Form 1:
Air and Combustion

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Chemical properties of Oxygen /combustion

Oxygen is a very reactive non metal.

Many elements react with oxygen through burning to form a group of compounds called Oxides.

Burning/combustion is the reaction of Oxygen with an element/substances.

Reaction in which a substance is added oxygen is called Oxidation reaction.

Burning/combustion are an example of an oxidation reaction.

Most non metals burn in Oxygen/air to form an Oxide which in solution / dissolved in water is acidic in nature.

They turn blue litmus red.e.g. Carbon (IV) oxide/CO2, Nitrogen (IV) oxide/ NO2, Sulphur (IV) oxide/ SO2

Some non metals burn in Oxygen/air to form an Oxide which in solution / dissolved in water is neutral in nature. They don’t turn blue or red litmus. E.g. Carbon (II) oxide/CO, Water/ H2O

All metals burns in Oxygen/air to form an Oxide which in solution/dissolved in water is basic/alkaline in nature. They turn red litmus blue.e.g. Magnesium oxide/MgO, Sodium Oxide/ Na2O, Copper (II) oxide/CuO

Elements/substances burn faster in pure Oxygen than in air

Air contains the inactive part of air that slows the rate of burning of substances/elements.

(i)Reaction of metals with Oxygen/air

The following experiments show the reaction of metals with Oxygen and air.

I. Burning Magnesium

Procedure

(a)Cut a 2cm length piece of magnesium ribbon.

Using a pair of tongs introduce it to a Bunsen flame.

Remove it when it catches fire.

Observe.

Place the products in a beaker containing about 5cm3 of water.

Test the solution/mixture using litmus papers

(b)Cut another 2cm length piece of magnesium ribbon.

Using a pair of tongs introduce it to a Bunsen flame.

When it catches fire, lower it slowly into a gas jar containing Oxygen.

Place about 5cm3 of water into the gas jar.

Test the solution/mixture using litmus papers.

Test the solution/mixture using litmus papers

Observations

(a)In air

Magnesium burns with a bright blindening flame in air forming white solid/ash /powder.

Effervescence/bubbles/ fizzing Pungent smell of urine.

Blue litmus paper remains blue.

Red litmus paper turns blue

(b) In pure Oxygen

Magnesium burns faster with a very bright blindening flame pure oxygen forming white solid/ash /powder.

No effervescence/bubbles/ fizzing.

No pungent smell of urine.

Blue litmus paper remains blue.

Red litmus paper turns blue

Explanation

Magnesium burns in air producing enough heat energy to react with both Oxygen and Nitrogen to form Magnesium Oxide and Magnesium nitride.

Both Magnesium Oxide and Magnesium nitride are white solid/ash /powder.

Chemical equations

Magnesium + Oxygen -> Magnesium Oxide,
2Mg(s) + O2(g) -> 2MgO(s)

Magnesium + Nitrogen -> Magnesium Nitride 3Mg(s) + N2(g) -> Mg3N2 (s)

Magnesium Oxide dissolves in water to form a basic/alkaline solution of Magnesium hydroxide

Chemical equations

Magnesium Oxide + Water -> Magnesium hydroxide
2Mg(s) + O2 (l) -> 2MgO(s)

Magnesium Nitride dissolves in water to form a basic/alkaline solution of Magnesium hydroxide and producing Ammonia gas. Ammonia is also an alkaline/basic gas that has a pungent smell of urine.

Chemical equations

Magnesium Nitride + Water -> Magnesium hydroxide + Ammonia gas
Mg3N2 (s) + 6H2O (l) -> 3Mg (OH)2 (aq) + 2NH3(g)

II. Burning Sodium

Procedure

(a)Carefully cut a very small piece of sodium. Using a deflagrating spoon introduce it to a Bunsen flame.

Remove it when it catches fire.

Observe.

Place the products in a beaker containing about 20cm3 of water. Test the solution/mixture using litmus papers

(b) Carefully cut another very small piece of sodium.

Using a deflagrating spoon introduce it to a Bunsen flame.

When it catches fire, lower it slowly into a gas jar containing Oxygen.

Place about 20 cm3 of water into the gas jar. Test the solution/mixture using litmus papers. Test the solution/mixture using litmus papers

Observations

(a)In air

Sodium burns with a yellow flame in air forming a black solid. Blue litmus paper remains blue. Red litmus paper turns blue

(b) In pure Oxygen

Sodium burns faster with a golden yellow flame in pure oxygen forming a yellow solid.

Effervescence/bubbles/ fizzing. Gas produced relights glowing splint.

Blue litmus paper remains blue. Red litmus paper turns blue.

Explanation

(a)Sodium burns in air forming black Sodium Oxide

Chemical equations

Sodium + Oxygen/air -> Sodium Oxide
4Na(s) + O2 (g) -> 2Na2O(s)

Sodium Oxide dissolves in water to form a basic/alkaline solution of Sodium hydroxide

Chemical equations

Sodium Oxide + Water -> Sodium hydroxide
Na2O(s) + H2O (l) -> 2NaOH (aq)

(b)Sodium burns in pure oxygen forming yellow Sodium peroxide

Chemical equations

Sodium + Oxygen -> Sodium peroxide
2Na(s) + O2 (g) -> Na2O2 (s)

Sodium peroxide dissolves in water to form a basic/alkaline solution of Sodium hydroxide. Oxygen is produced.

Chemical equations

Sodium Oxide + Water -> Sodium hydroxide + Oxygen 2Na2O2(s) + 2H2O (l) -> 4NaOH (aq) + O2 (l)

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