2. Acids, Bases and salts

Acids: All acids are sour in taste. 

Type of acids: 

(I)   Natural acids: This type of acid is obtained from ntural sources e.g Ascorbic acid (Amla and Guava), Lactic acid (Milk), Oxalic acid (Tomato), Citric acid (Lemon and Orange) etc. 0

(II)  Mineral acids: such type of acid is obtained from minerals e.g Hydrochlorice acid, Sulphuric acid and Nitric acid. 

Properties of acids:

(1)   Acids are sour in taste.

(2)   All acids give H⁺ hydrogen ion with water. 

(3)   It turns colour of blue litmus to red. 

(4)   Acids conduct electricity current.

(5)   It reacts with metals produces salt and hydrogen gas. 

Bases: Bases are bitter in taste and soapy touch. 

Properties of Bases:

(1)   Bases are bitter in taste.

(2)    All bases give (OH^- ) hydroxide ion with water.

(3)    It turns colour of red litmus into blue. 

(4)    Bases conduct electricity current. 

(5)    It also reacts with metals gives salt and hydrogen gas. 

 

Acid-Base Indicators : These are substances which indicate by chnaging in colour after testing acids and bases. 

Acid-base indicators are two types: 

(I)    Natural Indiccators : Litmus paper, turmeric and red cabbage leaves are natural indicators. 

(II)   Synthetic Indicators : Methyle orange and phenolphthalein are synthetic indicators. 

These indicators tell us whether a substance is acidic or basic by change in colour.

Olfactory Indicator: There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. 

Which of these – vanilla, onion and clove, can be used as olfactory indicators

Universal Indicator: Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Chemical Properties of Acids and Bases: 

 

Reaction with acids and metals: 

Acids react with metals give corresponding salt and hydrogen gas; 

   Acid          +              Metal       →             Salt           +         Hydrogen Gas  

Hydrochloric acid reacts with zinc and produces zinc chloride and hydrogen gas.

 2 HCl          +                Zn           →           ZnCl₂          +              H₂

Hydrochloric acid                 Zinc                               Zinc Chloride                   Hydrogen Gas

Hydrochloric acid reacts with sodium and gives sodium chloride and hydrogen gas. 

2 HCl           +            2 Na          →     2NaCl               +                H₂

_{Hydrochlorice acid                Sodium                Sodium Chloride              Hydrogen Gas} 

Hydrogen gas and zinc sulphate are formed when metal zinc reacts with sulphuric acid.

H₂ SO₄        +            Zn             →     ZnSO4              +                H₂  

Sulphuric acid               Zinc                     Zinc Sulphate                  Hydrogen gas 

 

Testing of Hydrogen Gas: 

When we react any metal with an acid the reaction gives corresponding salt and produces hydrogen gas. During this period of reaction, when we place a burnt candle near by gas it produces a pop sound. The pop sound indicates that the produced gas is hydrogen. 

Reaction of Metal Carbonate/Metal Hydrogencarbonate with Acids:

Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
Thus, the reaction can be summarised as –

Metal carbonate + Acid → Salt + Carbon dioxide + Water

Example: 

Calcium Chloride reacts with hydrochloric acid and gives calcium chloride, carbon dioxide and water. 

CaCO₃            +                2HCl              →         CaCl₂      +        CO₂        +       H₂O

(Calcium Carbonate)   (Hydrochloric Acid)   (Calcium Chloride)  (Carbon Dioxide)             (Water)  

Nitric Acid reacts with sodium carbonate and gives sodium nitrate carbon dioxide and water.

2NHO₃       +           Na₂CO₃                →                 NaNO₃        +        CO₂         +      2H₂O

(Nitric Acid)        (Sodium Carbonate)                     (Sodium Nitrate)    (Carbon dioxide)     (water)

Similarly;

Sodium Carbonate  +  Hydrochloric Acid  →  Sodium Chloride + Carbon Dioxide  +  Water

Calcium Carbonate + Sulphuric Acid  →  Calcium Sulphate + Carbon Dioxide  +  Water

 

Reaction of Metal hydrogencarbonate With Acids
 

General Formulla; 
 

Metal Hydrogen Carbonate (Bicarbonate)  + Acid → Salt   +   Carbondioxide   +   water 
 

Example: 

Sodium Bicarbonate reacts with Hydrochloric Acid produces Sodium Chloride Carbon dioxide and water.  
NaHCO₃        +      2HCl     →      NaCl      +    CO₂   +     H₂O  
(Sodium bicarbonate)   (Hydrochloric Acid)    (Sodium Chloride)   (Carbon dioxide)     (Water)

 

Reaction with bases and Metals: 

 

Bases react with metals gives corresponding salts and hydrogen gas. 

