Understanding The Chemical Properties Of Acids And Bases

Acids and bases exhibit characteristic chemical behaviours which can be studied through their reactions with various substances.

---

Acids And Bases In The Laboratory

Common laboratory acids include hydrochloric acid (HCl), sulphuric acid (H$_2$SO$_4$), nitric acid (HNO$_3$), and acetic acid (CH$_3$COOH). Common laboratory bases (alkalis) include sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)$_2$], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)$_2$], and ammonium hydroxide (NH$_4$OH).

Indicators are used to identify whether a solution is acidic or basic by showing a change in colour or odour.

• **Colour Indicators:** Litmus (red and blue), phenolphthalein, methyl orange, turmeric, red cabbage extract, flower petals.
• **Olfactory Indicators:** Substances whose odour changes in acidic or basic media. Examples include onion, vanilla essence, and clove oil. The smell of onions is diminished or disappears in a basic solution (like NaOH) but remains detectable in an acidic solution (like HCl). Vanilla essence and clove oil lose their characteristic smell in basic solutions but retain it in acidic solutions.

Answer:

---

How Do Acids And Bases React With Metals?

**Acids react with most metals** to produce a corresponding **salt** and **hydrogen gas**.

General reaction:

Acid + Metal $\to$ Salt + Hydrogen gas

Example: Reaction between dilute sulphuric acid and zinc granules:

Zn(s) + H$_2$SO$_4$(aq) $\to$ ZnSO$_4$(aq) + H$_2$(g)

(Zinc) (Sulphuric acid) (Zinc sulphate) (Hydrogen gas)

When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved, seen as bubbles around the zinc granules. This gas, when passed through soap solution, forms bubbles. When a burning candle is brought near these bubbles, the hydrogen gas burns with a pop sound, which is a characteristic test for hydrogen.

Different acids (like HCl, HNO$_3$, CH$_3$COOH) also react with metals to produce hydrogen gas, but the reactivity varies depending on the metal and the acid (e.g., dilute nitric acid usually does not produce hydrogen with metals due to its strong oxidising nature, except for very dilute solutions with Mg or Mn).

**Bases react with some metals** to produce salt and hydrogen gas, but this is not as common as with acids.

Example: Reaction between sodium hydroxide solution and zinc granules:

2NaOH(aq) + Zn(s) $\to$ Na$_2$ZnO$_2$(aq) + H$_2$(g)

(Sodium hydroxide) (Zinc) (Sodium zincate) (Hydrogen gas)

Hydrogen gas is again produced. However, not all metals react with bases in this manner.

---

How Do Metal Carbonates And Metal Hydrogencarbonates React With Acids?

**Acids react with metal carbonates and metal hydrogencarbonates** to produce a corresponding **salt**, **carbon dioxide gas**, and **water**.

General reaction:

Metal carbonate + Acid $\to$ Salt + Carbon dioxide + Water

Metal hydrogencarbonate + Acid $\to$ Salt + Carbon dioxide + Water

Example 1: Reaction between sodium carbonate and dilute hydrochloric acid:

Na$_2$CO$_3$(s) + 2HCl(aq) $\to$ 2NaCl(aq) + CO$_2$(g) + H$_2$O(l)

(Sodium carbonate) (Hydrochloric acid) (Sodium chloride) (Carbon dioxide) (Water)

Example 2: Reaction between sodium hydrogencarbonate and dilute hydrochloric acid:

NaHCO$_3$(s) + HCl(aq) $\to$ NaCl(aq) + CO$_2$(g) + H$_2$O(l)

(Sodium hydrogencarbonate) (Hydrochloric acid) (Sodium chloride) (Carbon dioxide) (Water)

The carbon dioxide gas evolved can be tested by passing it through lime water (calcium hydroxide solution). Carbon dioxide reacts with lime water to form calcium carbonate, a white precipitate, which makes the lime water milky or cloudy.

