ACIDS: ­-

An acid is defined as substances which ionizes and gives hydrogen ions (H⁺) when if it is dissolved in water. The turn “acid” has been derived from the Latin word “acids” our therefore it is also said that anything which sour in taste is acid.

General Properties of acids:-

v  The acid is a substance that turns blue turns red.

v An acid neutralize a base to form salt & water.

v An acid gives out hydrogen (H₂) when it reacts with reactive metals.

v An acid librates carbon dioxide (CO₂) from carbonates and bicarbonates.

v An acid reacts with metal oxide to form salt and water.

Example:-

v  HCl (ag) g H⁺(aq)+ Cl^- (aq)

v H₂SO₄ (Aq)š 2H⁺(aq)+SO₄^-2(aq)

v H₃PO₄(aq) g 3H⁺(aq)+PO₄³ (aq)

BASES:-

A base is a substance that produces a hydroxide or hydrogen only ion (OH^-) when it is dissolved in water. It is called that anything that is bitter in taste and soapy in touch is called “base”.

General Properties of base:-

v A base is a substance that changes red litmus blue.

v A base reacts with an acid to produce a salt.

Base+AcidgSalt+Water

v  A base combines with carbon dioxide to produce a carbonate.

v A base is slipping like soap. It tastes unpleasant and bitter.

The definition of acid and base stated above was purposed by Arrhenius known as “Arrhenius Theory”

Indicators:-

An indicates is a chemical compound which helps us to identify an acid or a basic solution. It imparts one colour is an acidic solution and a different colour is a basic solution.

Indicates are of two types:-

v Natural indicators – China rose, turmeric , beetroot

v Artificial/ Synthetic indicators – Litmus, Methyl orange, Phenolphthalein, red cabbage

Olfactory Indicators:-

An olfactory indicator is a substance which changes its odour (smell) with the change of the nature of the solution. It help us to identify it the given solution is acidic or basic.

Eg: onion. Vanilla, clove oil, etc.

All of them have characteristic odours. These odours change when some acid or base solution is brought in their contact and we can identify with this:

v Red cabbage:  Its colour remains red in acidic medium out changes to green in basic or alkaline medium.

v Turmeric: It is a yellow dye and its colour remains the same in acidic or neutral medium but changes to reddish brown in basic medium.

Some common indicators with characteristic colours:

Indicators	Colour in neutral Solution	Colour in neutral Solution	Colour in basic Solution
Litmus	Red	Purple	Blue
Phenolphthalein	Colourless	Colourless	Pink
Methyl Orange	Red	Orange	Yellow

Sample	Blue Litmus	Red Litmus	Phenolphthalein	Methyl Orange
Hcl	Changes to Red	No colour change	Colourless	Changes to red
HNO3	Changes to red	No colour change	Colourless	Changes to red
NaOH	No colour change	Changes to blue	Changes to light pink	Changes to yellow
KOH	No colour change	Changes to blue	Changes to light pink	Changes to yellow

Litmus: - Litmus is a natural dye. Its original colour is purple. It is extracted from a plant called lichen, which belong to the Thallophytic family.

All the artificial indicators are known as “acid-base indicator” as the indicate the presence of an acid or a base in aqueous solution.

Chemical Properties of Acid:-

v Acid reacts with metals, metal oxides, hydroxides Carbonates, bicarbonates, sulphides, sulphites, nitrate and chlorides.

(a)  Acid Metals g Metallic salt+ hydrogen gas.

eg :     (i) H₂SO₄(aq)+Zn(s)             g ZnSO₄(aq)+H₂ (gas)

            (ii) 2Hcl (aq) + Mg(s)          gMgcl₂ (aq) +H₂ (g)

            (iii) 3H₂SO₄ (aq) +2Al(s)    gMgcl₂ (aq) +H₂ (g)

            (iv) 2Hcl (aq)+Fe(s)                        gFecl₂(aq)+H₂(g)

(b)   Acid metal carbonate g Salt+Water+CO₂

eg:       (i) Na₂CO₃(s)+2Hcl(cq)      g2Nacl(aq)+H₂O(l)+CO₂(g)

            (ii) CaCO₃(s) +2Hcl (aq)     gCaCl₂(aq)+H₂O(l)+CO₂(g)

(c)  Acid +metal bicarbonate g Salt+Water+CO₂

eg:       (i) NaHCO₃(s)+Hcl(aq)       gNacl(aq)+H₂O(l)+CO₂(g)

            (ii) Ca (HCO₃)(s)2Hcl(aq)  gCacl₂(aq)+2H₂O(l)+2CO₂(g)

The CO₂ Gas produced when allowed to pass through  lime water, turns it milky due to the formation of insoluble calcium carbonate.

