TEXTUAL QUESTIONS
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QUESTIONS
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Why should a magnesium ribbon be cleaned before burning in air?
A magnesium ribbon should be cleaned before burning in air to remove the protective layer of magnesium oxide (and sometimes magnesium carbonate) that forms on its surface due to reaction with atmospheric oxygen (and carbon dioxide). This layer can prevent the magnesium from burning properly or efficiently. Cleaning, usually by rubbing with sandpaper, exposes the fresh magnesium metal for a more vigorous reaction. -
Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
H₂(g) + Cl₂(g) → 2HCl(g)
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) -
Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
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QUESTIONS
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A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.-
Substance ‘X’ is Calcium oxide.
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Its formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
(Calcium oxide reacts with water to form calcium hydroxide, also known as slaked lime)
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Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
In Activity 1.7 (electrolysis of water), the gas collected in one test tube (at the cathode) is hydrogen (H₂), and the gas collected in the other test tube (at the anode) is oxygen (O₂).
The amount of hydrogen gas collected is double the amount of oxygen gas collected because in a water molecule (H₂O), the ratio of hydrogen atoms to oxygen atoms is 2:1. When water is electrolyzed, it decomposes into its constituent elements, hydrogen and oxygen, in the same stoichiometric ratio.
The balanced equation for the electrolysis of water is: 2H₂O(l) → 2H₂(g) + O₂(g). This equation clearly shows that 2 volumes of hydrogen gas are produced for every 1 volume of oxygen gas.
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QUESTIONS
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Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
When an iron nail is dipped in a copper sulphate solution, the colour of the solution changes (from blue to light green) because iron is more reactive than copper. Iron displaces copper from the copper sulphate solution, forming iron(II) sulphate (which is light green) and depositing copper metal on the iron nail (which appears brownish).
Reaction: Fe(s) + CuSO₄(aq) (blue) → FeSO₄(aq) (light green) + Cu(s) (brownish) -
Give an example of a double displacement reaction other than the one given in Activity 1.10.
Activity 1.10 likely refers to the reaction: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).
Another example of a double displacement reaction is the reaction between silver nitrate and sodium chloride:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
(In this reaction, a white precipitate of silver chloride is formed). -
Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O₂(g) → 2Na₂O(s)-
Substance oxidised: Sodium (Na) – It gains oxygen.
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Substance reduced: Oxygen (O₂) – It combines with sodium (effectively gaining electrons, though this example focuses on oxygen gain/loss).
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l) -
Substance oxidised: Hydrogen (H₂) – It gains oxygen (to form H₂O).
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Substance reduced: Copper oxide (CuO) – It loses oxygen (to form Cu).
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MAIN EXERCISE BASED QUESTIONS
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Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b) -
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a
(d) displacement reaction. -
What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced. -
What is a balanced chemical equation? Why should chemical equations be balanced?
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A balanced chemical equation is an equation where the number of atoms of each element is the same on both the reactant and product sides of the equation.
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Chemical equations should be balanced to satisfy the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction.
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Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
3H₂(g) + N₂(g) → 2NH₃(g)
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) -
Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
(c) NaCl + AgNO₃ → AgCl + NaNO₃
NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq) (Already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HCl(aq) -
Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq) -
Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)
Type of reaction: Double displacement reaction (specifically, a precipitation reaction).
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
ZnCO₃(s) → ZnO(s) + CO₂(g)
Type of reaction: Decomposition reaction.
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
H₂(g) + Cl₂(g) → 2HCl(g)
Type of reaction: Combination reaction.
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Type of reaction: Displacement reaction. -
What does one mean by exothermic and endothermic reactions? Give examples.
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Exothermic reactions: Reactions in which heat is released along with the formation of products.
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Example: Burning of natural gas: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat
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Endothermic reactions: Reactions in which energy is absorbed.
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Example: Decomposition of calcium carbonate: CaCO₃(s) + Heat → CaO(s) + CO₂(g)
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Why is respiration considered an exothermic reaction? Explain.
Respiration is considered an exothermic reaction because energy is released during this process. During digestion, carbohydrates from food (like rice, potatoes, bread) are broken down into simpler substances like glucose. This glucose combines with oxygen in the cells of our body and undergoes oxidation, releasing energy (in the form of ATP), carbon dioxide, and water. Since energy is released, it is an exothermic process.
