Physical and Chemical Properties of Acids

Acids and Bases in the Laboratory

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Some common acids that are found in laboratories are Hydrochloric acid (HCl), Sulphuric acid (H2SO4) and Nitric acid (HNO3). Some of the lesser used acids are Acetic acid (CH3COOH), Hydrofluoric acid (HF), Carbonic acid (H2CO3). Hydrofluoric acid is a highly corrosive acid and is used to etch glass. Some acids are found in nature, like citric acid in certain fruits. These acids are called ‘organic acids’.

Examples are:

  • Citric acid (from citrus fruits and vegetables,)
  • Ascorbic acid (vitamin C, as from certain fruits)
  • Acetic acid (Vinegar)
  • Lactic acid (in buttermilk)
  • Tartaric acid (Tamarind)
  • Oxalic acid (Tomatoes)

 

Commonly found bases in laboratories and in our daily life are: Caustic soda, NaOH; Caustic potash, KOH; Milk of magnesia, Mg(OH)2; Liquor ammonia, NH3; Washing powder, Tooth paste.

One can familiarize with the acid base nature of some of the common substances by using litmus solution.

Activity :

 

Make solutions of the following substances:

‘Borax’, ‘Blood’, ‘Milk’, ‘caustic soda’, ‘Lemon Juice’, ‘Bleaching powder’, ‘Tomatoes’, ‘Vinegar’, ‘detergent’, ‘Pure

Water’, ‘Coffee’, Baking Soda’.

Put a drop of each of the above solutions on a watch-glass and test by adding a drop of litmus to it.

Answer :

 

Lemon juice, vinegar, tomatoes, coffee and milk turn litmus colour red indicating the acidic nature of these substances.

Pure Water does not show any change in colour of the litmus indicating its neutral character.

Blood, baking soda, borax, detergent, household ammonia, bleaching powder, caustic soda turn litmus colour blue showing the basic nature of these substances.

How do Acids React with Metals ?

 

Let us do more activities to understand the chemical nature of acids and bases.

Action on Metals :

 

Let us look at the way acids react with metals.

a) All metals above hydrogen in the metal reactivity series generally react with dilute acids to form their respective salt and liberate hydrogen.

Metal + Acid image Salt + Hydrogen

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b) Very active metals like potassium, sodium and calcium also react similarly, but tend to explode when combining with acids.

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c) Nitric acid (of various concentrations) usually exhibits oxidizing property, rather than acidic properties. Metals such as magnesium combine with extremely dilute (1%) nitric acid to liberate hydrogen.

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How do Metal Carbonates and Metal Hydrogen Carbonates React with Acids?

 

Acids react with carbonates and hydrogen carbonates (bicarbonates) to form their respective salt, water and carbon dioxide.

Carbonate/Bicarbonate + Acid image Salt + Water + Carbon dioxide

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How do Acids and Bases React with Each Other?

Acids and bases are the chemical ‘opposites’ of each other and the action of an acid and a base nullifies the effect of one on the

other. This is known as a neutralization reaction.

 

Neutralization : In a neutralization reaction an acid combines with a base, neutralizing the power of acid with a base and forms a molecule of water and the respective salt.

In general, a neutralization reaction can be written as:

Base + Acid ® Salt + Water

Example :

 

example for neutralization

 

 

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Reaction of Metallic Oxides with Acids

Action with Basic Oxides : Oxides that can add on hydroxyl ions (OH) to their molecules are called basic oxides. These oxides get neutralized when they react with acids.

Basic oxide + Acid image Salt + Water

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Action with Basic Hydroxides : Acids undergo neutralization reaction with basic hydroxides to form salt and water.

Basic hydroxide + Acid image Salt + Water

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Reaction of Non-metallic Salts with Base : Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt

and water. Since this is similar to the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic

in nature.

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Action of Alkalis/Base with Ammonium Salts : Alkalis combine with ammonium salts to liberate ammonia.

Alkali + Ammonium salt image Salt + Water + Ammonia

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  • It has a sour taste.
  • It turns blue litmus to red.
  • It turns methyl orange to red.
  • Acids are electrolyte.
  • Strong acids destroy fabric.
  • Strong acids cause burn on skin.
CHEMICAL PROPERTIES OF ACIDS
 
NEUTRALIZATION
An acid when reacts with a base, salt & water are produced. This reaction is called neutralization
HCl + NaOH èNaCl + H2O

HNO3 + NaOH è NaNO3 + H2O

HCl + KOH è KCl + H2O

REACTION WITH CARBONATES
Acid and carbonates are combined to produce salt, water and carbon dioxide
MgCO3 + 2HCl è MgCl2 + CO2 + H2O

CaCO3 + 2HCl è CaCl2 + CO2 + H2O

Na2CO3 + H2SO4 è Na2SO4 + CO2 + H2O

CaCO3 + H2SO4 è CaSO4 + CO2 + H2O

REACTION WITH BICARBONATES
Acid and bicarbonates are combined to produce salt, water and carbon dioxide
NaHCO3 + HCl è NaCl + CO2 + H2O
REACTION WITH METAL
With Zinc:
Zn + 2HCl è ZnCl2 + H2
With Aluminum:
2Al + 6HCl è 2AlCl3 + 3H2
Reaction with iron oxide:
6HCl + Fe2O3 è 2FeCl3 + 3H2O

 

 

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