2. Acids, Bases and Salts | Class 10 CBSE | Web Notes

2. ACIDS, BASES AND SALTS

-  Acids, bases and salts are used in our daily life.

-  Sour and bitter tastes of food are due to the presence of acids & bases. Acids are sour in taste and bases are bitter.

-  There are natural and synthetic acid-base indicators used to test whether a substance is acid or base.

-  Natural indicators: E.g. Litmus, red cabbage leaves, turmeric, coloured petals of some flowers (Hydrangea, Petunia & Geranium).

-  Litmus solution is a purple dye extracted from lichen (a plant under division Thallophyta).

When the litmus solution is neutral, its colour is purple.

Acids change the colour of blue litmus to red.

Bases change red litmus to blue.

-  A stain of curry on a white cloth turns reddish-brown when soap (basic) is rubbed on it because the curry contains an indicator turmeric. If washed with water, it turns yellow again.

-  Synthetic indicators: E.g. methyl orange and phenolphthalein.

UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS & BASES

Acids and Bases in the Laboratory

Put a drop of each solution given below on a watch-glass one by one and test with a drop of different indicators.

• Hydrochloric acid (HCl)
• Calcium hydroxide [Ca(OH)2]
• Sulphuric acid (H2SO4)
• Potassium hydroxide (KOH)
• Nitric acid (HNO3)
• Magnesium hydroxide [Mg(OH)2]
• Acetic acid (CH3COOH)
• Ammonium hydroxide (NH4OH)
• Sodium hydroxide (NaOH)

Results:

Sample solution	Red litmus solution	Blue litmus solution	Phenolphthalein solution	Methyl orange solution
HCl	No change	Red	Colourless	Pinkish red
H2SO4	No change	Red	Colourless	Pinkish red
HNO3	No change	Red	Colourless	Pinkish red
CH3COOH	No change	Red	Colourless	Pinkish red
NaOH	Blue	No change	Red to pink	Remains yellow
KOH	Blue	No change	Red to pink	Remains yellow
Mg(OH)2	Blue	No change	Red to pink	Remains yellow
NH4OH	Blue	No change	Red to pink	Remains yellow
Ca(OH)2	Blue	No change	Red to pink	Remains yellow

Olfactory indicators:

These are some substances whose odour changes in acidic or basic media. E.g. Onion, clove oil and vanilla.

Test with Onion:

-  Take some finely chopped onions in a plastic bag along with some clean cloth strips. Tie up the bag tightly and leave overnight in the fridge.

-  Take two of these cloth strips and check their odour.

-  Put a few drops of dilute HCl solution on one strip and a

few drops of dilute NaOH solution on the other.

-  Rinse both cloth strips with water and again check their odour. The odour of onion cloth vanishes in NaOH. The odour remains unchanged in HCl.

Test with vanilla essence:

-  Take some dilute HCl solution in one test tube and dilute NaOH solution in another.

-  Add a few drops of dilute vanilla essence to both test tubes and shake well. The odour of vanilla essence vanishes in NaOH. The odour remains unchanged in HCl.

Test with clove oil:

-  Repeat the same test using clove oil.

-  The odour clove oil vanishes in NaOH. The odour remains unchanged in HCl.

How do Acids & Bases react with Metals?

Reaction of Acids with metal:

-  Set the apparatus as shown in Figure.

-  Take 5 mL dilute sulphuric acid in a test tube and add some zinc granules.

-  Hydrogen gas bubbles are formed on the surface of zinc granules. Pass this gas through soap solution. Soap bubbles filled with hydrogen are formed.

-  Take a burning candle near a gas filled bubble. Hydrogen burns with a pop sound.

-  Repeat this using HCl, HNO₃ and CH₃COOH.

-  Bubbles come out vigorously with strong acids.

-  Zinc also reacts with weak acids like acetic acid, but gas is formed slowly.

-  When an acid reacts with a metal, the metal displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called salt.

