H2CO3 is formed when CO2(g) is dissolved in water. H2CO3 is a weak dibasic acid. It forms two series of salts:

  1. Normal trioxocarbonate (iv)
  2. Acidic hydrogen trioxocarbonate (iv)


Normal trioxocarbonate (iv)

Normal trioxocarbonate (iv) may be regarded as salts derived from H2CO3 by the complete replacement of the hydrogen by a metal or cationic radical.


Preparation of soluble trioxocarbonates (iv)

The CO32- of Na+, K+, and NH4+ are soluble in water. They are prepared in the laboratory by:

Bubbling CO2 through a solution of corresponding alkali.

2KOH(aq)     +    CO2(g)          K2CO3(aq)      +      H2O(l)

Decomposition of corresponding hydrogen trioxocarbonates (iv).

2KHCO3(s)            K2CO3(aq)       +       H2O(l)     +     CO2(g)

Preparation of insoluble trioxocarbonates (iv)

The insoluble metallic trioxocarbonates (iv) can be prepared by adding a solution of Na2CO3 or NaHCO3 to a solution of the corresponding metallic salt.

CaCl2(aq)      +     Na2CO3(aq)       CaCO3(s)    +   2NaCl(aq)

CaCl2(aq)    +    2NaHCO3(aq)      CaCO3(s)   +   2NaCl(aq)  +  H2O(l)  +  CO2(g) 2AgNO3(aq)   +  Na2CO3(aq)          Ag2CO3(s)  +   2NaNO3(aq)

2AgNO3(aq)  +  2NaHCO3(aq)       Ag2CO3(s)   +   2NaNO3(aq)  + H2O(l)  + CO2(g)

Note: When preparing the CO32- of the less electropositive metals like Cu, use NaHCO3


Properties of CO32- Salts

Solubility: The trioxocarbonate (iv) of alkali metal and NH4+ are soluble while the other trioxocarbonate (iv) are insoluble in water.

Na2CO3(s)   +  2H2O(l)        2NaOH(aq)   +     H2CO3(aq)

  1. Action of heat: The trioxocarbonate (iv) of Na, K and Barium cannot be decomposed by heat while all other CO32- decompose on heating to liberate CO2.

ZnCO3(s)            ZnO(s)   +   CO2(g)

2Ag2CO3(s)           4Ag(s)       +   2CO2(g)     +     O2(g)

(NH4)2CO3(s)        2NH3(g)  +    CO2(g)    +     H2O(l)

  1. Reaction with dilute acids: All trioxocarbonates (iv) react with dilute acids to form CO2, H2O and a salt.

Na2CO3(aq)  +  H2SO4(aq)     Na2SO4(aq)  +  H2O(l)  +  CO2(g)

ZnCO3(s)       +  2HCl(aq)       ZnCl2(aq)    +  H2O(l)  +  CO2(g)

MetalSolubility/effect of heatReaction with acids
K, NaSoluble in water. Does not decompose on heating 

These trioxocarbonate (iv) react with dilute acids to give a salt, water and carbon (iv) oxide.

Ca, Mg, Al, Zn

Fe, Sn

Pb, Cu

Insoluble in water. Decompose to yield the oxide and carbon (iv) oxide. Al2(CO3)3 does not exist.
Hg, Ag


Insoluble in water. Decomposed to the metal, CO2 and oxygen


Test for any CO32-

The unknown substance is placed in a test-tube and dilute trioxonitrate (v) acid is added into the test tube. If a CO32- is present, there will be effervescence and the gas which evolved will turn calcium hydroxide solution (lime water) milky.

CO32-(s)     +    2H+(aq)                    H2O(l)     +       CO2(g)



  1. Describe the laboratory preparation of soluble trioxocarbonate (iv) salts
  2. Giving suitable equation, state two properties of trioxocarbonate (iv) salts.



HCO3 may also be regarded as salts derived from H2CO3 by the partial replacement of the hydrogen by a metal or cationic radical.


Preparation of HCO3

HCO3 can be prepared by passing CO2 through a cold solution of the corresponding OH or CO32-.

  1. 2OH(aq) + CO2(g)                       CO32-(aq)       +    H2O(l)
  2. CO32-(aq) + CO2 (g)      +       H2O (l)                    2HCO3(aq)


Properties of HCO3

  1. Solubility: All hydrogen trioxocarbonate (IV) are soluble in water.
  2. Action of heat: They can all be decomposed by heat.

2NaHCO3(s)                      Na2CO3(s)   +   H2O(l)      +     CO2(g)

  1. Reaction with dilute acids: All HCO3 reacts with dilute acid to produce CO2, H2O and a salt. 2NaHCO3 (aq) + H2SO4 (aq)        Na2SO4(aq)   +    2H2O(l)  +  2CO2(g)

NOTE: This reaction is used to test for HCO3



  1. Describe the laboratory preparation of HCO3
  2. 2. List the properties of HCO3



  1. State Boyle’s law and Charles’ law
  2. A certain mass of gas occupies 300cm3 at 35oC. At what temperature will it have its volume reduced by half, assuming its pressure remains constant
  3. An enclosed vessel contains 2.8g of nitrogen and 14.2g of chlorine at atmospheric pressure and 0oC. What will be the partial pressure of nitrogen, if the temperature is raised to 180oC?
  4. A certain mass of hydrogen gas collected over water at 10oC and 760mmHg pressure has a volume of 37cm3. Calculate the volume when it is dry at s.t.p (saturated vapour pressure of water at 10oC = 9.2mmHg)
  5. State Graham’s law of diffusion



  1. All are decomposed by heat except (a) BaCO3 (b) CuCO3 (c) ZnCO3 (d) Ag2CO3
  2. The liquid from a bee sting is found to be acidic. Which of the following solutions will neutralize the bee sting? (a) Solution of pH 9 (b) Solution of pH 7 (c) Solution of pH 5 (d) Solution of pH 1
  1. What is the IUPAC name of K4Fe(CN)6? (a) Potassium hexacyanoferrate (ii) (b) Potassium hexacyanoferrate (iv) (c) Potassium hexacyanoferrate (vi) (d) Potassium hexacyanoferrate (vii)
  1. Which of the following trioxocarbonates (iv) are not decomposed by heat? (a) Calcium and sodium trioxocarbonate (iv) (b) Potassium and zinc trioxocarbonate (iv) (c) Magnesium and potassium trioxocarbonate (iv) (d) Sodium and potassium trioxocarbonate (iv)
  2. An aqueous solution of CaCl2 is (a) acidic solution. (b) alkaline solution (c) a buffer solution (d) a neutral solution



  1. State the function of each of the following substances in the laboratory preparation of dry carbon (iv) oxide: (a) potassium hydrogen trioxocarbonate (iv) solution; (b) fused calcium chloride.
  2. (a) Draw a labelled diagram for the laboratory preparation of dry carbon (iv) oxide. (b) Write a balanced equation for the decomposition of potassium hydrogen trioxocarbonate (iv).


See also






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