ACIDS, BASES AND SALTS

ACID -BASE:

• Acids are SOUR while bases are Bitter in taste.
• Without tasting, we can check them by LITMUS Test as acids turn blue litmus to RED and bases turn red litmus to BLUE [we have studied in previous classes]
•  If someone is suffering from ACIDITY, what will you give to that person, Lemon Juice,Vinegar or Baking Soda solution?......[ we know that Acids and Bases Neutralise each other]

Acids                                                           Bases

– Sour in taste                                                                         – Bitter in taste

– Change the blue litmus to red                                             – Change red litmus to blue

– eg.Hydrochloric Acid (HCl)                                                   eg. Sodium hydroxide (NaOH)

– Sulphuric Acid (H2SO4 )                                                                                Potassium hydroxide   (KOH)

– Nitric Acid  (HNO3)                                                                                        Calcium hydroxide  Ca(OH)2

– Acetic Acid  (CH3 COOH)                                                    – Ammonium hydroxide (NH4OH)

Some Naturally occurring acids

Vinegar –                                 Acetic Acid

Orange –                                  Citric Acid

Lemon –                                   Citric Acid

Tamarind –                              Tartaric Acid

Tomato –                                 Oxalic Acid

Sour milk (Curd) –                  Lactic Acid

Ant and Nettle sting –          Methanoic Acid

[Introductory VIDEO for Understanding ]

ACID-BASE INDICATORS:

NATURAL INDICATORS

                                        

(    1.   Litmus Solution :  A purple dye obtained from Lichen (symbiotic association of fungus and algae).
     2.   Turmeric (turn reddish brown with base & unchanged with acids).
     3. Red cabbage leaves (originally purple, turns reddish in acid, greenish in base)
      4. Some other examples are petals of  Petunia, Hibiscus (China rose), Geranium and Hydrangea. Grape Juice
    OLFACTORY INDICATORS

(  Olfactory indicators – Show odour/smell changes when mixed in acidic or basic media.

1. ONION ( Paste or juice of onion loses its smell when added with base. It does not change its smell with acid)
2. Vanilla ( The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid)
3. CLOVE OIL (smell same in acidic medium but didn't smell in base)

SYNTHETIC INDICATORS

Indicators that are synthesized in laboratory are known as synthetic indicators.
Eg. Methyl Orange, Phenolphthalein

1. Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
2. Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base.

UNIVERSAL INDICATORS

Using a litmus paper, phenolphthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.

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, phenolphthalein, 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.

(pH OF SOME EDIBLE INDICATORS FROM THE KITCHEN)

Chemical Properties of Acids and Bases

1. REACTION WITH METALS:

Acid + Metal gives Salt + Hydrogen   [ video ]

(Refer activity 2.3 on page No. 19 of NCERT Book)

2HCl + Zn  è ZnCl₂ + H₂

2HNO₃ + Zn  è Zn (NO₃)₂ + H₂

H₂SO₄ + Zn  è ZnSO₄ + H₂

2CH₃COOH + Zn  è  (CH₃COO)₂ Zn + H₂

Base + Metal also produce Salt + Hydrogen [ video ]

NaOH(aq.) + Zn(s)  è  Na₂ZnO₂(s) + H_2 (g)

                              _{(Sodium Zincate)}

2. Action of Acids with metal Carbonates and metal bicarbonates/ Hydrogencarbonates

[ NCERT text book Activity 2.5 for class X ]

Take 0.5g Sodium Carbonate in test tube -A and 0.5g Sodium Hydrogen Carbonate in test tube - B, Add 2 ml of dilute Hydrochloric Acid in it. Pass the produced gas through Lime Water (NaOH) or Calcium Hydroxide or Potassium Hydroxide solution and Observe.