Acid + Metal → Salt + Hydrogen gas

H₂SO₄ + Zn → ZnSO₄ + H₂

Reaction of Bases with metal:

-  Place some granulated zinc metal in a test tube.

-  Add 2 mL sodium hydroxide solution and warm the contents of the test tube.

-  Repeat the rest of the steps as in previous experiment.

-  Here also, hydrogen is formed.

2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)

         (Sodium zincate)

-  However, such reactions are not possible with all metals.

How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

- Take 0.5 g of sodium carbonate (Na₂CO₃) in test tube A and 0.5g of sodium hydrogencarbonate (NaHCO₃) in test tube B. Add 2 mL dilute HCl to both the test tubes.

-  CO₂ gas is produced in both test tubes.

Test tube A:

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

Test tube B:

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

-  Pass CO₂ through lime water (calcium hydroxide).

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

(Lime water)                        (White precipitate)

On passing excess CO₂:  

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

         (Soluble in water)

-  Limestone, chalk & marble are different forms of calcium carbonate. All metal carbonates & hydrogencarbonates react with acids to give corresponding salt, CO₂ & water.

Metal carbonate/Metal hydrogencarbonate + Acid →

Salt + Carbon dioxide + Water

How do Acids and Bases React with each other?

-  Take 2 mL dilute NaOH solution in a test tube and add two drops of phenolphthalein solution.

-  The solution becomes pink colour.

-  To this, add dilute HCl solution drop by drop. The pink colour changes and disappears.

-  Now add a few drops of NaOH to the above mixture. The pink colour of phenolphthalein reappears.

-  The effect of a base is nullified by an acid and vice-versa.

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

-  The reaction between an acid and a base to give a salt and water is called neutralisation reaction.

Base + Acid → Salt + Water

Reaction of Metallic Oxides with Acids

-  Take some copper oxide (CuO) in a beaker and add dilute hydrochloric acid slowly while stirring.

-  The solution becomes blue-green colour and the copper oxide dissolves. The colour is due to the formation of copper(II) chloride (CuCl₂).

CuO + 2HCl → CuCl₂ + H₂O

-  General reaction between a metal oxide and an acid is

Metal oxide + Acid → Salt + Water

-  Metallic oxides react with acids to give salts & water, like the reaction of a base with an acid. So metallic oxides are basic oxides.

Reaction of a non-metallic oxide with Base

-  Calcium hydroxide (base) reacts with carbon dioxide to produce a salt and water.

-  This is similar to the reaction between a base and acid. It means non-metallic oxides are acidic in nature.

WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?

-  All acids have similar chemical properties.

-  E.g. All acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids.

-  But all compounds containing hydrogen are not acidic. It can be proved by the following experiment.

·   Take solutions of glucose, alcohol, HCl, H₂SO₄, etc.

·   Fix two nails on a cork and place it in a 100 mL beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.

·   Pour some dilute HCl in the beaker and switch on the current. Repeat with dilute H₂SO₄.

·   In both cases, bulb glows. It means there is an electric current through the acidic solution by ions.

·   Repeat the experiment using glucose & alcohol solutions. In these cases, bulb does not glow because glucose & alcohol solutions do not conduct electricity.

-  Acids contain H⁺ ion as cation and anion (Cl^– in HCl, NO₃^– in HNO₃, SO²₄^– in H₂SO₄, CH₃COO^– in CH₃COOH).

-  Acidic properties are due to H⁺(aq) ions in solution.

-  Repeat the same Activity using alkalis such as sodium hydroxide, calcium hydroxide, etc. NaOH & KOH conduct electricity as they are broken down into ions when dissolved in water. The movement of the ions, Na⁺, K⁺ and OH^- in solution generate electricity.

What Happens to an Acid or a Base in a Water Solution?

Acids produce ions only in aqueous solution. It can be proved by the following experiment.

·    Take 1g solid NaCl in a clean dry test tube.

·    To this, add some conc. sulphuric acid.

·    HCl gas comes out of the delivery tube. [In very humid climate, pass the HCl gas through a guard tube (drying tube) containing calcium chloride to dry the gas].