Ca(OH)$_2$(aq) + CO$_2$(g) $\to$ CaCO$_3$(s) + H$_2$O(l)

(Calcium hydroxide) (Carbon dioxide) (Calcium carbonate) (Water)

If excess carbon dioxide is passed through lime water, the calcium carbonate precipitate reacts further with carbon dioxide and water to form calcium hydrogencarbonate, which is soluble in water, causing the milky appearance to disappear.

CaCO$_3$(s) + H$_2$O(l) + CO$_2$(g) $\to$ Ca(HCO$_3$)$_2$(aq)

(Calcium carbonate) (Water) (Carbon dioxide) (Calcium hydrogencarbonate - soluble)

Limestone, chalk, and marble are all different forms of calcium carbonate and react with acids similarly to produce CO$_2$.

---

How Do Acids And Bases React With Each Other? (Neutralisation Reaction)

**Acids and bases react with each other** to form a **salt** and **water**. This type of reaction is called a **neutralisation reaction** because the acid neutralizes the effect of the base, and vice versa.

General reaction:

Acid + Base $\to$ Salt + Water

Example: Reaction between sodium hydroxide (a base) and hydrochloric acid (an acid):

NaOH(aq) + HCl(aq) $\to$ NaCl(aq) + H$_2$O(l)

(Sodium hydroxide) (Hydrochloric acid) (Sodium chloride) (Water)

If phenolphthalein indicator (pink in basic solution, colourless in acidic/neutral solution) is added to a base like NaOH, the solution is pink. Adding dilute HCl drop by drop neutralizes the base, and as it becomes neutral or slightly acidic, the pink colour disappears. Adding a few drops of NaOH again makes it basic, and the pink colour reappears, demonstrating the neutralization process.

---

Reaction Of Metallic Oxides With Acids

**Metallic oxides** are compounds formed between a metal and oxygen (e.g., copper oxide, magnesium oxide). **Metallic oxides react with acids** to produce **salt** and **water**.

General reaction:

Metal oxide + Acid $\to$ Salt + Water

Example: Reaction between copper(II) oxide (black) and dilute hydrochloric acid:

CuO(s) + 2HCl(aq) $\to$ CuCl$_2$(aq) + H$_2$O(l)

(Copper(II) oxide) (Hydrochloric acid) (Copper(II) chloride) (Water)

When copper oxide is added to dilute HCl, the black solid dissolves, and the solution turns blue-green due to the formation of copper(II) chloride. This reaction is similar in form to the reaction between a base and an acid (neutralization reaction). Since metallic oxides react with acids to produce salt and water, they are considered **basic oxides**.

---

Reaction Of A Non-Metallic Oxide With Base

**Non-metallic oxides** are compounds formed between a non-metal and oxygen (e.g., carbon dioxide, sulphur dioxide). **Non-metallic oxides react with bases** to produce **salt** and **water**.

Example: Reaction between carbon dioxide and calcium hydroxide (lime water):

Ca(OH)$_2$(aq) + CO$_2$(g) $\to$ CaCO$_3$(s) + H$_2$O(l)

(Calcium hydroxide) (Carbon dioxide) (Calcium carbonate) (Water)

This reaction between a base (calcium hydroxide) and a non-metallic oxide (carbon dioxide) producing salt and water is similar to the reaction between a base and an acid. This similarity indicates that non-metallic oxides are **acidic in nature**.

Answer:

Answer:

Answer:

What Do All Acids And All Bases Have In Common?

Despite their differences in strength and properties, all acids share some fundamental characteristics, and all bases share others. This similarity is due to the types of ions they produce when dissolved in water.

All acids contain hydrogen atoms. When acids dissolve in water, they produce **hydrogen ions (H$^+$)**. It is the presence of these H$^+$ ions (specifically, hydrated H$^+$ ions) that is responsible for the acidic properties of acids.

Similarly, bases contain hydroxide groups (OH). When bases dissolve in water, they produce **hydroxide ions (OH$^-$)**. The presence of these OH$^-$ ions is responsible for the basic properties of bases.

---

What Happens To An Acid Or A Base In A Water Solution?

