Properties of Acids and Bases

Our Objective

To study the properties of acids and bases (HCl and NaOH) by their reaction with:

  • Litmus solution (blue/Red)
  • Zinc metal
  • Solid sodium carbonate

Objective(A)

How to carry out the reactions of an acid (HCl) with (i) litmus solution (blue and red), (ii) zinc metal (iii) solid sodium carbonate.

The Theory

Some examples of acids and bases from our daily life:

The sour and bitter tastes we find in food are due to the presence of acids and bases. Some naturally occurring acids are: vinegar (acetic acid), citric acid (present in orange and lemon) and tartaric acid (present in tamarind). Our stomach also produces hydrochloric acid which helps in the digestion of food. Some commonly used bases are baking soda and tooth paste. Tooth paste is a basic substance used for cleaning the teeth and it neutralizes the excess of acids present in the mouth and prevents tooth decay.

 What are different concepts about acids and bases? 

  • According to Arrhenius concept of acid and bases,an acid is a substance which furnishes H+ ions, when dissolved in water. For example, 

    

  • HCl is an acid and turns blue litmus to red.
  • HCl reacts with zinc metal to form zinc chloride and hydrogen.         
  • HCl reacts with sodium hydroxide, which is a strong alkali, to form sodium chloride. 

 

  • HCl reacts with sodium carbonate to give rise to CO2

Objective(B)

 To carry out the reactions of a base (NaOH) with (i) litmus solution (blue and red), (ii) zinc metal (iii) solid sodium carbonate.

The Theory

 What are bases?

  • Sodium hydroxide (NaOH) is a strong alkali. Its pH is much higher than 7. So, it turns red litmus to blue.
  • NaOH reacts with zinc to form sodium zincate and hydrogen.

  

  • Phenolphthalein is an indicator. In alkaline solution it gives pink colour, whereas it becomes colourless in acidic medium. So, NaOH gives pink colour with phenolphthalein. 

 The strength of an acid depends on its ability to donate a proton and the strength of a base depends on its ability to donate a hydroxide ion. A strong acid dissociates completely in an aqueous solution by losing protons, while a weak acid does not dissociate completely. Also a strong base ionizes completely in an aqueous solution, but a weak base does not ionize completely in the same.

The hydrogen ion concentration is measured using a scale called pH scale. It was introduced by Sorensen in 1909. 

Do you know who Sorensen was?

Soren Peder Lauritz Sorensen was a Danish chemist, famous for the introduction of the concept of pH, a scale for measuring acidity and basicity.   While working at the Carlsberg Laboratory, he studied the effect of ion concentration on proteins, and understood the concentration of hydrogen ions was particularly important. To express the hydronium ion (H3O+) concentration in a solution he devised a logarithmic scale known as pH scale.

Define pH:

pH is defined as the negative logarithm (base ten) of hydrogen ion concentration in moles/ litre.

We can measure the pH value from 0 to 14 on a pH scale. The pH value for an acidic solution is always less than 7 and the pH for a basic solution is always greater than 7. The pH of a neutral solution is 7. The higher the concentration of H+ ions, the lower is the pH value. The increase of pH from 7 to 14 indicates the increase in the concentration of OH - ions. The pH scale is shown below.

                  
The acidic and basic nature of various substances can be studied using different acid-base indicators.

Acidic and basic substances can be identified by noting the change in the colour of the indicator in the acidic and basic medium. Some commonly used acid-base indicators are given below.

Indicator
Acidic medium
Basic medium

Methyl orange

Orange-red

Yellow

Phenolphthalein

Colourless

Pink

Methyl red

Red

Yellow

Phenol red

Yellow

Red

 Action of Litmus solution in an acid and a base:

  • The Litmus solution is another indicator used to identify the acidic and basic nature of a substance. It is a solution of different dyes extracted from lichen plants.
  • Acids have the property to change the colour of blue litmus to red.  Bases change the colour of red litmus to blue.                                                                                         

Learning Outcomes

  1. Students understand  terms like acids & bases based on the given chemical reactions.
  2. Students acquire skills to classify the given compounds as acids and bases using the following materials.
    • Litmus solution
    • Zinc metal
    • Solid sodium carbonate.
  3. Based on acquired skills the student will be able to analyze the given samples & classify them as acids or bases in the future.
  4. Students obtain the skill to safely handle the acids and bases in the lab.
  5. Students acquire a general understanding of the common requirements to perform this experiment & how to arrange it in the lab.

 

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