Magnesium ribbon should be cleaned before burning in air because it reacts with oxygen in the air to form a thin layer of magnesium oxide (MgO) on its surface. This layer prevents the magnesium from burning properly. By cleaning it with sandpaper, we remove this protective layer, allowing the magnesium to burn efficiently with a dazzling white flame.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
(i) Name the substance 'X' and write its formula.
Substance X: Calcium oxide (Quick lime)
Formula: CaO
(ii) Write the reaction of the substance 'X' named in (i) above with water.
This reaction produces calcium hydroxide (slaked lime) which is used for whitewashing walls.
In Activity 1.7 (electrolysis of water), the gas collected in one test tube is hydrogen and in the other is oxygen. The amount of hydrogen gas collected is double the amount of oxygen gas because water (H₂O) contains two hydrogen atoms for every oxygen atom. During electrolysis, water decomposes as:
This shows that 2 molecules of hydrogen gas are produced for every 1 molecule of oxygen gas, hence the volume of hydrogen is double that of oxygen.
The gas collected in double amount is Hydrogen (H₂).
When an iron nail is dipped in copper sulphate solution, the colour changes from blue to green because of a displacement reaction. Iron, being more reactive than copper, displaces copper from copper sulphate solution. The reaction is:
The blue colour of copper sulphate (CuSO₄) fades and changes to the light green colour of iron sulphate (FeSO₄). Additionally, a brown coating of copper metal gets deposited on the iron nail.
An example of a double displacement reaction is the reaction between sodium carbonate and calcium chloride:
In this reaction, calcium carbonate (CaCO₃) is formed as a white precipitate, and sodium chloride (NaCl) remains in solution.
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
Oxidised: Sodium (Na) - because it gains oxygen to form sodium oxide
Reduced: Oxygen (O₂) - because it gets incorporated into sodium oxide (loses its elemental form)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Oxidised: Hydrogen (H₂) - because it gains oxygen to form water
Reduced: Copper oxide (CuO) - because it loses oxygen to form copper metal
Correct answer: (i) (a) and (b)
Explanation:
So statements (a) and (b) are incorrect.
Correct answer: (d) displacement reaction
Explanation: This is a displacement reaction where aluminium (Al) displaces iron (Fe) from iron oxide (Fe₂O₃). This is also known as the thermite reaction.
Correct answer: (a) Hydrogen gas and iron chloride are produced.
Explanation: The reaction is:
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
Iron reacts with dilute hydrochloric acid to produce iron(II) chloride and hydrogen gas.
Balanced chemical equation: A chemical equation in which the number of atoms of each element is equal on both sides of the equation (reactants and products).
Why chemical equations should be balanced:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(This equation is already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Type: Double displacement reaction
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Type: Decomposition reaction
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Type: Combination reaction
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Type: Displacement reaction
Exothermic reactions: Reactions in which heat is released along with the formation of products. The reaction mixture becomes warm.
Examples:
Endothermic reactions: Reactions in which energy is absorbed from the surroundings. The reaction mixture becomes cold.
Examples:
Respiration is considered an exothermic reaction because it involves the breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and energy. This energy is used by our body for various life processes. The chemical equation for respiration is:
Since energy is released during this process, respiration is an exothermic reaction.
Decomposition reactions are called the opposite of combination reactions because:
Examples:
Combination reaction: 2H₂(g) + O₂(g) → 2H₂O(l)
Decomposition reaction: 2H₂O(l) → 2H₂(g) + O₂(g)
As we can see, the decomposition reaction is exactly the reverse of the combination reaction.
Decomposition by heat (Thermal decomposition):
Decomposition by light (Photochemical decomposition):
Decomposition by electricity (Electrolysis):
Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its compound.
Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Double displacement reaction: A reaction in which there is an exchange of ions between two reactants to form new compounds.
Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Key difference: In displacement reactions, one element replaces another in a compound, while in double displacement reactions, ions are exchanged between two compounds.
In this reaction, copper displaces silver from silver nitrate solution because copper is more reactive than silver. The silver metal gets deposited, which is then recovered and refined.
Precipitation reaction: A reaction in which an insoluble solid (called precipitate) is formed when two solutions are mixed. Any reaction that produces a precipitate can be called a precipitation reaction.
Example 1:
Here, barium sulphate (BaSO₄) is the white precipitate formed.
Example 2:
Here, lead iodide (PbI₂) is the yellow precipitate formed.
(a) Oxidation: The process in which a substance gains oxygen or loses hydrogen during a reaction.
Examples:
(b) Reduction: The process in which a substance loses oxygen or gains hydrogen during a reaction.
Examples:
Element X: Copper (Cu)
Black coloured compound: Copper(II) oxide (CuO)
Reaction: 2Cu + O₂ → 2CuO
The shiny brown copper metal reacts with oxygen in air when heated to form black copper(II) oxide.
We apply paint on iron articles to prevent corrosion (rusting). Paint forms a protective layer on the surface of iron, preventing it from coming in contact with air and moisture. Since both air (oxygen) and moisture are required for rusting to occur, painting iron articles effectively prevents rust formation and increases their lifespan.
Oil and fat containing food items are flushed with nitrogen to prevent them from becoming rancid. Rancidity is the oxidation of fats and oils present in food, which causes a change in their smell and taste. Nitrogen is an inert gas that does not support oxidation. By flushing food packages with nitrogen, oxygen is displaced, thus preventing oxidation and keeping the food fresh for a longer time.
(a) Corrosion: The process of slowly eating away of metals due to attack by atmospheric gases, moisture, acids, etc.
Example: Rusting of iron - When iron is exposed to moist air, it forms a reddish-brown coating of hydrated ferric oxide (rust) on its surface.
(b) Rancidity: The oxidation of fats and oils present in food materials, resulting in a change in their taste and smell.
Example: When butter is left exposed to air for a long time, it develops an unpleasant smell and taste due to oxidation.