Acids produce ions when dissolved in water, and it is the movement of these ions that allows acid solutions to conduct electricity. Experiments show that solutions of acids (like HCl, H$_2$SO$_4$) conduct electricity, causing a bulb in a circuit to glow. However, solutions of compounds containing hydrogen but not classified as acids (like glucose and alcohol) do not conduct electricity. This indicates that acids produce mobile charged particles (ions) in water, while glucose and alcohol do not.

Acids, such as HCl, donate a proton (H$^+$) to water molecules when dissolved. The H$^+$ ion cannot exist alone; it immediately combines with a water molecule (H$_2$O) to form a **hydronium ion (H$_3$O$^+$)**.

HCl(aq) + H$_2$O(l) $\to$ H$_3$O$^+$(aq) + Cl$^-$(aq)

Alternatively, the acidic proton is represented as H$^+$(aq), implying it is hydrated.

So, acids produce H$^+$(aq) or H$_3$O$^+$(aq) ions in water. It is these hydronium ions (or hydrated hydrogen ions) that cause the acidic properties.

Bases also dissociate in water to produce ions, specifically hydroxide ions (OH$^-$).

• NaOH(s) $\xrightarrow{\text{H}_2\text{O}}$ Na$^+$(aq) + OH$^-$(aq)
• KOH(s) $\xrightarrow{\text{H}_2\text{O}}$ K$^+$(aq) + OH$^-$(aq)
• Mg(OH)$_2$(s) $\xrightarrow{\text{H}_2\text{O}}$ Mg$^{2+}$(aq) + 2OH$^-$(aq)

Bases that are soluble in water are called **alkalis**. Alkalis are soapy to touch, bitter, and corrosive.

The neutralisation reaction can be viewed as the combination of hydrogen ions from the acid and hydroxide ions from the base to form water:

H$^+$(aq) + OH$^-$(aq) $\to$ H$_2$O(l)

Dissolving an acid or a base in water is typically a highly exothermic process, releasing significant heat. It is important to add concentrated acids (especially sulphuric acid or nitric acid) slowly to water with constant stirring, and never add water to the concentrated acid. This prevents excessive heat build-up and splashing. This process of mixing an acid or a base with water is called **dilution**, which decreases the concentration of H$_3$O$^+$ or OH$^-$ ions per unit volume.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

How Strong Are Acid Or Base Solutions?

The strength of an acid or base solution is determined by the concentration of H$^+$ ions (or H$_3$O$^+$ ions) it produces in water. A universal indicator, which is a mixture of several indicators, can show different colours corresponding to different concentrations of hydrogen ions in a solution. This allows us to quantitatively assess the strength.

A **pH scale** has been developed to measure the concentration of hydrogen ions in a solution. The 'p' in pH stands for 'potenz' (German for power). The pH scale generally ranges from 0 to 14.

• **pH = 7:** Represents a **neutral solution** (e.g., pure water).
• **pH < 7:** Represents an **acidic solution**. Lower the pH value (closer to 0), higher the concentration of H$^+$ ions, and stronger the acid.
• **pH > 7:** Represents a **basic (alkaline) solution**. Higher the pH value (closer to 14), lower the concentration of H$^+$ ions (and higher the concentration of OH$^-$ ions), and stronger the base.

The strength of an acid (or base) is not just about its concentration in the solution, but how completely it dissociates to produce H$^+$ (or OH$^-$) ions. **Strong acids** (e.g., HCl, H$_2$SO$_4$, HNO$_3$) dissociate almost completely in water, producing a high concentration of H$^+$ ions. **Weak acids** (e.g., acetic acid, carbonic acid) dissociate only partially, producing a lower concentration of H$^+$ ions for the same initial concentration. Similarly, strong bases dissociate completely to produce OH$^-$ ions, while weak bases dissociate partially.

---

Importance Of PH In Everyday Life

pH plays a vital role in many biological and environmental processes.

