CBSE Maths Class Xth Polynomials Concept

CBSE Class X Electricity Board Practice Questions

ACIDS AND BASES SHORT NOTES - CLASS X CBSE

ACIDS   AND  BASES

11.       Problem of acidity can be resolved with : (a) Juice (b) Vinegar (c) baking soda.

22.       Acids : Sour in taste . It Changes blue litmus to red.

33.       Base : Bitter in taste . It changes red litmus to blue.

44.       Natural indicators : Litmus( When the Litmus solution is neither  acidic or basic , its colour is purple) , Turmeric , red cabbage , coloured petals of Hydrangea, Petunia and Geranium)

55.       Synthetic indicators : Methyl Orange , Phenolpthalein.

66.       Litmus is extracted from Lichen, a plant belonging to division Thallophyta

77.       Olfactory Indicators : Some substances whose odour changes in acidic or basic media they are called olfactory indicators. Eg Vanilla , Onion and Clove .

88.       Acid + Metal --> Salt  + Hydrogen Gas

Metal  displaces hydrogen from the acids and combines with the remaining part of the ACID and forms a compound called SALT.  Eg (i) SULPHURIC ACID +  Zn --à Zinc Sulphate  +  Hydrogen (g)

Zn + H₂SO₄ ⇨ ZnSO₄ + H₂

(ii)  Nitric Acid + Calcium  ---à Calcium Nitrate + Hydrogen (g)

(iii) 2Na + 2HCl ⇨ 2NaCl + H₂

(iv) 2K + H₂SO₄ ⇨ K₂SO₄ + H₂

(v) Mg + 2HCl ⇨ MgCl₂ + H₂

(vI) 2Al + 6HCl ⇨ 2AlCl₃ + 3H₂

COPPER , GOLD and SILVER are called NOBLE METALS  as they do not react with water or dil Acids.

99.       Base + Metal  -->  Salt  + Hydrogen (g)

          (i) Sodium hydroxide gives hydrogen gas and sodium zincate when reacts with zinc metal.

              2NaOH + Zn ⇨ Na₂ZnO₂ + H₂

              _(ii)  Sodium aluminate and hydrogen gas are formed when sodium hydroxide reacts with    
              aluminium metal.

              2NaOH + 2Al + 2H₂O ⇨ 2NaAlO₂ + 2H₂

_{NOTE  :}  Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a lighted candle near it. If the gas burns with pop sound, then it confirms the evolution of hydrogen gas. Burning with pop sound is the characteristic test for hydrogen gas.

10.      ACIDS +  Metal Carbonates / Metal Hydrogen Carbonates ----> Salt  + Carbon Dioxide + Water

Acids give carbon dioxide gas and respective salts along with water when they react with metal carbonates / Metal Hydrogen Carbonates ( Bicarbonates )

(i)                 Na₂CO₃ + 2HCl ⇨ 2NaCl + CO₂ + H₂O  ( Sodium Carbonate reacts with HCL)

(ii)               MgCO₃ + 2HCl ⇨ MgCl₂ + CO₂ + H₂O  ( Magnesium Carbonate  )

(iii)             CaCO₃ + H₂SO₄ ⇨ CaSO₄ + CO₂ + H₂O ( Calcium carbonate )

(iv)             Na₂CO₃ + H₂SO₄ ⇨ Na₂SO₄ + CO₂ + H₂O ( Sodium carbonate producing Sodium Sulphate)

(v)               2HNO₃ + Na₂CO₃ ⇨ NaNO₃ + 2H₂O + CO₂   (Nitric Acid with Sodium Carbonate -> Sodium Nitrate)

(i)                 NaHCO₃ + HCl ⇨ NaCl + CO₂ + H₂O  ( Sodium Bicarbonate )

(ii)                2NaHCO₃ + H₂SO₄ ⇨ Na₂SO₄ + 2CO₂ + 2H₂O

Test for evolution of carbon dioxide gas: Carbon dioxide turns lime water milky when passed through it. This is the characteristic test for carbon dioxide gas.

The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.

Ca(OH)₂ + CO₂ ⇨ CaCO₃ + H₂O  (  CaCO₃   is the white precipitate )

when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.

CaCO₃ + CO₂ + H₂O ⇨ Ca(HCO₃ )₂

Reaction of acid with marble and egg shell: Since, marble and egg shell are made of calcium carbonate, hence when acid is poured over marble or egg shell, bubbles of carbon dioxide are formed.

11.  How ACID rain is  formed ? What  are its  affects ?

 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.

12.                        ACID  and Metal Carbonates and Bicarbonates are used in FIRE EXTINGUISHERS

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.

 Neutralisation Reaction:

13.                         Acid + Base ⇨ Salt + Water

An acid neutralizes a base when they react with each other and respective salt and water are formed.

