Acids, Bases and Salts - pH scale, properties of acids/bases, preparation of washing soda, baking soda, and plaster of paris

Welcome, students! Acids, bases, and salts are all around us—from the sour lemon juice in your kitchen to the antacid tablet you take for stomach relief, and even the plaster cast used to heal a fractured bone.

In this comprehensive guide, we will break down the essential concepts from NCERT Class 10 Science Chapter 2: Acids, Bases and Salts. We will cover chemical properties, the pH scale, and the step-by-step preparation of essential salts like baking soda, washing soda, and plaster of paris.

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1. Physical & Chemical Properties of Acids and Bases

Before diving into complex reactions, let's establish a clear picture of what acids and bases actually are.

What are Acids and Bases?

Acids are substances that release hydrogen ions ($H^+$ or $H_3O^+$ hydronium ions) when dissolved in water. They turn blue litmus red and taste sour.

Bases are substances that release hydroxide ions ($OH^-$) in water. Bases that dissolve in water are called alkalis (e.g., $NaOH$, $KOH$). They turn red litmus blue, taste bitter, and feel slippery/soapy to the touch.

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Indicators: Chemical Detectives

Indicators change color or smell when exposed to acidic or basic solutions.

Indicator Type	Indicator	Color/Smell in Acid	Color/Smell in Base
Natural	Litmus Paper	Red	Blue
Natural	Turmeric	Yellow (No change)	Reddish-Brown
Synthetic	Phenolphthalein	Colorless	Pink
Synthetic	Methyl Orange	Red	Yellow
Olfactory	Vanilla / Onion	Retains smell	Loses smell

💡 Teacher’s Tip: Olfactory indicators are extremely useful for visually impaired students because they rely on the sense of smell rather than sight!

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Key Chemical Reactions

1. Reaction with Metals

When an acid reacts with a active metal, it produces salt and hydrogen gas.

$$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen Gas}$$

$$\text{Zn (s)} + 2\text{HCl (aq)} \rightarrow \text{ZnCl}_2\text{ (aq)} + \text{H}_2\text{ (g)} \uparrow$$

Pop Test: Bring a burning candle near the escaping gas. If it burns with a "pop" sound, the gas is confirmed to be Hydrogen ($H_2$).

2. Reaction of Acids with Metal Carbonates & Hydrogen Carbonates

Acids react with carbonates ($CO_3^{2-}$) and hydrogen carbonates ($HCO_3^-$) to yield salt, water, and carbon dioxide gas.

$$\text{Metal Carbonate/Bicarbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide}$$

$$\text{Na}_2\text{CO}_3\text{ (s)} + 2\text{HCl (aq)} \rightarrow 2\text{NaCl (aq)} + \text{H}_2\text{O (l)} + \text{CO}_2\text{ (g)} \uparrow$$

Lime Water Test: Pass the $CO_2$ gas through lime water ($Ca(OH)_2$). It turns milky due to the formation of an insoluble precipitate, calcium carbonate ($CaCO_3$).

3. Neutralization Reaction

When an acid and a base react together, they neutralize each other's effects to form salt and water.

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

$$\text{HCl (aq)} + \text{NaOH (aq)} \rightarrow \text{NaCl (aq)} + \text{H}_2\text{O (l)}$$

4. Reaction of Metallic and Non-Metallic Oxides

Metallic oxides are basic in nature (e.g., $CuO + 2HCl \rightarrow CuCl_2 + H_2O$).

Non-metallic oxides are acidic in nature (e.g., $CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O$).

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2. The pH Scale: Measuring Solution Strength

What is the pH Scale?