• **Living Organisms:** Our bodies and those of other living beings function optimally within a narrow pH range (typically 7.0 to 7.8 in humans). Significant changes in pH can disrupt biological processes.
• **Acid Rain:** Rainwater with a pH below 5.6 is considered acid rain. It is caused by atmospheric pollutants like sulphur dioxide and nitrogen oxides. Acid rain flowing into rivers and lakes lowers their pH, making it difficult for aquatic life to survive.
• **Soil pH:** Plants require a specific pH range in the soil for healthy growth and nutrient absorption. Soil pH can vary and might need adjustment (e.g., adding quicklime or slaked lime to acidic soil) for optimal crop yield.
• **Digestive System:** Our stomach produces hydrochloric acid, which is essential for digesting food. During indigestion, excess acid production can cause pain. Antacids (mild bases like magnesium hydroxide) are taken to neutralize this excess acid.
• **Tooth Decay:** Tooth decay begins when the pH in the mouth drops below 5.5. Bacteria in the mouth break down sugars, producing acids that corrode the tooth enamel (made of calcium hydroxyapatite). Using basic toothpastes helps neutralize these acids and prevent decay.
• **Self-defence by Animals and Plants:** The stings of insects like honey-bees and plants like nettles contain acids (e.g., methanoic acid) that cause pain and irritation. Using a mild base (like baking soda on a bee sting or dock leaf extract on a nettle sting) can neutralize the acid and provide relief.

Answer:

Answer:

Answer:

Answer:

More About Salts

**Salts** are ionic compounds formed when an acid reacts with a base (neutralisation reaction). They consist of a positive ion (cation), typically from a metal or ammonium (NH$_4^+$), and a negative ion (anion), typically from an acid.

Salts are diverse in their properties and uses. Common salt, sodium chloride (NaCl), formed from hydrochloric acid (HCl) and sodium hydroxide (NaOH), is used in food and as a raw material for various chemicals.

---

Family Of Salts

Salts can be grouped into families based on the common positive or negative radical they contain. For example, salts containing the same cation belong to the same metal family (e.g., NaCl, Na$_2$SO$_4$, NaNO$_3$ are sodium salts). Salts containing the same anion belong to the same acid family (e.g., NaCl, KCl, CaCl$_2$ are chloride salts). Identifying these families helps in understanding their properties and preparation.

---

Ph Of Salts

The pH of a salt solution depends on the strength of the acid and base from which the salt was formed:

• Salts formed from a **strong acid and a strong base** are **neutral** with a pH of **7**. Example: NaCl (from HCl and NaOH).
• Salts formed from a **strong acid and a weak base** are **acidic** with a pH **less than 7**. Example: Ammonium chloride (NH$_4$Cl) from HCl (strong acid) and NH$_4$OH (weak base).
• Salts formed from a **weak acid and a strong base** are **basic** with a pH **more than 7**. Example: Sodium carbonate (Na$_2$CO$_3$) from carbonic acid (H$_2$CO$_3$, weak acid) and NaOH (strong base).

Checking the pH of various salt solutions using a universal indicator paper helps in identifying their nature (acidic, basic, or neutral) and inferring the strengths of the parent acid and base.

---

Chemicals From Common Salt

Sodium chloride (common salt) obtained from seawater or rock salt deposits is a crucial raw material for the production of several important chemicals used in industry and daily life.

• **Sodium Hydroxide (NaOH):** Produced by passing electricity through an aqueous solution of sodium chloride (brine). This process is called the **chlor-alkali process** because it produces chlorine (chlor) and sodium hydroxide (an alkali).

  2NaCl(aq) + 2H$_2$O(l) $\xrightarrow{\text{Electricity}}$ 2NaOH(aq) + Cl$_2$(g) + H$_2$(g)

  Chlorine gas is produced at the anode, hydrogen gas at the cathode, and sodium hydroxide solution near the cathode. All three products have various uses (e.g., NaOH for soaps, paper, detergents; Cl$_2$ for water treatment, PVC, bleaching powder; H$_2$ for fuels, ammonia).

• **Bleaching Powder (CaOCl$_2$):** Manufactured by the action of chlorine gas on dry slaked lime [Ca(OH)$_2$].

