In this chapter, you will learn about acids, bases, and salts, the basis on which a compound is classified as an acid or a base and the importance of maintaining pH in our lives.
An acid is a molecule or ion, capable of contributing a hydron (proton or hydrogen ion H+) or capable of forming a covalent bond with an electron pair. Acids form an aqueous solution with a sour taste, can turn blue litmus to red, and react with bases and certain metals (like calcium) to form salts. The word acid is derived from the Latin acidus/acēre meaning sour.
Classification of Acids:
Acids are classified on the basis of:
- Origin: organic and mineral acids
- Organic acids: Are derived from living organisms like plants and animals. For example, citric acid present in lemons and oranges, tartaric acid in tamarind and lactic acid present in the curd.
- Mineral acids: Also called as inorganic acids, they are derived from one or more inorganic compounds. They are either oxygenless or oxoacids. All mineral acids form hydrogen ions and the conjugate base ions when dissolved in water. For example, hydrochloric acid, hydrogen sulphide, nitric acid and sulphuric acid.
- Strength: strong and weak acids
- Strong acids: Acids that completely disassociate into their ions in aqueous solutions. For example, nitric acid and sulphuric acid.
- Weak acids: Acids that do not completely disassociate into their ions in aqueous solutions. For example, carbonic acid.
- Concentration: dilute and concentrated acids
- Dilute acids: Have a low concentration of acids in aqueous solutions.
- Concentrated acids: Have a high concentration of acids in aqueous solutions. For example, commercially available Hydrochloric acid and Sulphuric acid.
- The number of hydrogen atoms present: monoprotic, diprotic, triprotic, and polyprotic
- Monoprotic acid: Acids that produce one mole of H+ ions per mole of acid. For example, hydrochloric acid and nitric acid.
- Diprotic acid: Acids that produce two moles of H+ ions per mole of acid. For example, sulphuric acid.
- Triprotic acid: Acids that produce three moles of H+ ions per mole of acid. For example, phosphoric acid.
- Polyprotic acid: Acids that produce more than three H+ ions per mole of acid.
Some Important Reaction of Acids:
The reaction of acids with carbonates and bicarbonates:
- Metal Carbonate + Acid → Salt + Carbon dioxide + Water
- Metal Bi Carbonate + Acid → Salt + Carbon dioxide + Water
E.g., Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
The reaction of acids with oxides:
- Metal Oxide + Acid → Salt + Water
E.g., CuO + HCl → CuCl2 + H2O
The reaction of acids with water:
E.g., HCl + H2O → H3O + + Cl–
The reaction of acids with metals:
- Acid + Metal → Salt + Hydrogen
E.g., 2HCl + Zn → ZnCl2 + H2
Have you ever wondered what is soap made of?
Soaps are actually bases that are formed by the reaction of fatty acids with sodium hydroxide or potassium hydroxide. You can conduct a simple litmus test to find if your soap is mild and suitable for use on human skin by simply placing a red litmus paper against wet soap. A soap that turns red litmus blue is considered a good soap.
Let us now study about bases, their properties and some important reactions of bases.
A base is either a metallic hydroxide or a metallic oxide or aqueous ammonia which reacts with an acid to form salt and water only.
Bases:
- Are slippery to touch
- Taste bitter
- Change red litmus to blue
- React with acids to form salts
- Promote chemical reaction
- Contain displaceable OH- ions
- Accept protons from a proton donor
Classification of Bases:
They are classified on the basis of:
- Strength: strong and weak bases
- Strong bases: A basic chemical compound that can remove a proton (H+) from a molecule of a very weak acid in an acid-base reaction. Example: Sodium hydroxide (NaOH), Potassium hydroxide (KOH).
- Weak bases: A chemical base that does not ionise fully in an aqueous solution. Weak bases tend to build up in acidic fluids. Example: Ammonia.
- Concentration: diluted and concentrated bases
- Concentrated and dilute refer to the concentration of an acidic or basic substance in a solvent.