The term pH stands for *“potenz”* (German for power) of Hydrogen. It measures the concentration of hydrogen ions ($H^+$) present in a solution.

$$\text{Higher } H^+ \text{ ion concentration} \implies \text{Lower pH value (More Acidic)}$$

$$\text{Lower } H^+ \text{ ion concentration} \implies \text{Higher pH value (More Basic)}$$

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Acidic Range Neutral Basic (Alkaline) Range

0 <------------------------ 7 ------------------------> 14

[High H+ ions] [Equal] [High OH- ions]

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pH < 7: Acidic solution

pH = 7: Neutral solution (Pure water)

pH > 7: Basic solution

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Importance of pH in Everyday Life

Plants and Animals are pH Sensitive: Human bodies function optimumly within a narrow pH range of 7.0 to 7.8. When rain pH drops below 5.6, it is called acid rain, which lowers river water pH and harms aquatic life.

pH in our Digestive System: Stomach cells produce hydrochloric acid ($HCl$) to help digest food. Overeating causes excess acid production (acidity). We take antacids like Milk of Magnesia ($Mg(OH)_2$) to neutralize the excess acid.

Tooth Decay caused by pH Changes: Bacteria in our mouth break down sugar remnants and produce acids. When mouth pH drops below 5.5, tooth enamel (made of calcium hydroxyapatite) begins to corrode.

Self-Defense by Animals and Plants: Honeybee stings inject methanoic acid (formic acid), causing pain and swelling. Applying a mild base like baking soda provides relief. Nettle plant leaves inject methanoic acid on contact, but nature provides a remedy nearby: rubbing leaves of the dock plant neutralizes the pain!

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3. Important Chemical Compounds from Common Salt

Sodium Chloride ($NaCl$) is not just table salt—it is a raw material for synthesizing several crucial chemical compounds.

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Compound 1: Baking Soda

Chemical Name: Sodium Hydrogen Carbonate / Sodium Bicarbonate

Chemical Formula: $\text{NaHCO}_3$

Method of Preparation (Solvay Process)

Baking soda is prepared on an industrial scale by reacting cold, concentrated sodium chloride solution (brine) with ammonia ($NH_3$) and carbon dioxide ($CO_2$).

$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

Ammonium Chloride: $\text{NH}_4\text{Cl}$

Sodium Hydrogen Carbonate: $\text{NaHCO}_3$

Action of Heat

When cooking, baking soda is heated and undergoes thermal decomposition:

$$2\text{NaHCO}_3 \xrightarrow{\text{Heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \uparrow$$

The released $CO_2$ gas forms bubbles in dough, making cakes and bread soft and spongy.

Key Uses

Making Baking Powder: Baking powder is a mixture of baking soda and a mild edible acid like tartaric acid. When mixed with water/heated, the acid reacts with sodium bicarbonate, releasing $CO_2$ without leaving a bitter taste of sodium carbonate ($\text{Na}_2\text{CO}_3$).

Antacid: Being mildly alkaline, it neutralizes stomach acidity.

Soda-Acid Fire Extinguishers: Releasing $CO_2$ cuts off the oxygen supply to put out fires.

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Compound 2: Washing Soda

Chemical Name: Sodium Carbonate Decahydrate

Chemical Formula: $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$

💡 What is Water of Crystallization?

It is the fixed number of water molecules bound inside one formula unit of a salt crystal. The $10\text{H}_2\text{O}$ keeps the crystal structure intact; it does not make the salt wet!

Method of Preparation (3-Step Process)

Production of Baking Soda:

$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$

Thermal Decomposition: Heating baking soda yields anhydrous sodium carbonate (Soda Ash).

$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$

Recrystallization: Dissolving soda ash in water and recrystallizing it yields washing soda.

$$\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$$

Key Uses

Used in glass, soap, and paper industries.

Used in manufacturing sodium compounds like Borax ($\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$).

Used as a cleaning agent for domestic laundry.

Removes permanent hardness of water by precipitating out dissolved calcium and magnesium ions.

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Compound 3: Plaster of Paris (POP)

Chemical Name: Calcium Sulphate Hemihydrate

Chemical Formula: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ *(or $2\text{CaSO}_4 \cdot \text{H}_2\text{O}$)*

Method of Preparation

Plaster of Paris is produced by carefully heating Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) at $373\text{ K}$ ($100^\circ\text{C}$).

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}$$

⚠️ Crucial Precaution: If heated above $373\text{ K}$, gypsum loses all its water of crystallization to form Anhydrous Calcium Sulphate ($\text{CaSO}_4$), known as "Dead Burnt Plaster", which loses the property of setting into a hard mass upon adding water.