(i)                HCl + NaOH ⇨ NaCl + H₂O

(ii)               2HCl + Ca(OH)₂ ⇨ CaCl₂ + 2H₂O

(iii)           H₂SO₄ + 2NaOH ⇨ Na₂SO₄ + 2H₂O

(iv)           HNO₃ + NaOH ⇨ NaNO₃ + H₂O

  Reaction of Acid with Metal Oxides:

14.                          Acid + Metal Oxide ⇨ Salt + Water

Metal oxides are basic in nature. Thus, when an acid reacts with a metal oxide both neutralize each other. In this reaction, respective salt and water are formed.

(i)                2HCl + CaO ⇨ CaCl₂ + H₂O

(ii)              H₂SO₄ + ZnO ⇨ ZnCl₂ + H₂O

(iii)            Al₂O₃ + 6HCl ⇨ 2AlCl₃ + 3H₂O

Copper does not react with  HCl , but copper oxide does react. A metal- Acid reaction is a REDOX reaction. Since Copper has higher reduction potential than  hydrogen , it does not react with non oxidizing acids like HCL and dil H₂SO_4.         

But Metal oxides are basic in nature and And they generally react with acid to form salt and water. Such acid – base reactions ae non redox reactions in nature

Copper Oxide is a weak base and it readily reacts with HCl to form Copper (II)  Chloride and water

CuO(s) + 2HCl(aq) = CuCl2(aq) + H2O(l)

Reaction of base with oxides of non-metals:

15.                        Base + Non-metal oxide ⇨ Salt + Water

Non-metal oxides are acidic in nature. For example; carbon dioxide is a non-metal oxide. When carbon dioxide is dissolved in water it produces carbonic acid.

Therefore, when a base reacts with non-metal oxide both neutralize each other resulting respective salt and water are produced.

(i)                2NaOH + CO₂ ⇨ Na₂CO₃ + H₂O

(ii)              Ca(OH)₂ + CO₂ ⇨ CaCO₃ + H₂O

16.                          Common in all ACIDS and what happens to an ACID in a Water Solution

Acids give hydrogen gas when they react with metal. This shows that all acids contain hydrogen. For example: Hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), etc.

When an acid is dissolved in water, it dissociates hydrogen. The dissociation of hydrogen ion in aqueous solution is the common property in all acids. Because of dissociation of hydrogen ion in aqueous solution, an acid shows acidic behavior.

i)                    Hydrochloric acid (HCl) gives hydrogen ion (H⁺) and chloride ion (Cl⁻) when it is dissolved in water.

                               HCl (aq) ⇨ H⁺ + Cl⁻

ii)                  Sulphuric acid (H₂SO₄) gives hydrogen ion (H⁺) and sulphate ion (SO₄^{− −}) in water.

                              H₂SO₄ (aq) ⇨ H⁺ + SO₄^{− −}         

iii)                Nitric acid (HNO₃) gives hydrogen ion (H⁺) and nitrate ion (NO₃⁻) in water.

                               HNO₃ (aq) ⇨ H⁺ + NO₃⁻

iv )      Acetic acid (CH₃COOH) gives acetate ion (CH₃COO⁻) and hydrogen ion (H⁺).

                               CH₃COOH (aq) ⇨ CH₃COO⁻ H⁺

Hydrogen ion which is produced by acid (when acid is combined with water molecule), exists in the form of hydronium ion (H₃O+) in aqueous solution. That’s why hydrogen ion is always written with suffix (aq), such as H⁺ (aq).

HCl + H₂O ⇨ H₃O+    + Cl⁻

H₂SO₄ + H₂O ⇨ H₃O+   + SO₄^{− −}                                         

Acids conduct electricity in their aqueous solution because of dissociation of hydrogen ion. Hydrogen ion in aqueous solution conducts electricity.
A dry acid, such as dry hydrochloric acid does not change the colour of blue litmus paper to red because a dry acid does not dissociate hydrogen ion. This is the cause that a moist litmus paper is used to check the acidic or basic character of a gas.

Are all compounds which contain hydrogen, necessarily acids?

No, all compounds which contain hydrogen are not acid. For example; glucose (C₆H₁₂O₆), methyl alcohol (CH₃OH), etc. are not acid in spite of the fact that they contain hydrogen. This is because these compounds do not dissociate hydrogen ion in their aqueous solution.

17.                        Common in all BASES and what happens to a BASE in a Water Solution

A base dissociates hydroxide ion in water, which is responsible for the basic behavior of a compound.

Example: When sodium hydroxide is dissolved in water, it dissociates hydroxide ion and sodium ion.

NaOH (aq) ⇨ Na⁺ + OH⁻

Similarly, when potassium hydroxide is dissolved in water, it dissociates hydroxide ion and potassium ion.

KOH (aq) ⇨ K⁺ + OH⁻

Thus, base shows its basic character because of dissociation of hydroxide ion.

18.                      Dilution of Acid and Base :

The concentration of hydrogen ion in an acid and hydroxide ion in a base; per unit volume; shows the concentration of acid or base.