·  When HCl gas is tested with wet blue litmus paper, it becomes red colour. But with dry litmus paper, no colour change occurs.

·    It means dry HCl gas (absence of water) cannot produce H⁺ ions. So it does not behave as an acid.

·    HCl solution (presence of water) can produce H⁺ ions and behave as an acid.

HCl + H₂O → H₃O⁺ + Cl^–

-  H⁺ ions cannot exist alone. They exist after combining with water molecules. Thus H⁺ ions must be shown as H⁺(aq) or hydronium ion (H₃O⁺).

H⁺ + H₂O → H₃O⁺

Action of base with water:

-  Bases generate hydroxide (OH^–) ions in water.

-  All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy, bitter and corrosive. Never taste or touch them as they cause harm.

-  NaOH, KOH, Mg(OH)₂, NH₄OH etc. are alkalis.

Neutralisation reaction:

Acid + Base → Salt + Water

H  X + M  OH → MX + HOH

H⁺(aq) + OH^– (aq) → H₂O(l)

Mixing of acid or base with water:

This process is highly exothermic. E.g.

· Take 10 mL water in a beaker. Add a few drops of concentrated H₂SO₄ and swirl the beaker slowly.

·  Touch the base of the beaker. It is hot. So the reaction is exothermic.

·  Repeat this activity with sodium hydroxide pellets. It is also exothermic reaction.

-  Mixing of concentrated nitric acid or sulphuric acid with water must be done carefully. Add the acid slowly to water with constant stirring. Otherwise, the excessive local heating may cause the mixture to splash out and cause burns. The glass container may also be broken.

-  Mixing an acid or base with water results in decrease in the concentration of ions (H₃O⁺/OH^–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

HOW STRONG ARE ACID OR BASE SOLUTIONS?

-   Amount of H⁺ ions present in a solution can be measured by using a universal indicator. It is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ions.

-  A scale for measuring H⁺ ion concentration in a solution is called pH scale (p = ‘potenz’ in German = power).

-   pH is measured from 0 (very acidic) to 14 (very alkaline).

-   Higher the H⁺ ion concentration, lower is the pH value.

-   pH of a neutral solution is 7.

-   Values less than 7 represent an acidic solution.

-   Values more than 7 represent an alkaline solution.

- pH value from 7 to 14 represents an increase in OH^– ion concentration, i.e., increase in the strength of alkali.

-   Generally, paper impregnated with the universal indicator is used for measuring pH.

pH values of various solutions:

Solution	Colour of pH paper	Approximate pH value	Nature of substance
1.    Saliva (before meal)	Green	6.8 – 7.4	Slightly acidic to basic
2.    Saliva (after meal)	Yellow green	5.8	Acidic
3.    Lemon juice	Orange	2.2	Acidic
4.    Colourless aerated drink	Yellow	4.0	Acidic
5.    Carrot juice	Yellow green	6.0	Acidic
6.    Coffee	Yellow	4.5	Acidic
7.    Tomato juice	Yellow	4.3	Acidic
8.    Tap water	Green	6 – 8.5	Varied
9.    1M NaOH	Dark blue	14	Basic
10. 1M HCl	Red	0	Acidic

pH of some common substances shown on a pH paper

-  The strength of acids and bases depends on the number of H⁺ ions and OH^– ions produced, respectively. E.g. 1 molar hydrochloric acid and 1 molar acetic acid (same concentration) produce different amounts of H⁺ ions.

-  Strong acids: Produce more H⁺ ions. E.g. HCl, H₂SO₄.

-  Weak acids: Produce less H⁺ ions. E.g. CH₃COOH.

-  Strong bases: Produce more OH^- ions. E.g. NaOH, KOH.

-  Weak bases: Produce less OH^- ions. E.g. NH₄OH, Ca(OH)₂.

Importance of pH in Everyday Life

-  Plants and animals are pH sensitive. They can survive only in a narrow range of pH change.

-  Our body works at the pH range of 7.0 to 7.8.

