🎓 Scholarpad

Carbon and its compounds

Hydrogen Trioxocarbonates

Hydrogen trioxocarbonates are salts formed when only one hydrogen atom of carbonic acid (H₂CO₃) is replaced by a metal or ammonium ion. Their general formula is MHCO₃, where M represents a monovalent metal such as sodium, potassium, or ammonium.

These compounds are also known as bicarbonates and are intermediate salts between carbonates and carbonic acid.

Examples

Sodium hydrogen trioxocarbonate (NaHCO₃)
Potassium hydrogen trioxocarbonate (KHCO₃)
Ammonium hydrogen trioxocarbonate (NH₄HCO₃)

Lab. Preparation of NaHCO₃

Principle: Sodium hydrogen carbonate is prepared by passing excess carbon dioxide through a cold, concentrated solution of sodium carbonate.

Equation:

$$ \small{Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3} $$

Process: The reaction is carried out in a cooled saturated solution of sodium carbonate. The white crystalline sodium hydrogen carbonate precipitates out because it is less soluble in water than sodium carbonate.

Physical Properties

Property	Description
Appearance	White crystalline solid or powder
Solubility	Moderately soluble in cold water; decomposes slightly in hot water
Taste	Alkaline but mildly salty taste
Decomposition Temperature	Decomposes on heating to form sodium carbonate, carbon dioxide, and water

Chemical Properties

Action of Heat
On heating, sodium hydrogen trioxocarbonate decomposes:

$$\tiny{ 2NaHCO_3 \xrightarrow. Na_2CO_3 + H_2O + CO_2 }$$
Reaction with Acids
It reacts with dilute acids to liberate carbon dioxide gas:

$$ \tiny{NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2} $$
Reaction with Bases
On heating with strong alkalis, it forms carbonate and water:

$$ \tiny{NaHCO_3 + NaOH \rightarrow Na_2CO_3 + H_2O} $$
Reaction with Limewater
Bicarbonates react with calcium hydroxide to form calcium carbonate:

$$ \tiny{2NaHCO_3 + Ca(OH)_2 \rightarrow CaCO_3 + Na_2CO_3 + 2H_2O }$$

Chemical Test

Add a few drops of dilute acid (HCl or H₂SO₄) to the sample.
Effervescence occurs — a gas (CO₂) is evolved.
Pass the gas through limewater.
Limewater turns milky, confirming the presence of a hydrogen trioxocarbonate.

Equation:

$$ \tiny{NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2 }$$

$$ \small{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O} $$

Differences

Feature	Trioxocarbonates (Carbonates)	Hydrogen Trioxocarbonates (Bicarbonates)
Formula	M₂CO₃	MHCO₃
Reaction with Acids	Effervescence occurs rapidly and vigorously	Effervescence occurs gently
Reaction with Magnesium tetraoxosulphate (VI)	Forms white precipitate of magnesium trioxocarbonate (IV)	No visible reaction
Thermal Decomposition	Stable to moderate heat (except some like CaCO₃)	Decomposes easily on heating to form carbonate, CO₂, and H₂O
Reaction with Limewater	Forms precipitate directly	Forms precipitate more slowly
Solubility	Some are insoluble (e.g., CaCO₃)	All are soluble in water

General Uses

As baking soda (NaHCO₃): Used in baking to make dough rise by releasing CO₂ on heating.
In fire extinguishers: Releases CO₂ to smother flames.
As an antacid: Neutralizes excess stomach acid (HCl) in medicine.
In cleaning agents: Used as a mild abrasive and deodorizer.
In beverages: Source of carbon dioxide in fizzy drinks.
In laboratories: Used to produce pure CO₂ gas.

Summary Table

Compound	Formula	Key Properties	Main Uses
Sodium hydrogen trioxocarbonate	NaHCO₃	White crystalline solid, decomposes on heating	Baking, fire extinguishers, medicine
Potassium hydrogen trioxocarbonate	KHCO₃	Soluble in water, mild alkali	Fire extinguishers, pH regulation
Ammonium hydrogen trioxocarbonate	NH₄HCO₃	Unstable, releases NH₃ and CO₂ on heating	Baking powder, leavening agent, smelling salts