Some Important Reaction of Bases:
- The reaction of Bases with Oxides
- Non-metallic oxide + Base → Salt + Water
- Example: Ca(OH)2 + CO2 → CaCO3 + H2O
- The reaction of Bases with Water
- Example: NaOH(s) → Na+ (aq) + OH – (aq)
- The reaction of Bases with Metal
- Base + Metal → Salt + Hydrogen
- Example: NaOH + Zn → Na2 ZnO2 + H2
- Neutralization Reaction
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3. Strength of Acids and Bases.
While a small number of acids can ionise completely, all acids and bases do not ionise or disassociate completely in an aqueous solution. Which means, all acids and bases do not produce H+ and OH– equally in a solution. The terms 'strong' and 'weak' give an indication of the strength of an acid or base.
Comparison of Acids and Bases
Neutralization Reaction: This reaction takes place when the effect of a base is nullified by an acid and vice versa to give salt and water.
- Base + Acid → Salt + Water
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Strength of Acids and Bases:
- The strength of acids and bases depends on the number of H+ ions and OH– ions produced respectively.
- With the help of a universal indicator, we can find the strength of an acid or a base. This indicator is called the pH scale.
- pH = Potenz in German means power.
- This scale measures from 0 (very acidic) to 14 (very alkaline) to 7 Neutral (water in neutral).
Importance of pH in our daily life:
- Our body functions between the pH ranges of 7.0 to 7.8. Living organisms can survive only in the narrow range of a pH change.
- Importance of pH in our digestive system:
- The pH level of our body regulates our digestive system.
- In case of indigestion, our stomach produces acid in very large quantities that results in pain and irritation in our stomach.
- Antacids are used to relieve the pain as they can neutralise excess acid, due to which we get relief.
- Tooth decay and pH: Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth. Using toothpaste which is generally basic can neutralise the excess acid and prevent tooth decay.
- pH of Acid Rain: When the pH of rain water is less than 5.6, it is called acid rain. When this acidic rain flows into rivers the water in the rivers get acidic, which causes a threat to the survival of aquatic life.
- pH of Soil: Plants require a specific range of pH for a healthy growth. If the pH of soil in any particular place is less or more than normal, then farmers add suitable fertilisers to balance the pH.
Salt or table salt as we commonly know it, is a white crystalline substance that is used to season and enhance the flavour of the food. The same salt gives seawater its characteristic taste. This salt is sodium chloride, NaCl, and is derived from reaction between sodium hydroxide and hydrochloric acid. This salt forms when chlorine and sodium ions bind together.
Table salt is just one variety of the many. Salts come in different colours, have different properties and have different uses as well. In this chapter you will learn about the different types of salts and a few salts that are used for domestic purposes.
So what is salt?
Salt is an ionic compound formed that is formed in the neutralization reaction between an acid and a base. In this reaction, all or part of the hydrogen of the acid is replaced by a metal or other cations.
Salts are a combination of cations, which are positively charged and anions, which are negatively charged. Hence the resultant product is electrically neutral without a net charge.
Types of Salts:
- Neutral Salts: Strong Acid + Strong Base (pH value is 7)
Example: Sodium chloride (NaCl), Calcium sulphate (span id="MathJax-Element-3-Frame" class="MathJax" tyle="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="CaSO4">CaSO4CaSO4)
- Acidic Salts: Strong Acid + Weak Base (pH value is less than 7)
Example: Ammonium chloride (span id="MathJax-Element-4-rame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="NH4Cl">NH4ClNH4Cl), Ammonium nitrate (span id="MathJax-Eleent-5-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="NH4NO3">NH4NO3NH4NO3)
- Basic Salts: Strong Base + Weak Acid (pH value is more than 7)
Example: Calcium Carbonate (span id="MathJax-Element-6-rame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="CaCO3">CaCO3CaCO3), Sodium Acetate (span id="MathJax-Eement-7-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="CH3COONa">CH3COONaCH3COONa)
- Common Salts:
- Sodium chloride is called as common salt and is used in our food. It is derived from seawater.
- Common salt is an important raw material for many materials of daily use such as sodium hydroxide, washing soda and bleaching powder.
- Rock salt comes in the form of brown coloured large crystals and is mined like coal.
