ACID, BASES AND SALTS

 ACID, BASES AND SALTS 

Introduction
→ The sour and bitter tastes of food are due to acids and bases are present in them.
→ Acids are sour in taste and change the colour of blue litmus to red.

→ Litmus solution is a purple dye, which is extracted from lichen. When the litmus solution is neither acidic nor basic, its colour is purple.

→ Other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution.
Properties of Acids

• The term ‘acid’ has been derived from the Latin word, 'acidus' which means sour.

• Acids have sour taste.

• They turn blue litmus solution red.

• They give H⁺ ions in aqueous solution.

Strong Acids: HCl, H₂ SO₄ , HNO₃

Weak Acids: CH₃COOH, Oxalic acid, Lactic acid

Concentrated Acids: More amount of acid + Less amount of water

Dilute Acids: More amount of water + Less amount of acid

Properties of Bases

• These are the substances which are bitter in taste and soapy in touch.

• They turn red litmus solution blue.

• They give OH^- ions in aqueous solution.

Strong Bases: NaOH, KOH, Ca (OH)₂

Weak Bases: NH₄OH

Alkalis: These are bases which are soluble in water. Examples: NaOH, KOH, Ca(OH)₂.

Types of Indicators and its properties

Indicators: Substances which change their colour/smell in different types of substances (like acids and bases).

Types of Indicators:

(i)                 Natural indicators: Found in nature in plants. Examples: Litmus, red cabbage leaves extract, flowers of hydrangea plant, turmeric.

(ii)               Synthetic indicators: These are chemical substances. Examples: Methylorange, phenolphthalein.

(iii)             Olfactory indicators: These substances have different odour in acid and bases. Examples: Clove, garlic, onion, etc.

Important table of indicators

 

Indicator	Original Colour	Acid	Base
Red litmus	Red	No Change	Blue
Blue litmus	Blue	Red	No change
Turmeric	Yellow	No Change	Reddish brown
Red cabbage juice	Purple	Reddish	Greenish yellow
Phenolphthalein	Colourless	Colourless	Pink
Methyl Orange	Orange	Red	Yellow
Onion	n/a	No change	Smell vanishes
Vanilla	n/a	No change	Smell vanishes

 Chemical Reactions of acid and bases

 Reaction of Acids and Bases with Metals

→ Reaction of Acids with Metals

• Acids react with metal to form metal salt and releases Hydrogen Gas.

Acid  + Metal → Salt + Hydrogen Gas

• Example: Zinc granules react with dilute Hydrochloric acid in a test tube.

2HCl + Zn → ZnCl₂ + H₂

→ Reaction of Bases with Metals

• Bases react with metal to evolve hydrogen Gas. Also, note that all metals do not react with bases. The metal must be more reactive than the metals present in the base for the reaction to take place.

Base + Metal → Salt + Hydrogen gas

• Example: Zinc granules react with NaOH solution to form sodium zincate and evolve hydrogen gas.
2NaOH + Zn → Na₂ZnO₂ + H₂

• Hydrogen gas released can be tested by bringing burning candle near gas bubbles, it burst with pop sound.

Reaction of Acids with Metal Carbonates and Metal Hydrogencarbonates

• Acids reacts with Metal Carbonates and Metal Hydrogencarbonates to form Salt, Carbon dioxide and water.

Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water
• Examples: (i) 2HCl + Na₂CO₃ → 2NaCl + CO₂ + H₂O

(ii) HCl + NaHCO₃ → NaCl + CO₂ + H₂O

• CO₂ can be tested by passing it through lime water. It turns lime water milky.

Ca (OH)₂ + CO₂ → CaCO₃ + H₂O

• When excess CO2 is passed, milkiness disappears.

CaCO₃ + CO₂ + H₂O → Ca(HCO)₃

• Bases do not react with Metal Carbonates and Metal Hydrogencarbonates.

Base + Metal Carbonate/Metal Hydrogen Carbonate → No Reaction

Reaction of Acids and Bases with each other

• Acids and Bases react to form salt and water.