-  When pH of rain water is less than 5.6, it is called acid rain. When it flows into rivers, it lowers the pH of river water. It adversely affects the survival of aquatic life.

What is the pH of the soil in your backyard?

To find out the pH for healthy growth of a plant, collect the

soil from various places and check their pH as given below.

·    Put about 2 g soil in a test tube and add 5 mL water.

·    Shake the contents of the test tube.

·    Filter the contents and collect the filtrate in a test tube.

·    Check the pH of filtrate using universal indicator paper.

Ideal soil pH for the growth of plants = 6 to 7.

pH in our digestive system

-  Stomach produces hydrochloric acid.

-  During indigestion, the stomach produces too much acid and causes pain and irritation.

-  To get rid of pain, bases called antacids are used. They neutralise the excess acid. E.g. Magnesium hydroxide (Milk of magnesia), a mild base.

pH change as the cause of tooth decay

-  Tooth enamel is made up of calcium hydroxyapatite (a crystalline form of calcium phosphate). It is the hardest substance in the body. It does not dissolve in water.

-  Bacteria in the mouth produce acids by degradation of sugar and food particles. Thus enamel is corroded at the pH below 5.5. It is called tooth decay.

-  Using toothpastes (basic) can neutralise the excess acid and prevent tooth decay.

Self-defence by organisms through chemical warfare

-  Bee-sting leaves formic acid which causes pain and irritation. Applying mild base like baking soda on the stung area gives relief.

-  Stinging hair of nettle leaves inject methanoic acid causing burning pain. A traditional remedy is rubbing the area with the leaf of the dock plant.

Some naturally occurring acids

Natural source	Acid	Natural source	Acid
Vinegar	Acetic acid	Sour milk (Curd)	Lactic acid
Orange	Citric acid	Lemon	Citric acid
Tamarind	Tartaric acid	Ant sting	Methanoic acid
Tomato	Oxalic acid	Nettle sting	Methanoic acid

Acids in other planets: The atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric acid.

MORE ABOUT SALTS

Family of Salts

-  Salts having same positive or negative radicals belong to a family. E.g. NaCl & Na₂SO₄ belong to family of sodium salts. NaCl & KCl belong to the family of chloride salts.

Salts & their Chemical formulae	Formed from which Acids & bases?
Potassium sulphate (K2SO4)	H2SO4 & KOH
Sodium sulphate (Na2SO4)	H2SO4 & NaOH
Calcium sulphate (CaSO4)	H2SO4 & CaCO3
Magnesium sulphate (MgSO4)	H2SO4 & Mg(OH)2
Copper sulphate (CuSO4)	H2SO4 & Cu(OH)2
Sodium chloride (NaCl)	HCl & NaOH
Sodium nitrate (NaNO3)	HNO3 & NaOH
Sodium carbonate (Na2CO3)	H2CO3 & NaOH
Ammonium chloride (NH4Cl)	HCl + NH4OH

 

pH of Salts

-  Salts of a strong acid & a strong base are neutral (pH = 7).  

-  Salts of a strong acid and weak base are acidic (pH < 7).

-  Salts of a strong base and weak acid are basic (pH > 7).

-  pH of some salt samples (soluble in distilled water) and the acid & base used to form the salts are given below:

Salt	pH	Acid used	Base used
Sodium chloride	7	HCl	NaOH
Potassium nitrate	7	HNO3	KOH
Aluminium chloride	7	HCl	Al(OH)3
Zinc sulphate	7	H2SO4	Zn(OH)2
Copper sulphate	< 7	H2SO4	Cu(OH)2
Sodium acetate	> 7	CH3COOH	NaOH
Sodium carbonate	> 7	H2CO3	NaOH
Sodium hydrogen carbonate	> 7	H2CO3	NaOH

Chemicals from Common Salt

-  Common salt (sodium chloride, NaCl) is a neutral salt formed by reaction of HCl & NaOH solution.

-  NaCl is separated from seawater containing many salts.