Ca (OH)₂(aq)+CO₂(g)          g CaCO₃(s)+H₂O(l)

On passing excess carbon dioxide through lime water the milkiness disappears because of the formation of calcium bicarbonate which is soluble in water.

CaCO₃(s) +H₂O(l)+CO₂(g)gCa(HCO₃)₂ (aq)

                                                            (Calcium bicarbonate)

(d)  Acid +metallic oxide g Salt+Water

eg:       (i) Na₂O(s)+2Hcl(aq)g2Nacl(aq)+H₂O(l)

            (ii) Cuo(s) +H₂SO₃(aq)gCuSO₄(aq)+H₂O(l)

Since this reaction is similar that of neutralization reaction and the metal is behaving like a base, we can say that metallic oxides are basic in nature.

Type of Acids:-

(i)                On the basis of extent of Ionization, acid are classified as                            (a)Strong Acid, (b)Weak Acid

(a)  Strong acids: - An acid called strong if it completely ionizes in solution for eg:  H+ and Cl- ions.

Example of strong acids Hcl, H₂SO₄, HNO₃, etc.

(b)  Weak Acids: - An acid is called weak if its ionization in solution is partial. Thus the solution of a weak acid contains ions and unionized molecules both eg.  CH₃COOH (acetic Acid)  g  H⁺+CHCOO^-

acetic acid, oxalic acid, carbonic acid, Phosphoric acid, lactic acid, citric acid, citric acid etc.

(ii)             On the basis of concentration: (i) A solution containing larger amount of the acid per unit volume is called a “concentrated solution. A concentrated solution of an acid produces a larger no. of H⁺ ions.

Dilute acid: A solution containing smaller amount of acid per unit volumes is called “dilute acid”. A dilute solution of an acid produces smaller no. of H+ ions.

(iii)           On the basis of Origin :-

(a)  Minerals acid – HCl,  H₂SO₄, H₃PO₂, H₂CO₃

(b)  Organic acids – Acetic acid            Lactic acid                 Citric acid

Formula                    Chemical Name                   Common Name

NaOH                         Sodium hydroxide               Caustic Soda

KOH                           Potassium hydroxide           Caustic potash

Ca (OH)₂                    Calcium Hydroxide             Lime water

NH₄OH                       Ammonium hydroxide        Aqua ammonia

Types of Bases:-

It is not necessary that every base contains hydroxide ions (OH^-) ion. There are many substances that act as bases. For example:

(i)                Metallic oxide acting as bases : Na₂O, K₂O, CaO, ZnO, Cuo,

(ii)             Metallic hydroxide acting as base : NaOH, KOH₂, Ca(OH)₂, Al(OH)₃

(iii)           Water soluble bases : NaOH, KOH, Ca(OH)₂, Na₂O, K₂O, CaO

(iv)           Water insoluble bases : Cu(OH)₂, Al(OH)₃, CuO, Zno

Alkalis:-

A base is called “alkali” if it is soluble in water in aqueous solution; an alkali gives hydroxide ion (OH^-) as the only onion.

Example of alkali:-

Metallic hydroxide, namely NaOH, KOH, and Ca(OH)₂ are examples of alkalis Thus :

·        Sodium hydroxide (NaOH) is an alkali. It is soluble in water. An aqueous solution of sodium hydroxide contains sodium ions (Na⁺) as cations & hydroxide ion (OH^-) as anions.

            NaOH (aq)                             g        Na⁺ (aq)+ OH^-(aq)

            Sodium hydroxide                           Sodium ion    Hydroxide ion

            (Alkali)                                              (cation)                      (Anion)

·        Pottasium hydroxide (KOH) is an alkali. It is soluble in water. An aqueous solution of potassium hydroxide contains potassium ions (K⁺ ion) as cation and hydroxide ions (OH ^-ions) as anions.

KOH (aq)                               g        K⁺ (aq) +OH^-(aq)

                        Potasium hydroxide                         Potassium ion          hydroxide ion

                        (Alkali)                                              (cation)                      (Anion)

 Basic nature of (NH₃):

Ammonia (NH₃) is neither a metallic oxide nor a metallic hydroxide, but it has basic nature. It is confirmed by the following experiments.

·        Ammonia is base because it reacts with hydrochloric acid (HCl) so that ammonium chloride is found

NH₃ (g)   +   HCl (aq)                       g      NH₄Cl (aq)

Ammonia       hydrochloric acid                 Ammonium chloride

(Base)             (Acid)                                     (Salt)

·        Ammonia is highly soluble in water. Its aqueous solution is called ammonium hydroxide (NH₄OH)

NH₃     +          H₂O (l)                                   g        NH₄OH (aq)

·        Ammonium hydroxide is a weak base. It is only particularly ionized in solution.

