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The following observations were made by student A, B, C, and D when they rubbed solid baking soda on dry litmus paper : Student Effect on dry red litmus paper Effect on dry blue litmus paper A Color Changed to blue No change B No change Color changed to red C Color changed to blue Color changed to red D No change No change The correct observation was made by the student

The following observations were made by student A, B, C, and D when they rubbed solid baking soda on dry litmus paper : Student Effect on dry red litmus paper Effect on dry blue litmus paper A Color Changed to blue No change B No change Color changed to red C Color changed to blue Color changed to red D No change No change The correct observation was made by the student Correct Answer D

Key Points

  • Litmus paper is an acid/base indicator that changes colour and is used to distinguish between bases and acids.
  • These are used extensively to verify a substance's acidity or alkalinity in laboratories and educational institutions.
  • Litmus is a pigment produced from naturally occurring compounds, like lichens (fungi), of which Roccella tinctoria is the most widely used species. 
  • Under acidic conditions, blue litmus paper turns red and stays the same in basic conditions.
  • But, the litmus paper will only change its colour if either of litmus paper or the chemical on which it has to be tested is in aqueous solution form. 
  • If the chemical and litmus paper both are dry then there will be no change in colour. 
  • An acid is material in an aqueous solution that releases hydrogen ions.
  • Blue litmus paper is prepared by treating plain paper with lichen-blue litmus.
  • Acids such as sulphuric acid, hydrochloric acid, nitric acid, ethanoic acid, citric acid, vinegar, etc. are usually tested with blue litmus paper. 

Explanation:

  • Sodium hydrogen carbonate (NaHCO3) commonly known as Baking Soda is prepared when Sodium Chloride is reacted with water along with Ammonia (NH3) and carbon dioxide.
  • It can be given as in the form of the equation as


NaCl + H2O + CO2 + NH3 ⇒ NaHCO3 + NH4Cl

  • Where NH4Cl is ammonium chloride.
  • Baking Soda is widely used for cooking etc.
  • When Baking Soda is mixed with a mild acid like tartaric acid in presence of water or carbon dioxide we get Baking Powder.
  • Its Chemical equation can be given as NaHCO3 + H+ ⇒ H2O + CO2 + Sodium Acid Salt
  • The reaction of Baking Soda with water produces water, carbon dioxide and Sodium carbonate.
  • The baking soda solution is a basic solution and has a pH > 7.
  • The solution thus becomes basic in nature.
  • But, here baking soda is solid and not in aqueous form. Also litmus paper is dry. Either of them needed to be in an aqueous state. 
  • So, there will be no change. The correct option is D. 

Additional Information

  • Some indicators and their colour change are given in the table below:
Indicator Colour Change in Acidic Solution Colour Change in Basic Solution
Phenolphthalein Colourless Pink
Methyl Orange Red or Orange Yellow
Litmus Paper Red Blue
Turmeric Yellow Red
China Rose Dark pink Green

Related Questions

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