Equation: C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy -
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Decomposition reactions are called the opposite of combination reactions because:-
In a combination reaction, two or more substances combine to form a single new substance.
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Equation: A + B → AB
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Example: 2Mg(s) + O₂(g) → 2MgO(s)
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In a decomposition reaction, a single substance breaks down to give two or more simpler substances.
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Equation: AB → A + B
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Example: 2H₂O(l) –(Electricity)–> 2H₂(g) + O₂(g)
Thus, one involves synthesis (combination) and the other involves breakdown (decomposition).
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Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
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Heat: CaCO₃(s) –(Heat)–> CaO(s) + CO₂(g) (Thermal decomposition)
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Light: 2AgCl(s) –(Sunlight)–> 2Ag(s) + Cl₂(g) (Photolytic decomposition)
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Electricity: 2H₂O(l) –(Electricity)–> 2H₂(g) + O₂(g) (Electrolytic decomposition)
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What is the difference between displacement and double displacement reactions? Write equations for these reactions.
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Displacement Reaction: In a displacement reaction, a more reactive element displaces a less reactive element from its compound.
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Equation: A + BC → AC + B
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Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces copper)
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Double Displacement Reaction: In a double displacement reaction, there is an exchange of ions between two reacting compounds to form two new compounds.
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Equation: AB + CD → AD + CB
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Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Exchange of sulphate and chloride ions)
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In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s) -
What do you mean by a precipitation reaction? Explain by giving examples.
A precipitation reaction is a type of chemical reaction in which two soluble salts in aqueous solution combine, and one of the products is an insoluble salt called a precipitate, which separates out from the solution.-
Example 1: AgNO₃(aq) + NaCl(aq) → AgCl(s) (precipitate) + NaNO₃(aq) (Formation of white precipitate of silver chloride)
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Example 2: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) (precipitate) + 2NaCl(aq) (Formation of white precipitate of barium sulphate)
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Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
Oxidation is a process that involves the gain of oxygen by a substance or the loss of hydrogen from a substance.-
Example 1 (Gain of oxygen): 2Cu(s) + O₂(g) → 2CuO(s) (Copper gains oxygen to form copper oxide)
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Example 2 (Gain of oxygen): C(s) + O₂(g) → CO₂(g) (Carbon gains oxygen to form carbon dioxide)
(b) Reduction
Reduction is a process that involves the loss of oxygen from a substance or the gain of hydrogen by a substance. -
Example 1 (Loss of oxygen): CuO(s) + H₂(g) → Cu(s) + H₂O(l) (Copper oxide loses oxygen to form copper)
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Example 2 (Loss of oxygen): ZnO(s) + C(s) → Zn(s) + CO(g) (Zinc oxide loses oxygen to form zinc)
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A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
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Element ‘X’ is Copper (Cu).
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The black coloured compound formed is Copper(II) oxide (CuO).
Reaction: 2Cu(s) + O₂(g) –(Heat)–> 2CuO(s)
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Why do we apply paint on iron articles?
We apply paint on iron articles to prevent them from rusting. Paint creates a barrier between the iron surface and the atmospheric oxygen and moisture, which are necessary for rusting (corrosion of iron) to occur. -
Oil and fat containing food items are flushed with nitrogen. Why?
Oil and fat containing food items are flushed with nitrogen to prevent them from becoming rancid. Rancidity occurs when fats and oils oxidize, leading to unpleasant smells and tastes. Nitrogen is an inert gas and does not react with the fats and oils, thereby displacing oxygen and preventing oxidation, thus increasing the shelf life of the food items. -
Explain the following terms with one example each.
(a) Corrosion
Corrosion is the process of gradual deterioration of metals by the action of air, moisture, or chemicals (like acids) on their surface.-
Example: Rusting of iron: Iron objects when exposed to moist air get coated with a reddish-brown flaky substance called rust (Fe₂O₃.xH₂O).
(b) Rancidity
Rancidity is the condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste. -
Example: Potato chips or oily snacks left exposed to air for a long time develop an off-flavor and smell due to the oxidation of the oils they contain.
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