Downward delivery method

The gas collected is passed through a delivery tube connected to a gas jar in an upright position. The gas displaces the air in the gas jar upwardly and the method is also referred to as upward displacement of air.


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This method is used to collect gases that are:

  • Denser than air
  • Required dry
  • Soluble in water
    Examples of gases collected by this method include: carbon dioxide, sulphur dioxide, chlorine and hydrogen chloride.
  1. Use of a gas syringe
    This method collects any gas irrespective of its density or solubility (i.e. whether the gas is soluble in water or not; whether the gas is denser than air or not). It is however suitable for collecting gases produced in small quantities.
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Advantages of using syringe method

  • It is easy to know if the syringe is full of the gas or not.
  • It is easy to transport the gas.
  • Air does not easily mix with the gas collected, therefore it is almost pure.
  • Collects any gas irrespective of its properties like solubility and density.
  • Volume of gas collected is known and correct volumes of gases are used.
    Substances that contain water can be dried by drying agents or desiccants.

Desiccants are substances that have very high affinity for water (water loving compounds) and therefore remove water from other substances. Desiccants can either be hygroscopic or deliquescent substances.

  1. Drying gases
    When gases are required dry, they are passed through liquid and solid desiccants. Examples of these desiccants include: concentrated sulphuric acid, anhydrous calcium chloride, calcium oxide and silica gel.
    Concentrated sulphuric acid placed in a vessel is used for drying gases like sulpur dioxide, oxygen, carbondioxide, chlorine and oxygen. The gas is bubbled through concentrated sulphuric acid using a delivery tube as shown below.
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Gases like ammonia and hydrogen sulphide react with concentrated sulphuric acid and are not dried by it. Instead solid desiccants like fused calcium chloride and calcium oxide (quick lime) placed in a U-tube or quick lime tower is used.

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Gases dried through the U- tube include oxygen, carbon dioxide, hydrogen and hydrogen sulphide. Ammonia is dried is a quick lime tower as shown below.

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  1. Drying liquid
    Organic liquids are the only liquids that can be dried as some contains little amount of water. The liquid can be dried by the process of fractional distillation or by use of desiccants such as fused calcium chloride, calcium oxide, magnesium sulphate e.t.c. placed in a desiccator as shown below.
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A desiccator consists of a lid, upper chamber and lower chamber. The substance to be dried is placed in the upper chamber and the drying agent is placed in the lower chamber. The circumference of the lid is smeared with oil or grease to make it ai air proof and the substance remains dry as long as it is in the desiccators.

  1. Drying solids
    Solids with moisture can be dried by slight warming or by evaporating water from the solid at a temperature slightly below the melting point of the solids. Solids can also be dried by placing them in a special drying apparatus (desiccators) as for liquids.
    Sample questions on the atmosphere and Oxygen

The atmosphere

1.What is the approximate percentage composition of the atmosphere by volume?. Descried a laboratory experiment by which you could verify the figure you give for the percentage of one element in the atmosphere. Name four substances found in the atmosphere in small quantities.

  1. Air is considered a mixture. Briefly explain why it is regarded as a mixture.
  2. What is rust?. State the conditions necessary for rusting to occur. What steps can be taken to prevent iron from rusting?. Descried experiments to prove that iron does not rust unless when both air and water are present. State briefly three ways in which rust differs from a mixture of iron and oxygen.
  3. What is air pollution? Mention at least five environmental pollutants and briefly explain how each pollutes the environment. Give at least four sources of environmental pollutants. Outline some measures that can be taken t reduce on environmental pollution.
  4. Describe an experiment you can carry out to investigate the products formed when a burning candle is burnt in air.
  5. How would you prove experimentally that air contains (a) water vapour (b) carbon dioxide (c) oxygen in the laboratory.


  1. Describe with the aid of labeled drawings how you can prepare oxygen in the laboratory from: hydrogen peroxide, potassium chlorate and sodium peroxide. During the preparation of oxygen a catalyst is added, name the catalyst and mention its role. What is the proof that the named substance is actually acting as a catalyst.
  1. Describe a simple experiment to show what fraction of air is oxygen. State briefly how oxygen is prepared from air on a large scale. Mention three uses of oxygen.
  2. With the aid of equations, describe the reaction of oxygen/air with sodium, potassium, calcium, magnesium, copper, carbon, sulphur, and phosphorus.
  3. Non metals react with oxygen to form acid anhydrides, what is an acid anhydride? What is an oxide? Describe with the aid of equations and examples the different types of oxides formed by elements.
  1. Explain the observation that
    a) Magnesium burns in carbon dioxide to form a mixture of white ash and black solid particles. Use an equation to illustrate.
    b) When a mixture of magnesium and copper (II) oxide is heated, a red glow spreads through the mixture leaving a white ash and a brown solid.
    c) When a mixture of carbon and copper (II) Oxide is heated in a test tube, a brown solid is formed and a colorless gas that turns lime water milky ( C02) is liberated.

6.Sodium is higher in the electrochemical series than copper. Explain what this statement means.

  1. Describe the different ways of collecting gases, mentioning clearly the examples of gasses collected by each method and why. Drawings must be included as part of the description. Outline the advantages of using a syringe over other methods to collect gases.
  2. What is a desiccant? Explain briefly how gases, liquids and solids are dried.