Acid + Base → Salt + H₂O

Neutralisation Reaction: Reaction of acid with a base is called as neutralization reaction.

Example: HCl + NaOH → NaCl + H₂O

• Strong Acid + Weak Base → Acidic salt + H₂O

• Weak Acid + Strong Base → Basic salt + H₂O

• Strong Acid + Strong Base → Neutral salt + H₂O

• Weak Acid + Weak Base → Neutral salt + H₂O

Reaction of Metallic Oxides with Acids

→ Metallic oxides are basic in nature.

Example: CaO, MgO are basic oxides.

Metallic Oxide + Acid → Salt + H₂O

CaO + 2HCl → CaCl₂ + H₂O

Reaction of Non-metallic Oxides with Bases

→ Non-metallic oxides are acidic in nature.

→ Non-metallic Oxide + Base → Salt + H₂O

CO₂ + Ca (OH)₂ → CaCO₃ + H₂O

Reaction of Acid

(i)                  Acid + Metal Carbonate → Salt + CO₂ + Water

(ii)                Acid + Metal → Salt + H₂

(iii)              Acid + Metal Hydrogen Carbonate → Salt + CO₂ + H2O

(iv)              Acid + Metallic oxide(basic in nature) → Salt +H₂O

(v)                Acid + Base → Salt + H₂O

Reaction Of Base

(i)                  Base + Metal → Salt + H₂

(ii)                Base + Metal Carbonate → No Reaction

(iii)              Base + Metal Hydrogen Carbonate → No Reaction

(iv)              Base + Acid → Salt + H₂O

(v)                Base + Non-Metallic oxide(acidic in nature) → Salt + H₂O

Similarities between all Acids and all Bases

→ All acids have H⁺ ions in common. All acids produce H⁺ ions
→ Acids produce H⁺ ions in solution which are responsible for their acidic properties.
→ All bases have OH^- (hydroxyl ions) in common. All bases produce OH^- ions
Acid or Base in Water Solution

→ Acids produce H⁺ ions in presence of water.

→ H⁺ ions cannot exist alone, they exist as H₃O⁺ (hydronium ions).

H⁺ + H₂O → H₃O⁺

HCl + H₂O → H₃O⁺ + Cl^-

→ Bases when dissolved in water gives OH − ions.

→ Bases soluble in water are called alkali.

→ While diluting acids, it is recommended that the acid should be added to water and not water to acid because the process of dissolving a acid or a base in water is highly exothermic.

Universal Indicator

 Strength of Acids and Base solutions

→ Strength of acid or base can be estimated using universal indicator.

→ Universal indicator: It is a mixture of several indicators. It shows different colours at different concentrations of H⁺ ions in the solution. A universal indicator has a pH range from 0 to 14 that indicates the acidity or alkalinity of a solution. A neutral solution has pH=7

→ pH Scale: A scale for measuring H⁺ ion concentration in a solution. p in pH stands for ‘potenz’ a German word which means power.

• If value of pH is equal to 7 → neutral solution

• If value of pH is less than 7 → acidic solution

• If value of pH more than 7 → basic solution

The pH scale ranges from 0 to 14.
If pH < 7 → acidic solution
If pH > 7→ basic solution

Importance of pH in everyday life

(i)                  pH sensitivity of plants and animals: Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.

(ii)                pH of a soil: The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.

(iii)              pH in the digestive system: The process of digestion happens at a specific pH in our stomach which is 1.5 to 4. The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.

(iv)              pH in tooth decay: Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.

(v)                pH of self-defence by animals and plants: Acidic substances are used by animals and plants as a self-defence mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defence. These secreted acidic substances have a specific pH.

Chemicals from Common Salt

Sodium Chloride: Sodium chloride is a common salt. NaCl is its molecular formula. The fundamental element in our meals is sodium chloride. It is used in our food as a flavour enhancer as well as a preservative. From common salt, we may make the following four compounds.

