

Q.1Classify Acids Based on Source
✓ Answer

Organic Acids

Acids present in plants and animals.

Examples:

  • Citric acid
  • Acetic acid
  • Lactic acid

Inorganic Acids

Acids obtained from rocks and minerals.

Examples:

  • Hydrochloric acid
  • Sulphuric acid
  • Nitric acid

Q.2Uses of Acids
✓ Answer

Sulphuric Acid ((H_2SO_4))

  • Called King of Chemicals
  • Used in batteries
  • Used in manufacture of chemicals

Hydrochloric Acid ((HCl))

  • Used as cleaning agent

Citric Acid

  • Food preservative
  • Effervescent salts

Nitric Acid ((HNO_3))

  • Fertilizers
  • Dyes
  • Paints
  • Explosives

Oxalic Acid

  • Removes stains
  • Bleaching agent

Carbonic Acid

  • Used in aerated drinks

Tartaric Acid

  • Used in baking powder

Q.3Significance of Soil pH in Agriculture
✓ Answer

Answer:

Different crops require different soil pH.

CropPreferred Soil
RiceSlightly acidic (pH 5.5-6.5)
SugarcaneNeutral to slightly alkaline (pH 6.0-7.5)
Citrus fruitsSlightly acidic (pH 5.5-6.5)

Soil pH affects nutrient availability and crop growth.


Q.4Uses of Aqua Regia
✓ Answer

Aqua Regia

Mixture of:

$$HCl : HNO_3 = 3 : 1$$

Uses

  • Dissolves gold and platinum
  • Cleaning and refining gold

Q.5Uses of Plaster of Paris
✓ Answer

Uses

  • Plastering fractured bones
  • Making statues and casts

Q.6Identify Acids A and B
✓ Answer

Acid A

Gives one hydrogen ion per molecule.

Answer:

$$HCl$$

(Hydrochloric acid)


Acid B

Gives two hydrogen ions per molecule.

Answer:

$$H_2SO_4$$

(Sulphuric acid)


King of Chemicals

$$H_2SO_4$$

Sulphuric acid


Q.7Define Aqua Regia
✓ Answer

Answer:

Aqua regia is freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in ratio:

$$3 : 1$$

Q.8Correct the Mistakes
✓ Answer

(a) Incorrect statement: Washing soda is used for making cakes soft. Correct statement: Baking soda, which is sodium bicarbonate (NaHCO₃), is used for making cakes and bread soft and spongy. When baking soda is heated during baking, it decomposes to produce carbon dioxide gas, which creates bubbles in the batter and makes the final product light and fluffy. Washing soda, on the other hand, is sodium carbonate (Na₂CO₃) and is used for cleaning purposes. (b) Incorrect statement: Calcium sulphate hemihydrate is used in bleaching. Correct statement: Bleaching powder, which is calcium hypochlorite (CaOCl₂), is used in textile bleaching and water purification. Bleaching powder contains hypochlorite ions that have strong oxidizing properties, which remove color from fabrics and kill microorganisms in water.

Q.9Define Neutralization Reaction
✓ Answer

Answer:

Reaction between acid and base producing salt and water is called neutralization.

$$HCl + NaOH \rightarrow NaCl + H_2O$$


3III. Answer in Detail5 questions

Q.1Difference Between Hydrated and Anhydrous Salts
✓ Answer
Hydrated SaltsAnhydrous Salts
Contain water of crystallizationDo not contain water
Usually colouredUsually white or colourless
Lose water on heatingAbsorb water from atmosphere
Example: (CuSO_4.5H_2O)Example: (CuSO_4)
Correction made:
NaCl is not best example of anhydrous salt in this context.

Q.2Tests for Acids and Bases
✓ Answer
IndicatorAcidBase
Blue litmusTurns redNo change
Red litmusNo changeTurns blue
PhenolphthaleinColourlessPink
Methyl orangeRedYellow
Correction made:
Original answer incorrectly mentioned “pink” for acid in methyl orange test.

Q.3Uses of Bases
✓ Answer

Sodium Hydroxide ((NaOH))

  • Soap manufacture

Calcium Hydroxide ((Ca(OH)_2))

  • White washing

Magnesium Hydroxide ((Mg(OH)_2))

  • Antacid medicine

Ammonium Hydroxide ((NH_4OH))

  • Removes grease stains

Q.4Uses of Salts
✓ Answer

# Common Salt ((NaCl))

  • Food seasoning
  • Preservative

# Washing Soda ((Na_2CO_3))

  • Softens hard water
  • Glass and soap industry

# Baking Soda ((NaHCO_3))

  • Baking powder
  • Fire extinguishers
  • Antacid

# Bleaching Powder ((CaOCl_2))

  • Disinfectant
  • Bleaching agent

# Plaster of Paris ((CaSO_4 \cdot \frac{1}{2}H_2O))

  • Bone plaster
  • Statues

Q.5Why is Sulphuric Acid Called King of Chemicals?
✓ Answer

Sulphuric acid is called the King of Chemicals because of its extensive and diverse applications in the chemical and industrial sectors. Sulphuric acid is widely used in the manufacture of numerous other chemicals and industrial products, making it one of the most important chemicals in the world. It is used in the production of fertilizers, particularly in the manufacture of superphosphate fertilizers. Sulphuric acid is also essential in the refining of petroleum, the production of dyes and pigments, the manufacture of explosives, and in metal processing industries. Additionally, it is used in the production of plastics, synthetic fibers, and many other organic and inorganic compounds. The sheer volume of sulphuric acid produced and consumed globally, combined with its critical role in numerous industrial processes, justifies its designation as the King of Chemicals. Without sulphuric acid, modern industrial production would be severely hampered, making it indispensable to the chemical industry.


4Intext Activities0 questions


5Activity – 20 questions

Electrical Conductivity of Acids

Observation:

Bulb glows in:

  • Hydrochloric acid
  • Sulphuric acid

Bulb does not glow in:

  • Glucose solution
  • Alcohol solution

Conclusion:

Acids ionize in water and conduct electricity.



6Activity – 30 questions

Litmus and Indicator Test

SubstanceBlue LitmusRed LitmusPhenolphthaleinMethyl Orange
AcidRedNo changeColourlessRed
BaseNo changeBluePinkYellow


7Activity – 40 questions

Water of Crystallization

Examples:

SaltWater of Crystallization
(CuSO_4 \cdot 5H_2O)5
(Na_2CO_3 \cdot 10H_2O)10
(CaSO_4 \cdot \frac{1}{2}H_2O)1/2


8Activity – 50 questions

Deposits After Evaporation

Observation:

White deposits remain after boiling groundwater.

Conclusion:

Groundwater contains dissolved salts.


# Important Summary


Acids

  • Sour taste
  • Turn blue litmus red

Bases

  • Bitter and soapy
  • Turn red litmus blue

Salts

Products of acid-base reactions.


Neutralization

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

pH Scale

  • Acidic → below 7
  • Neutral → 7
  • Basic → above 7

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