Sodium hydroxide reacts with zinc gives sodium zincate and hydrogen gas. 

2NaOH(aq)         +            Zn(s)        →           Na₂ ZnO₂(aq)       +         H₂(g)   

(Sodium hydroxide)        (Zinc)          (Sodium Zincate)          (Hydrogen Gas) 

Sodium hydroxide reacts with alluminium and water gives sodium aluminate and hydrogen  gas. 

2NaOH(aq)        +       2 Al (s)  +  2H₂O      →       2 NaAlO₂(aq)          +      2H₂(g)   

(Sodium hydroxide)   (Aluminium)  (water)        (Sodium Aluminate)          (Hydrogen Gas) 

 

Historical Monuments and Acid Rain:

Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases; present in atmosphere because of pollution; they form acid rain.

Acid rain causes damage to the historical monuments and other buildings. For example Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded.

Use of Reaction between Acid and Metal Carbonate or Metal Bicarbonate in Fire Extinguisher:

Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire.

 

WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?

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Ions : charged atoms are known as ions.

Ions are two types: 

(I)   Cation: It carries (+) Positive charge on atom.

(II)   Anion: It carries (-) Negative charge on atom.

all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids. but all compounds having hydrogen are not acids. As acids have H⁺ Ion. For example we take three substances,

Vinegar, glucose and alcohal 

The chemical formula are;

Vinegar  → CH₃COOH,

Glucose → C₆H₁₂O_6, 

_Alcohal → _​ CH₂CH₃OH, 

​These all have hydrogen. 

 

Salts 

Salts: salts are ionic products of of acids and bases neutralisation reaction.

Acidic Salts are formed from a strong acid and a weak base.

Neutral Salts are formed from a strong acid and strong base.

Basic Salts are formed from a strong base and a weak acid.

Neutral Salt: NaCl  

HCl          +       NaOH          →           NaCl        +          H₂O 

Strong Acid     Strong Base            Neutral Salt 

Basic Salt: NaC₂H₃O₂ 

HC₂H₃O₂           +    NaOH           →       NaC₂H₃O₂     +     H₂O 

Weak Acid          Strong Base               Basic Salt 

Acidic Salt: NH₄Cl 

HCl                +       NH₄​OH          →        NH₄Cl          +     H₂O 
Strong Acid             Weak Base           Acidic Salt 
S
S

Our blood must maintain a pH around 7.35-7.45.  If the pH is above 7.45 you would have a condition called alkalosis.  If the pH is below 7.35, then one would suffer from acidosis.

Chlor-Alkali Process:

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process.

This process is taken place by electrolysis of aqueous sodium chloride. 

The chemical equation of this process is as folllow:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂ (g) + H₂(g) 

When electricity is passed through an aqueous solution of sodium chloride (called brine), Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

Products of chlor-alkali Process: 

(1)  Sodium Hydroxide

(2)  Chlorine Gas

(3)  Hydrogen Gas

Uses of Sodium Hydroxide : 

(i)   It is used for de-greasing Metals.

(ii)  In soaps and detergents.

(iii)  In Paper making. 

(iv)  For making of artificial fibres.

Uses of Chlorine gas: 

(i)    It is used In water treatment.

(ii)  In swimming pool. 

(iii) In manufactoring of PVC, CFCs and pesticides.

(iv) It is also used as disinfectants.

Uses of hydrogen:

(i)    It is used as fuels.

(ii)   It is used in Margarine. 

(iii)  In manufactoring of ammonia for fertilisers. 

Production of Hydrochloric acid: Chlorine and Hydrogen are the main products of chlor-alkali process. These are used for production of Hydrochloric acids. Hydrochloric acid is important chemical product which is used in manufacturing of medicines, cosmetics and ammonium chloride and also used for cleaning steel. 

Production of Bleaching powder: Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. This chlorine gas is obtained from electrolysis of aqueous sodium chloride of chlor-alkali process. 

The chemical equation of this process is 

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Bleaching powder is used –

(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) for disinfecting drinking water to make it free of germs.

Production of Baking soda: 

The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogencarbonate (NaHCO₃).

The chemical equation of this reaction is; 

NaCl + H₂O + CO₂ + NH₃ →  NH₄Cl         +            NaHCO₃

                                            _{(Ammoium Chloride)     (Sodium Hydrogencarbonate)} 

When Sodium hydrogencarbonate is heated during cooking it gives following products.

This reaction produces carbon dioxide which is responsible for making bread or cake risen, soft and spongy. 

When baking powder is heated or mixed in water, it gives carbon dioxide, water and Sodium salt of acid. 

The reacton is as; 

NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid

Uses of sodium hydrogencarbonate (NaHCO₃)

(i)  For Making Baking Powder.

(ii) Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
(iii) It is also used in soda-acid fire extinguishers.