Wednesday, April 28, 2021

Acids, Bases and Salts | Class 10 CBSE | Web Notes | Part 1 | Chemical Properties of Acids and Bases

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 an 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 it 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 the 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 & hydrogen carbonates react with acids to give corresponding salt, CO2 & water. 

Metal carbonate/Metal hydrogen carbonate + 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.


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