Following is a list of common salts that are used in our daily life:
Baking soda:
Sodium bicarbonate (NaHCO3) or commonly known as baking soda is a salt composed of sodium ions and bicarbonate ions.
- Sodium carbonate (Na2CO3) commonly known as washing soda, soda ash or soda crystals is a water-soluble sodium salt of carbonic acid.
Preparation:
span id="MathJax-Element-8-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="NaCl+H2O+CO2+NH3NH4Cl+NaHCO3">NaCl+H2O+CO2+NH3→NH4Cl+NaHCO3NaCl+H2O+CO2+NH3→NH4Cl+NaHCO3
(Recrystallisation of sodium carbonate)
Used in glass manufacturing, soap and paper industry, as cleaning agent for domestic purposes and removal of hardness of water and manufacture of boraxPreparation: - span id="MathJax-Element-9-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="NaHCO3+HeatNa2CO3+H2O+CO2">NaHCO3+Heat→Na2CO3+H2O+CO2NaHCO3+Heat→Na2CO3+H2O+CO2
- On heating NaHCO3, it produces sodium salt of acid (Na2CO3) and carbon dioxide (CO2)
- NaHCO3 + Heat → Na2CO3 + H2O + CO2
- CO2 produced causes the dough to rise and makes cakes and pastries spongy
- Used to make baking powder, antacids etc
Washing Soda:
Sodium carbonate (Na2CO3) commonly known as washing soda, soda ash or soda crystals is a water-soluble sodium salt of carbonic acid.
- Preparation:
- span id="MathJax-Element-10-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="NaCl+H2O+CO2+NH3NH4Cl+NaHCO3">NaCl+H2O+CO2+NH3→NH4Cl+NaHCO3NaCl+H2O+CO2+NH3→NH4Cl+NaHCO3
- (Recrystallisation of sodium carbonate)
- Used in glass manufacturing, soap and paper industry, as cleaning agent for domestic purposes and removal of hardness of water and manufacture of borax
Bleaching Powder:
Usually, in form of a white powder, bleaching powder is any of various mixtures of calcium hypochlorite, lime and calcium chloride. Chlorine is the basis for the most common bleaches. Bleaching powder is used as a whitening agent in laundering, as a disinfectant, and weed killer.
- Preparation: span id="Mathax-Element-11-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="Ca(OH)2+Cl2CaOCl2+H2O">Ca(OH)2+Cl2→CaOCl2+H2OCa(OH)2+Cl2→CaOCl2+H2O
- Uses: Used in textile factories, laundries and as a disinfectant
Plaster of Paris:
Commonly available plaster contains gypsum, lime or cement. Plaster is available in the form of a dry powder and mixed with water to form a stiff paste but eventually solidifies. Gypsum plaster or plaster of Paris (PoP) is produced by heating gypsum to about to about 150 °C or 300 °F.
- Preparation: On heating gypsum to 373K it becomes CaSO4.½H2O that is plaster of Paris
- span id="MathJax-Element-12-Frame" class="MathJax" style="box-sizing: border-box; margin: 0px; padding: 0px; word-break: break-word; display: inline; font-style: normal; font-weight: normal; line-height: normal; font-size: 1em; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" data-mathml="CaSO4.2H2OCaSO4.12H2O">CaSO4.2H2O→CaSO4.12H2OCaSO4.2H2O→CaSO4.12H2O
- When plaster of Paris is mixed with water, it changes to gypsum
- CaSO4.½ H2O + 1½ H2O → CaSO4.2H 2O
- Uses: Making toys, decorative materials and smooth surfaces
Water of Crystallisation:
Water of crystallisation or water of hydration or crystallisation water is the water that is found in the crystalline framework of a metal complex or salt, which is not directly bonded to the metal cation. It is defined as the fixed number of water molecules present in one formula unit of a salt. For example,
- On heating copper sulphate crystals, water droplets appear. Formula of hydrated copper sulphate: CuSO4.5H2O
- Gypsum also contains water of crystallisation. Formula of gypsum: CaSO4.2H2O