By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called dilution and the acid or base is called diluted.

The dilution of acid or base is exothermic.

 Thus, acid or base is always added to water and water is never added to acid or base.

If water is added to a concentrated acid or base a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.

Q1. Why do HCL ,  HNO₃ etc , show acidic  characters in aqueopus solutions while solutions of cpmpounds like alcohol and glucose do not show acidic character?

Ans : The acidic character of a substance is due to the presence of hydrogen ions in aqueous solution. HCL and Nitric acid  (HNO₃  ) release hydrogen ions in aqueous solution, hence show acidic character in aqueous solution .

Whereas alcohol and glucose do not ionize in water to produce hydrogen ions and hence doesn’t show acidic character.

Q2. Why does an aqueous solution of an acid conduct electricity ?

Ans : An aq solution of an acid conducts electricity because of the presence of  hydrogen ions  (H⁺) in the solution . The hydrogen ion is a charged particle and its movement causes the flow of current.

Q3. Why does HCL gas do not change the colour of the dry litmus paper ?

Ans : The change of litmus colour is due to the presence of  Hydrogen ions (H⁺) that are dissociated in water of aqueous solution of an acid. Gaseous HCL has no free hydrogen ions , so it will not change the colour of blue litmus.

Q4. How is the concentration of hydronium ions (H₃O+) affected when a solution of an acid is diluted ?

Ans :  When an acid is diluted the concentration of hydronium ions (H₃O+) decreases . This is because on dilution  the overall volume of the solution increases and so the number of hydronium ions per unit volume decreases.

Q5. How is the concentration of hydroxide ions (OH⁻)  affected when excess base is dissolved in a solution of sodium hydroxide ?

Ans : The concentration of hydroxide ions (OH⁻) would increase when base is dissolved in a solution of Sodium Hydroxide . Usually bases ( MOH -------àMetal hydroxide , where M is metal )  which dissolves in water dissociates as M+ and OH-  and hence furnish OH- ions .

MOH -------à M+   +     OH-

If we dissolve excess base in a solution of sodium hydroxide ( in which there are already  OH-  ions  present as NaOH ----à Na+   +  OH-    ) , there would be more amount of base that would release OH-  ions in solution . This will cause the increase in the concentration of OH- ions )

119.                      Indicators:

Substances which show the acidic or basic behavior of other substance by change in colour are known as indicator.

Type of Indicator: There are many types of indicators. Some common types of indicators are

• Natural
• Olfactory Indicator
• Synthetic Indicator
• Universal Indicator

1.      Natural Indicator: Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.

             Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.

• An acid turns blue litmus paper red.
• A base turns red litmus paper blue.

       Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.

        Red cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.

2.      Olfactory Indicators: Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.

            Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with    

                         acid.

          Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.
        Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.

3.      Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc.

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

.

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

Universal Indicator:

Using a litmus paper, phelophthalein, 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, 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.

 

20.                     Strength of Acid and Base

     Acids in which complete dissociation of hydrogen ion takes place are called strong acid.

    In mineral acids, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and             

hence they are considered as strong acid.

Since, in organic acids hydrogen ions do not dissociate completely, so they are weak acid.

The complete dissociation of hydrogen ions or hydroxide ions is shown by a single arrow. The incomplete dissociation of hydrogen ions or hydroxide ions is denoted by double arrow.

Example of complete dissociation:

NaOH (aq) ⇨ Na⁺ + OH⁻

HCl ⇨ H⁺ + Cl⁻

Example of incomplete dissociation:

Mg(OH)2 ⇨ Mg^{+ +} + OH^{− −}

CH₃COOH ⇨ CH₃COO⁻ + H⁺

Although acetic acid being an organic acid is a weak acid, but concentrated acetic acid is corrosive and can damage the skin if poured over it.

Bases in which complete dissociation of hydroxide ion takes place are called strong base.

Alkalis are water soluble base, thus in alkali; complete dissociation of hydroxide ions takes place and they are considered as strong base.

21.                     pH – Measurement of strength of Acid and Base

pH is a scale which quantifies the concentration of hydrogen ion in a solution. The range of pH scale is between 0 to 14.

In pH scale ‘p’ stands for ‘potenz’. Potenz is a German word which means ‘power’ or ‘potential’. Here; ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or power of hydrogen.

The pH value decreases with increase in hydrogen ion concentration. If the value of pH is 0, this shows maximum hydrogen ion concentration. pH value equal to 14 shows lowest hydrogen ion concentration. pH value equal to 7 shows the hydrogen ion concentration is equal to hydroxide ion concentration.

A neutral solution, such as distilled water has value of hydrogen ion concentration equal to 7 on pH scale.

The acidic solution has value of hydrogen ion concentration less than 7 on pH scale.

The basic solution has value of hydrogen ion concentration greater than 7 on pH scale.