Rehydration Reaction (Setting of POP)

When mixed with water, POP rehydrates back into hard Gypsum within 10 to 15 minutes:

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \text{ (Gypsum - Hard solid mass)}$$

Key Uses

Used by doctors as a plaster cast to support fractured bones in position.

Used for making decorative items, toys, statues, and smooth wall finishes.

Used as a fireproofing material.

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4. Practice Questions with Detailed Solutions

Here are 3 board-exam-style questions to test your understanding!

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Question 1 (Conceptual - pH Scale & Ion Concentration)

Five solutions A, B, C, D, and E show pH values of 4, 1, 11, 7, and 9 respectively when tested with a universal indicator.

Which solution is:

(a) Neutral?

(b) Strongly alkaline?

(d) Weakly acidic?

(e) Weakly alkaline?

Arrange the pH values in increasing order of Hydrogen ion ($H^+$) concentration.

Solution:

Identifying solution nature based on pH:

Neutral (pH = 7): Solution D

Strongly alkaline (pH = 11): Solution C

Strongly acidic (pH = 1): Solution B

Weakly acidic (pH = 4): Solution A

Weakly alkaline (pH = 9): Solution E

Arranging in increasing order of $H^+$ concentration:

*Concept:* Lower pH means higher $H^+$ ion concentration. Conversely, a higher pH means a lower $H^+$ ion concentration.

Increasing order of $H^+$ concentration: Lowest $H^+$ concentration $\rightarrow$ Highest $H^+$ concentration

$$\text{pH 11 (C)} < \text{pH 9 (E)} < \text{pH 7 (D)} < \text{pH 4 (A)} < \text{pH 1 (B)}$$

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Question 2 (Chemical Synthesis & Problem Solving)

A chemical compound 'X' of calcium is widely used in hospitals for setting fractured bones.

Identify the compound 'X' and write its chemical formula.

How is this compound prepared? Write the balanced chemical equation.

What happens when 'X' is mixed with water? Express the chemical reaction taking place.

Solution:

Identification:

Compound 'X' is Plaster of Paris (Calcium Sulphate Hemihydrate).

Chemical Formula: $\mathbf{\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}}$

Preparation:

It is prepared by heating Gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) at $373\text{ K}$ ($100^\circ\text{C}$).

Chemical Equation:

$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O}$$

Reaction with Water:

On mixing with water, Plaster of Paris absorbs water molecules and sets into a hard solid mass called Gypsum.

Chemical Equation:

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$

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Question 3 (Application-Based Reasoning)

While baking a cake, standard baking soda was used instead of baking powder. Explain:

How will this affect the taste of the cake and why?

How can this issue be corrected? Explain the chemistry behind the solution.

Solution:

Effect on Taste:

The cake will taste bitter.

Reason: When baking soda ($\text{NaHCO}_3$) is heated during baking, it decomposes to form sodium carbonate ($\text{Na}_2\text{CO}_3$), water, and carbon dioxide gas:

$$2\text{NaHCO}_3 \xrightarrow{\text{Heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \uparrow$$

Sodium carbonate ($\text{Na}_2\text{CO}_3$) is a basic salt, and basic substances are bitter in taste.

Correction:

Instead of baking soda, baking powder should be used.

Chemistry: Baking powder is a mixture of baking soda ($\text{NaHCO}_3$) and a mild edible acid like tartaric acid.

When water is added or the batter is heated, the hydrogen ions ($H^+$) from tartaric acid react with sodium bicarbonate, neutralizing the basic sodium carbonate to produce a pleasant sodium salt of tartaric acid, preventing any bitter taste.

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🌟 Quick Summary Mind-Map

Acids: Sour, $H^+$ ions, Blue $\rightarrow$ Red litmus, pH < 7.

Bases: Bitter/Slippery, $OH^-$ ions, Red $\rightarrow$ Blue litmus, pH > 7.

Baking Soda: $\text{NaHCO}_3$ (Used in baking powder & antacids).

Washing Soda: $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ (Used for cleaning & removing water hardness).

Plaster of Paris: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$ (Formed by heating gypsum at $373\text{ K}$; sets back to hard gypsum with water).