Metal Carbonate + Acid  produces Salt + Carbon dioxide + Water  [ video ]

Test-Tube-A:

Na₂CO₃(s)  + 2HCl (aq) è  2NaCl(aq) + H₂O(l) + CO₂(g)

Test-Tube-B:

NaHCO₃(s)  + HCl (aq) è  NaCl(aq) + H₂O(l) + CO₂(g)

On passing evolved gas through lime water:

Ca(OH)₂ (aq.) + CO₂(g)  è CaCo₃(s) + H₂O(l)

                                          [white ppt]

On passing excess gas, following reaction takes place:

CaCo₃(s) + H₂O (l) + CO₂(g)  è Ca(HCO₃)₂(aq.)

                                                    [soluble in water]

Neutralisation Reactions [Acid Base Reactions]

Base + Acid  forms Salt + Water

NaOH(aq) + HCl(aq)  will give NaCl(aq) + H2O(l)

Neutralisation reaction takes place when the effect of a base is nullified by an acid and vice versa to give salt and water.

(A)Reactions of metal oxides with acids   [ video ]

Metal Oxide + Acid  forms Salt + Water       

ACTIVITY: Take small amount of Copper Oxide in a beaker and add dilute HCl  slowly while stirring. Note the colour change. [ Metal oxides are BASIC in nature]

CuO + HCl  will give CuCl2 + H2O

(B)   Reaction of Non Metallic Oxide with Base

Non metallic oxide + Base gives Salt + Water

Ca(OH)2 + CO2 è  CaCO3 + H2O

[Refer activity 2.5 of NCERT Book]

[VIDEO ON REACTION OF METAL & NON METAL OXIDES WITH ACIDS 7 BASES ]

Alkalis:

All bases do not dissolve in water. An alkali is a base that dissolves in water.

Common alkalis are:

NaOH Sodium hydroxide

KOH Potassium hydroxide

Ca(OH)₂ Calcium hydroxide

NH₄OH :Ammonium hydroxide

[Note : All alkalis are bases but all bases are not alkalis.]

When an acid or a base is mixed with water they become dilute. This results in the decrease in the concentration of H3O+ or OH– per unit volume in acids and bases respectively.

Strength of an Acid or Base

Strength of acids and bases depends on the no. of H+ ions and OH– ions produced respectively.

With the help of a universal indicator we can find the strength of an acid or base. This indicator is called pH scale.

pH = Potenz in German means power.

This scale measures from 0 (very acidic) to 14 (very alkaline) 7 Neutral (water in Neutral).

pH paper : Is a paper which is used for measuring PH.

(Variation of PH)

S. No.  PH Value         Colour of the Paper       Nature of Solution    H₊ion Conc.       OH-ion

1.             0                  Dark red                                 highly acidic          very high        very low

2.             4                     Orange or yellow                       Acidic                      high                  low

3.             7                    Green                                            Neutral                   Equal                Equal

4.            10                    Bluish green or blue                    Alkaline                  low                 high

5.            14                   Dark blue or violet                    highly basic         very low         very high

– strongAcids give rise to more H+ions.

eg. HCl, H₂SO₄ and HNO₃

– Weak Acids give rise to less H+ ions

eg. CH₃ COOH, H₂CO₃ (Carbonic acid)

– Strong Bases – Strong bases give rise to more OH– ions.

eg. NaOH, KOH, Ca(OH)₂

– Weak Bases : give rise to less OH– ions.

eg. NH₄OH

More about Salts

(Salts and their derivation)

S. No                  Name of Salt                                 Formula                                    Derived from

1.                     Potassium Sulphate                        K2SO4                                                           KOH + H2SO4

2.                    Sodium Sulphate                              Na2SO4                                                       NaOH + H2SO4

3.                    Sodium Chloride                               NaCl                                            NaOH  +  HCl

4.                     Ammonium Chloride                       NH4Cl                                          NH4OH +  HCl

Note : NaCl and Na2 SO4 belong to the family of sodium salts as they have the same radicals. Similarly NaCl and KCl belong to the family of chloride salts.

Importance of pH in our daily life

(1)   Importance of pH in our digestive system – pH level of our body regulates

our digestive system (digestive enzymes acts at specific pH). In case of indigestion our stomach produces acid in a very large quantity because of which we feel pain and irritation in our stomach (Acidity). To get relief from this pain antacids are used. These antacids neutralizes the excess acid and we get relief.

(2)   pH of Acid Rain : When pH of rain water is less than 5.6 it is called Acid

Rain. When this acidic rain enters rivers and soil, decreasing their pH, which causes

 threat to the survival of plants & animals present there. It also causes Marble and skin cancer.

(3)   pH of Soil : Plants require a specific range of pH for their healthy growth. If pH of soil of any particular place is less or more than normal than the farmers add suitable fertilizers to it. Indiscriminate use of fertilizer can cause pH shift and poor yield.

(4)   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 neutralize the excess acid and prevent tooth decay.

(5)   INSECT bite or Bee sting or Nettle sting contains methanoic acid which causes pain and irritation. When we use a weak base like baking soda on it we get relief.

Chemicals from Common Salt

– Sodium chloride is called as common salt used in our food. It is derived

from seawater.

– Rock Salt is the brown coloured large crystals. This is mined like coal.

– Common Salt is an important raw ingredient for many materials of daily use

such as:

             (a)   Sodium hydroxide

             (b)   Washing Soda

 (c )Bleaching Powder.

Sodium Hydroxide

Preparation :Prepared by the method called chlor-alkali (when electricity is passed through aqueous solution of  NaCl or BRINE, it Decomposes to form NaOH)

It is Called chlor-alkali because we get chlorine and a base in this.

2NaCl(aq) + 2H₂O(l)  è2NaOH(aq) + Cl₂(g) + H₂(g)

(Chlor-Alkali Process)

BLEACHING - POWDER [ CaOCl₂ ]

Preparation:
            Ca(OH)₂ +  Cl₂    è CaOCl₂ + H₂
            calcium hydroxide         chlorine                bleaching powder
                 ( Dry Slaked Lime)

USES:
1. For Bleaching Cotton & Linen in Textile Industry, bleaching wood pulp in paper factories & for bleaching washed clothes in washing industries.
2. As an Oxidizing agent in chemical industries.
3. Purification of Drinking water.

Baking Soda [ NaHCO₃ ]

– Common name – Sodium Hydrogen Carbonate

Preparation:

 NaCl + H2O + CO2 + NH3 èNH4Cl + NaHCO3

                                                                                                                                (Sodiumhydrogen carbonate)

On heating NaHCO3 produces :

NaHCO3 è HeatèNa2CO3 + H2O + CO2

CO2 produced causes dough to rise and make cakes, pastries spongy.

Uses :

1.       In household, ingredients of antacid

2.      In making baking powder

3.      On heating baking powder produces

            NaHCO3 + H⁺ èCO2 + H2O + Sodium Salt of acid

Washing Soda [ Na2CO3.10H2O ]

Preparation : Re-crystallization of sodium carbonate

Na2CO3 + 10H2O è Heat è  Na2CO3. 10H2O

Uses

1.       Used in glass, soap and paper industry

2.      Cleaning agent for domestic purposes.

3.       Removal of hardness of water.

4.       Manufacture of borax.

Water of crystallisation :

 Fixed number of water molecules present in one formula unit of a salt.

(     On heating copper sulphate crystals water droplets appear, formula of hydrated copper sulphate – CuSO4. 5H2O.

    Plaster Of Paris

(   1.  Gypsum also contains water of crystallization.

(   2.  Formula of gypsum – CaSO4.2H2O

(   3. On heating gypsum at 373 K it becomes CaSO4.½H2O is plaster of Paris. 

    4. Plaster of Paris is used as plaster for fractured bones.

    5. When plaster of Paris is mixed with water it changes to gypsum.

                         CaSO4.½ H2O + 1½ H2O èCaSO4.2H2O

                          Uses of plaster of Paris :

                            Making toys,  decorative material and smooth surfaces.