NH₄OH (aq)                                      g        NH4⁺ (aq) + OH^-(aq)

·        An aqueous solution of ammonia contains ammonium ions as cations. OH- ions anions and unionized molecules.

Conclusion:-

 From the above examples we draw the following conclusions:-

·         Every alkali is a base but every base is not being alkali. NaOH is both an alkali & a base.

·        An alkali contains OH group but every substance containing OH group need not be an alkali for eg: NaOH is an alkali but CH₃O₄ is not an alkali.

Action of base on indicators:-

Water soluble bases affect indications in the following ways:-

·         Action on litmus: - An aqueous solution of a base changes red litmus to blue however bases do not affect blue litmus.

·        Action on Phenolphthalein: - A basic solution turns pin when phenolphthalein indicator is added to it.

·        Action on turmeric Solution: - Turmeric solution is yellow. When a base is added to it, reddish brown colour is produced.

·        Action on methyl orange: - Base does not change the colour of methyl indicator.

Strong and Weak Bases:-

·         A strong base is almost completely ionized in aqueous solution to give its constituent cations and anions. That is the extense of ionization of a strong base is very high. Thus the amount of hydroxide ions (OH^-) in the solution of single base is large.

NaOH (aq)                 g        Na+ (aq) + OH^- (aq)

·        A weal base is only partially ionized into its cations and anions in aqueous solution. That is the extent of ionization of a weak base is small. Thus, the amount of hydroxide ions (OH^-) in the solution of a strong base is small.

NH₄OH                       g        NH₄⁺ + OH^-

Example of strong and weak bases (partial Ionization)

Strong bases                                                              Weak bases

Name	Formula	Name	Formula
Sodium hydroxide	NaOH	Ammonium hydroxide	NH4OH
Potassium hydroxide	KOH	Magnesium hydroxide	Mg (OH)2

Note: - Pure water gives equal no. of H⁺ and OH^- ions on ionization. It does not change the colour of either blue or red litmus. Hence, it is considered neutral.

Chemical properties of Bases and Alkalis:-

(a)   Base + Metals                      g        basic salts + hydrogen

Eg: (i) Zn+2NaoH                      g        Na₂ZnO₂ +H₂ (g)

                                                                  (Sodium Zincate)

      (ii) 2Al +2 NaOH+2H₂O     g 2NaAlO₂ + 3H₂ (g)

                                                      (Sodium Aluminate)

(b)  Base +Non- metallic oxides           g        Salt + Water

Bases form salt and water when they react with non-metallic oxides.

  Eg:    CO₂ + 2NaOH                       g        Na₂CO₃ + H₂O

                                                                        (Sodium Carbonate)

This reaction is similar t neutralization reaction between acids and bases, where the non-metallic oxide is behaving like an acid. Hence, we can say that non-metallic oxides are acidic in nature.

(c)    Acid +Base                                      g         Salt + Water

Eg : NaOH+HCl                                g        Nacl + H₂O

This reaction is also known as “neutralization reaction”.

(d)    An alkali librates Ammonia from ammonium slats.

Eg : NH₄Cl +NaOH                          g        NaCl+H₂O+NH₃

        (Ammonium Chloride)                        (Ammonia)

(e)  When a base and a slat solution are mixed, a reaction takes place to produce another base and another salt.

Eg :     Base    (1)  +Salt(1)              g        base(2) + Salt (2)

(i)                3 NaOH +FeCl₃         g        Fe(OH)₃ + 3 NaCl    

            (ii)       Ca (OH)₂ + Na₂CO₃  g        2NaOH + CaCO 

Difference between acids and bases:-

ACIDS:-

(a)  The properties are due to the presence of hydrogen ions (H) + are water solution of an acid.

(b)   Turns blue litmus to red.

(c)   Reacts with active metals like Na, K Ca, & Zn to give hydrogen gas.

(d)  Aqueous solution conducts electricity.

(e)   Reacts with bases to form salt and water.

(f)   Frequently corrosive to stein.

(g)   Taste Sour.

(h)   The PH value is less than 7 at 25⁰ C.

(i)    Reacts with carbonates to give CO₂.

BASES :-

(a)   The properties are due to the presence of hydroxide ions (OH^-) in water solution of a base.

(b)   Turns red litmus to blue.

(c)  Does not react with metals except with Zn, Al, and Sn.

(d)  Aqueous solution conducts electricity.

(e)  Reacts with acids to form salt and water.

(f)   Absorb CO₂ to form carbonates.

(g)   Frequently corrosive to akin and feels slippery.

(h)  Unpleasant bitter taste.

(i)    The PH value is greater than 7 at 25⁰ C

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