1. Sodium hydroxide or lye or caustic soda
2. Baking soda or sodium hydrogen carbonate, or sodium bicarbonate
3. Washing soda or sodium carbonate decahydrate
4. Bleaching powder or calcium hypochlorite

 Sodium Hydroxide

Chemical formula – NaOH
Also known as – caustic soda

Preparation (Chlor-alkali process): The Chlor-alkali process is used for the electrolysis of Sodium chloride solution. In this process, from the aqueous solution of Sodium chloride electricity is passed which decomposes to form Sodium hydroxide. Brine is the aqueous solution of sodium chloride.

 

Electrolysis of brine (solution of common salt, NaCl) is carried out.

At anode: Cl₂ is released
At cathode: H₂ is released
Sodium hydroxide remains in the solution.

Bleaching Powder

Bleaching powder is soluble in water and is used as a bleaching agent in textile industries. It is also used as an oxidizing agent and a disinfectant in many industries. It should also be noted that bleaching powder is synthesized by the reacting chlorine gas on dry slaked lime i.e. Ca(OH)2.

Chemical formula – Ca(OCl)Cl or CaOCl₂
Preparation – Ca(OH)₂(aq)+Cl₂(g)→CaOCl₂(aq)+H₂O(l)

On interaction with water – bleaching powder releases chlorine which is responsible for bleaching action.

Uses of Bleaching Powder

• It is used for bleaching dirty clothes in the laundry, as a bleaching agent for cotton and linen in the textile industry.
• It is a strong oxidizing agent, hence used as an oxidizer in many industries.
• It is used as a disinfectant which is used for disinfecting water to make potable water.

Baking Soda

Sodium bicarbonate, also known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO₃ and the IUPAC designation sodium hydrogen carbonate. A sodium cation (Na⁺) and a bicarbonate anion (HCO₃) combine to form this salt. Sodium bicarbonate is a white, crystalline substance that is commonly found as a fine powder. It tastes slightly salty and alkaline, like washing soda (sodium carbonate).

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃

Preparation (Solvay process):
a. Limestone is heated: CaCO₃ → CaO + CO₂
b. CO₂ is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)  +  NH₃(g)  +  CO₂(g)  +  H₂O(l)  →  NaHCO₃(aq)  +  NH₄Cl(aq)

Uses:

• Reduces the acidity in the stomach
• Acts as an antacid which is used to treat stomach upset and indigestion
• Used in the process of washing as a water softener

Washing Soda

Chemical name – Sodium hydrogen carbonate
Chemical formula – NaHCO₃
Preparation (Solvay process) – 
a. Limestone is heated: CaCO₃  →  CaO  +  CO₂
b. CO₂ is passed through a concentrated solution of sodium chloride and ammonia:
NaCl(aq)  +  NH₃(g)  +  CO₂(g)  +  H₂O(l)  →  NaHCO₃(aq)  +  NH₄Cl(aq)

Uses
1. In the glass, soap and paper industries
2. Softening of water
3. Domestic cleaner

Crystals of Salts

Certain salts form crystals by combining with a definite proportion of water. The number of water molecules that combines with the salt is called water of crystallisation.

The process by which a solid form, in which the atoms or molecules are strongly arranged into a structure known as a crystal, is known as crystallisation. Precipitation from a solution, freezing, and, more rarely, direct deposition from a gas are some of the ways crystals form.

Example:

Table salt (sodium chloride or halite crystals), sugar (sucrose), and snowflakes are examples of common materials that form crystals. Many gemstones, such as quartz and diamond, are crystals.

Plaster of Paris

Plaster of Paris is a white powdery chemical compound that is hydrated calcium sulphate that is usually obtained by calcining gypsum. To put it another way, Plaster of Paris is often manufactured of heated gypsum at a high temperature.

Plaster of Paris is expressed as CaSO₄. ½ H₂O in the chemical formula.

Gypsum, CaSO₄.2H₂O (s) on heating at 100°C (373K) gives CaSO₄. ½ H₂O and 3/2 H₂O
CaSO₄. ½ H₂O is plaster of Paris.
CaSO₄. ½ H₂O means two formula units of CaSO₄ share one molecule of water.
Uses – cast for healing fractures, sculpting materials and gauze bandages