-  Deposits of large crystals of solid salt are also found in several parts. These are often brown due to impurities. This is called rock salt.

-  There are beds of rock salt formed when seas of bygone ages dried up. Rock salt is mined like coal.

-  Common salt is used in food. It is also a raw material to make sodium hydroxide, baking soda, washing soda, bleaching powder etc.

1. Sodium hydroxide (NaOH)

-  When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide. It is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.

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

-  Chlorine gas is given off at the anode, and hydrogen gas at the cathode. NaOH solution is formed near cathode.

-  All three products are useful.

-  Uses of NaOH: De-greasing metals, soaps & detergents, paper making, artificial fibres etc.

-  Uses of Cl₂: For water treatment, in swimming pools, PVC, disinfectants, CFCs, pesticides etc.

-  Uses of H₂: Fuels, margarine, ammonia for fertilisers.

Important products from the chlor-alkali process

2. Bleaching powder (CaOCl₂)

-  It is manufactured by using chlorine produced during the electrolysis of brine.

-  Chlorine acts on dry slaked lime to give bleaching powder.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

-  Actual composition of CaOCl₂ is quite complex.

Uses of Bleaching powder:

·    For bleaching cotton & linen in the textile industry.

·    For bleaching wood pulp in paper factories.

·    For bleaching washed clothes in laundry.

·    As an oxidising agent in many chemical industries.

·    To make drinking water free from germs.

3. Baking soda

-  Chemical name: Sodium hydrogencarbonate (NaHCO₃).

-  It is produced using sodium chloride.

-  It is a mild non-corrosive basic salt.

-  When it is heated for cooking, following reaction occurs.

Uses of Baking soda:

·   To make tasty crispy pakoras, etc.

·   It is added for faster cooking.

·   To make baking powder (baking soda + mild edible acid such as tartaric acid). When baking powder is heated or mixed in water, following reaction occurs:

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

                                             (From any acid)

CO₂ causes bread or cake to rise making soft and spongy.

·   It is an ingredient in antacids. Being alkaline, it neutralises excess acid (acidity) in stomach.

·   Used in soda-acid fire extinguishers.

4. Washing soda (Na₂CO₃.10H₂O)

-  A basic salt obtained from NaCl.

NaCl → NaHCO₃ → Na₂CO₃ (sodium carbonate).

-  Recrystallisation of sodium carbonate → washing soda.

Na₂CO₃     +     10H₂O     →    Na₂CO₃.10H₂O

Uses of washing soda:

·    Used in glass, soap and paper industries.

·    To manufacture sodium compounds such as borax.

·    Used as a cleaning agent for domestic purposes.

·    For removing permanent hardness of water.

Are the Crystals of Salts really Dry?

-   Heat few copper sulphate crystals in a dry boiling tube.

-   Copper sulphate becomes white and some water droplets appear in the boiling tube.

-   When 2-3 drops of water are added, copper sulphate crystals restore its blue colour.

-   Copper sulphate crystals contain water of crystallisation. When heated the crystals, this water is removed and the salt turns white.

-   Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of hydrated copper sulphate (Cu SO₄. 5H₂O).

-   Similarly, the molecule of Na₂CO₃.10H₂O is wet.

-   Gypsum is also a salt possessing water of crystallisation (CaSO₄.2H₂O).

Plaster of Paris

-  On heating gypsum at 373 K, it loses water molecules and becomes a white powder. This is called Plaster of Paris or Calcium sulphate hemihydrate (CaSO₄. ½ H₂O).

-  Plaster of Paris on mixing with water, it changes to gypsum again giving a hard solid mass.

CaSO₄. ½ H₂O +1½ H₂O → CaSO₄.2H₂O

(Plaster of Paris)                  (Gypsum)

-  Only half a water molecule is shown to be attached as water of crystallisation. It is written in this form because two formula units of CaSO₄ share one molecule of water.

-  Uses of Plaster of Paris:

o  Used as plaster to support fractured bones.

o  To make toys and materials for decoration.

o  To make surfaces smooth.

-  Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris.