The percentage composition of carbon (iv) oxide in the atmospheric air is about 0.03% by volume while in dissolved air is about 0.50% by volume.


Laboratory preparation

  1. Carbon (iv) oxide is prepared in the laboratory by the action of dilute acids on a trioxocarbonate (iv) or a hydrogen trioxocarbonate (iv). Usually CaCO3, in form of marble chips, shells or ‘potash’; is used with hydrochloric acid or trioxonitrate (v) acid. Reaction between CaCO3 and HCl can be carried out in a Kipp’s apparatus.

CaCO3(s)             +        2HCl(aq)                      CaCl2(aq) + H2O(l) + CO2(g)

NaHCO3(aq)       +     HNO3(aq)                    NaNO3(aq)   +  H2O(l)     +    CO2(g)

  1. It is also prepared by heating metallic trioxocarbonates (iv) [except those of Na and K], or the hydrogen trioxocarbonate (iv) of Na or K.

CuCO3(s)                        CuO(s)     +       CO2(g)

Note: If the gas is required  dry, it is pass through potassium hydrogen trioxocarbonate (iv) solution first to remove any acid fumes, and then through a U-tube containing fused Calcium chloride to remove the water vapour. The dry gas is then collected by downward delivery as it is heavier than air.


Method of collection of gases

The method of collection of gases depends on its:

  1. Density.
  2. Solubility.


There are two method of collecting gases:

(a) Downward delivery/upward displacement of air: This method is used for collecting gases that are denser than air e.g. CO2, SO2, H2S, NO2, Cl2 and HCl e.t.c.


(b) Upward delivery/downward displacement of air: This method is used for collecting gases that are less denser than air e.g NH3, H2, N2, methane and ethane.



CO2 is obtained industrially as a by product in fermentation processes and when limestone is heated to make quicklime.



(1) CO2 is a colourless, odourless gas with a sharp refreshing taste.

(2) It is about 1.5 times denser than air.

(3) It is soluble in water. At room temperature and standard pressure, water dissolves its own volume of the gas.

(4) It turns damp blue litmus paper pink because CO2 dissolves in water to yield trioxocarbonate (iv) acid.

(5) On cooling, it readily liquefies and solidifies (-780C) to form a white solid known as dry ice.



  1. Reaction with water: Carbon (iv) oxide is not very active chemically. It dissolves in water to form trioxocarbonate (iv) acid (Soda water). This is a weak, dibasic acid which ionizes slightly.

(a)  CO2(g)             +     H2O(l)            H2CO3(aq)

(b)  H2CO3(aq)     +     H2O(l)            H3O+(aq)       +    HCO3(aq)

(c)  HCO3 (aq)       +     H2O(l)                H3O+(aq)     +    CO32-(aq)

On heating, trioxocarbonate (iv) acid decomposes to form H2O(l) and CO2(g).


  1. Reaction with alkalis: It reacts directly with alkalis to yield trioxocarbonate (iv)

CO2(g)     +  2NaOH(aq)         Na2CO3(aq)       +       H2O(l)


Excess CO2 reacts with alkalis to produce Hydrogen trioxocarbonate (iv) salt.

CO2(g)       +  NaOH(aq)               NaHCO3(aq)


  1. Reaction with burning Na, K or Mg: CO2 is reduced to carbon by burning Na, K or Mg.

CO2(g)     +      2Mg(s)          C(s)      +       2MgO(s)

Note: CO2 does not support combustion.


  1. Reaction with red hot carbon: CO2 is reduced to CO, If the gas is passed over red hot carbon.

CO2(g)      +     C(s)                 2CO(g)

The reaction is of great importance in the blast furnace and in the manufacture of gaseous fuels.


Test for CO2

Bubble the unknown gas through a solution of lime water (Calcium hydroxide)if the lime water turn milky due to the formation of insoluble calcium trioxocarbonate (iv), then the unknown gas is CO2

Ca(OH)2(aq)        +      CO2(g)         CaCO3(s)     +       H2O(l).

If the gas is bubbled in excess, the milkiness disappears and turns to a clear solution due to the formation of soluble calcium hydrogen trioxocarbonate (iv).

CaCO3(s)    +   H2O(l)    +   CO2(g)          Ca(HCO3)(aq)

Finally, if the clear solution is heated, the milkiness reappears due to the decomposition of soluble Ca(HCO3)2 to form insoluble CaCO3

Ca(HCO3)2(aq)      CaCO3(s)    +  H2O(l)   +   CO2(g)


Uses of carbon (iv) oxide

  1. It is used as fire extinguishers since it does not support combustion.
  2. It gives carbonated (aerated) drinks their refreshing taste. Beer, cider and champagne contains CO2
  3. It is used in the manufacture of Na2CO3 (washing soda) by the Solvay process.
  4. It is used as a leavening agent in the baking of bread. Yeast and baking powder produces CO2 which make the dough of bread to rise.
  5. It is used in the manufacture of fertilizer (such as urea and (NH4)2SO4.
  6. Solid CO2 (i.e dry ice) is used as a refrigerant for perishable goods e.g ice cream. (It sublimes on warming and provides a lower temperature).
  7. Gaseous CO2 is used to preserve fruits.
  8. CO2 is also used as a coolant in nuclear reactors.



  1. Describe the laboratory preparation of dry carbon (iv) oxide.
  2. Write balanced equation to show the following reactions of CO2:

(a)  Reaction with sodium hydroxide

(b)  Reaction with burning magnesium




  1. Carbon (ii) oxide can be prepared by passing Carbon (iv) oxide through red-hot carbon while the Carbon (iv) oxide is itself reduced to Carbon (ii) oxide. The gaseous mixture is passed through concentrated NaOH to remove the excess Carbon (iv) oxide.

CO2(g)            +       C(s)            2CO(g)

The pure Carbon (ii) oxide is collected over water.


  1. Carbon (ii) oxide can also be prepared by the dehydration of methanoic (formic) acid or ethanedioic (oxalic) acid, using concentrated tetraoxosulphate (vi) acid.

HCOOH(l)          Conc. H2SO4    CO(g)   +   H2O

Methanoic acid

Note: The gaseous mixture is passed through concentrated NaOH to remove the CO2.

Caution: The preparation of CO must be done in a fume cupboard as the gas is poisonous.

The major air pollutants that can result from smoky vehicles are Carbon (ii) oxide and Carbon particles.


When CO is breath in for any length of time, even 1% of it in the air may cause death, which makes it clear how dangerous it can be to run a car engine in a closed garage or a generator in a closed room. Very often the victim collapse without warning so insidious is its effect.



(1) CO is a poisonous, colourless, tasteless and odourless gas.

(2) It is insoluble in water, but dissolves in a solution of ammoniacal copper (i) chloride.

(3) It is neither lighter nor heavier than air.

(4) It is neutral to litmus.


(1) As a reducing agent: CO is a strong reducing agent. It reduces some metallic oxides to the metals and it is oxidized to CO2.

PbO(s)   +    CO(g)       Pb(s)         +       CO2(g)

Fe2O3(s)  +   3CO(g)     2Fe(s)        +       3CO2(g)

CuO(s)    +    CO(g)      Cu(s)         +       CO2(g)

  1. Combination reaction

(a). With oxygen: CO burns in air with a faint pale blue flame to form CO2 .

2CO(g)      +      O2(g)                         2CO2(g)

(b).  With haemoglobin: CO combine irreversibly with haemoglobin in the  red blood cells to form carboxy-haemoglobin thereby preventing the red corpuscle from acting as oxygen carrier.

  1. CO mixed with NaOH at 1500C under 50atm pressure to form Sodium formate.

CO(g)       +     NaOH(aq)                   HCOONa(aq)

This product decomposes into Sodium oxalate and H2 at 4000C


2HCOONa(aq)      +    H2(g)  4000C   COONa

  1. CO combined with certain metals (e.g Ni, Fe, Cobalt) when hot to form a volatile carbonyl.

Ni(s)    +     4CO(g)            Ni(CO)4(aq)

  1. CO is absorbed readily by a solution of Copper (i) Chloride (CuCl) in aqueous NH3 or conc. HCl forming a compound, CuCl.CO.2H2O. This reaction is used to separate CO from a mixture of gases.

CO(g)  +  CuCl(s)   +  2NH4OH(aq)       CuCl.CO.2H2O(aq)    +   2NH3(g)

  1. CO combined with Chlorine gas when expose to ultra-violet light or passed over a catalyst of activated charcoal at 1500C to form carbonyl chloride.

CO(g)     +       Cl2(g)               COCl2(g)

This product, COCl2, is also known as Phosgene and was employed as a poisonous gas in the First World War. It is now use in the manufacture of dyestuff.


Test for Carbon (ii) oxide

When a lighted splint is inserted into a test tube containing CO(g) and some lime water added to it and shaken, the CO(g) burn with a pale-blue flame and turn lime water milky after burning but not before burning.


Uses of Carbon (ii) oxide

(1) CO is used in the extraction of metals from their ores.

(2) It is also an important constituent of gaseous fuels like producer gas and water gas.

(3) CO gas is used in the manufacture of methyl alcohol, synthetic petrol, carbonyl chloride, oxalate and formate.



  1. Describe the laboratory preparation of Carbon (ii) oxide.
  2. Explain why Carbon (ii) oxide cannot be collected by any method of delivery
  3. Write two equations to show the chemical properties of Carbon (ii) oxide



  1. What is the pH of a solution whose H+ concentration is 3.0 x 10-5moldm-3?
  2. Mention the separation technique that be used to recover a soluble salt that (a) decompose on heating (b) does not decompose on heating
  3. How many molecules are there in 14g of Nitrogen gas at s.t.p? [N=14, NA=6.0 x 102]
  4. What is the number of oxygen atoms in 32g of the gas?
  5. State Avogadro’s law



  1. Kipp’s apparatus is important in the laboratory because it (a) allows intermittent supply of gases. (b) is used for preparing poisonous gases. (c) is used to prepare light gas. (d) is used to prepare sensitive gas
  2. Gas prepared by the reaction between methanoic acid and concentrated tetraoxosulphate (vi) acid is (a) SO2 (b) CO (c) CO2           (d) H2S.
  3. Gas which dissolves in ammoniacal copper (i) chloride but insoluble in water is

(a) NH3 (b) CO (c) N2O (d) CO2.

  1. Where else is CO2 found in free state apart from the atmosphere?

(a) In carbonated drinks. (b) Dissolved form in water. (c) In corals. (d) In limestone region

  1. It is dangerous to stay in a badly ventilated room which has a charcoal fire because of the presence of (a) carbon (ii) oxide (b) carbon (iv) oxide (c) hydrogen sulphide (d) producer gas.



1(a) Why is the laboratory preparation of carbon (ii) oxide done in a fume chamber?

(b) State the property of CO2 that makes it to be used in (i) carbonated drinks (ii) fire extinguishers

2(a) Why it is not advisable to stay in a closed garage for a long time when racing a car engine.

(b). State what is observed when (i) excess CO2 is bubbled through lime water. (ii) the solution in b(i) above is heated.


See also






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