  Ca(OH)$_2$(s) + Cl$_2$(g) $\to$ CaOCl$_2$(s) + H$_2$O(l)

  Bleaching powder is used for bleaching textiles, paper pulp, and washed clothes; as an oxidising agent; and for disinfecting drinking water.
• **Baking Soda (Sodium Hydrogencarbonate, NaHCO$_3$):** Produced using sodium chloride, water, carbon dioxide, and ammonia.

  NaCl + H$_2$O + CO$_2$ + NH$_3$ $\to$ NH$_4$Cl + NaHCO$_3$

  Baking soda is a mild, non-corrosive basic salt. It is used in cooking (as baking powder, which is a mixture of baking soda and a mild edible acid like tartaric acid – heating or mixing with water produces CO$_2$ that makes dough rise), as an ingredient in antacids to neutralize stomach acid, and in soda-acid fire extinguishers.
• **Washing Soda (Sodium Carbonate Decahydrate, Na$_2$CO$_3 \cdot$ 10H$_2$O):** Sodium carbonate (soda ash) is obtained by heating baking soda. Recrystallization of sodium carbonate from water gives washing soda (hydrated sodium carbonate).

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

  Na$_2$CO$_3$ + 10H$_2$O $\to$ Na$_2$CO$_3 \cdot$ 10H$_2$O

  Washing soda is also a basic salt. It is used in the glass, soap, and paper industries; in the manufacture of sodium compounds like borax; as a cleaning agent; and for removing permanent hardness of water.

---

Are The Crystals Of Salts Really Dry? (Water Of Crystallisation)

Even though salt crystals appear dry to the touch, many contain a fixed number of water molecules associated with each formula unit. This water is called **water of crystallisation**.

Example: Copper sulphate crystals (blue). When heated, they lose their water of crystallisation and turn white, becoming anhydrous copper sulphate. If water is added back, the blue colour returns.

CuSO$_4 \cdot$ 5H$_2$O(s) $\xrightarrow{\text{Heat}}$ CuSO$_4$(s) + 5H$_2$O(g)

(Hydrated copper sulphate - blue) (Anhydrous copper sulphate - white)

CuSO$_4$(s) + 5H$_2$O(l) $\to$ CuSO$_4 \cdot$ 5H$_2$O(s)

The formula CuSO$_4 \cdot$ 5H$_2$O indicates that 5 water molecules are associated with one formula unit of copper sulphate. Similarly, washing soda (Na$_2$CO$_3 \cdot$ 10H$_2$O) has 10 molecules of water of crystallisation, making it a hydrated salt, but it is not 'wet' in the conventional sense.

Another salt with water of crystallisation is **gypsum** (CaSO$_4 \cdot$ 2H$_2$O), which has two water molecules of crystallisation.

---

Plaster Of Paris

**Plaster of Paris** is obtained by heating gypsum (CaSO$_4 \cdot$ 2H$_2$O) at 373 K (100°C). It loses water molecules to become calcium sulphate hemihydrate (CaSO$_4 \cdot \frac{1}{2}$H$_2$O).

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

(Gypsum) (Plaster of Paris)

The formula CaSO$_4 \cdot \frac{1}{2}$H$_2$O indicates that two formula units of CaSO$_4$ share one molecule of water. Plaster of Paris is a white powder. When mixed with water, it reacts with the added water to form gypsum again, setting into a hard solid mass.

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

(Plaster of Paris) (Water) (Gypsum)

Plaster of Paris is used by doctors for setting fractured bones, for making toys, decorative materials, and for making surfaces smooth (e.g., wall plastering). It should be stored in a moisture-proof container because it reacts with moisture in the air, setting into hard gypsum.

Answer:

Answer:

Answer:

Answer:

Answer:

Page No. 18

Answer:

---

Page No. 22

Answer:

Answer:

Answer:

---

Page No. 25

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

---

Page No. 28

Answer:

Answer:

Answer:

Answer:

---

Page No. 33

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: