All questions
14
Q1
What observations or changes help us determine that a chemical reaction has taken place? Give one example for each.
A chemical reaction is indicated by one or more of these observations: change in state, change in colour, evolution of a gas, change in temperature, and formation of a precipitate. For example, burning of magnesium ribbon produces a dazzling white light and a white ash (change in colour and state); reaction of zinc with dilute acid produces gas bubbles (evolution of hydrogen gas) and the flask becomes warm (change in temperature).
Q2
Why should a magnesium ribbon be cleaned with sandpaper before burning in air? Write the balanced chemical equation for the burning of magnesium ribbon.
Magnesium ribbon is cleaned before burning because it develops a thin layer of magnesium oxide (MgO) on its surface when exposed to air. This oxide layer prevents the magnesium from making direct contact with oxygen, making ignition difficult. Cleaning with sandpaper removes this layer so the fresh metal surface can react freely. Balanced equation: .
Q3
Distinguish between a word equation and a chemical equation. Why are chemical equations preferred for representing chemical reactions?
A word equation describes a chemical reaction using the names of reactants and products separated by an arrow, e.g., 'Magnesium + Oxygen → Magnesium oxide.' A chemical equation uses chemical formulae instead of names, e.g., '.' Chemical equations are preferred because they are concise, convey the exact identity and proportion of each substance, and allow balancing to satisfy the law of conservation of mass.
Q4
State the law of conservation of mass. Why must a chemical equation be balanced? Give an example of an unbalanced (skeletal) equation and show how it is balanced.
The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction — the total mass of reactants equals the total mass of products. A chemical equation must be balanced so that the number of atoms of each element is the same on both sides. Skeletal equation: . To balance: . Now atoms of Mg (2 on both sides) and O (2 on both sides) are equal.
Q5
Balance the following equation using the hit-and-trial method and write the final balanced equation with state symbols: . Why is H₂O written with the state symbol (g) in the balanced form?
Using the hit-and-trial method: Initial skeletal equation . Step I — balance oxygen: add coefficient 4 to H₂O → . Step II — balance hydrogen: add coefficient 4 to H₂ → . Step III — balance iron: add coefficient 3 to Fe → . Final balanced equation with state symbols: . H₂O is written as (g) because steam (water vapour) is used in this reaction.
Q6
What is a combination reaction? Describe the reaction of quicklime (calcium oxide) with water, give the balanced chemical equation, and explain its application in whitewashing walls. Is this reaction exothermic?
A combination reaction is a chemical reaction in which two or more substances (elements or compounds) combine to form a single product. Quicklime () reacts vigorously with water to form slaked lime () with the release of a large amount of heat, so it is exothermic. Balanced equation: . In whitewashing, a solution of slaked lime is applied to walls. It slowly reacts with from air to form a thin, hard layer of calcium carbonate (), which gives the walls a shiny finish after 2-3 days.
Q7
What are decomposition reactions? Explain thermal, electrolytic, and photolytic (photochemical) decomposition with one chemical equation for each type.
A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more simpler substances. Thermal decomposition uses heat: (ferrous sulphate decomposes into ferric oxide, sulphur dioxide, and sulphur trioxide). Electrolytic decomposition uses electricity: (water decomposes into hydrogen and oxygen). Photolytic decomposition uses light: (silver chloride decomposes into silver and chlorine).
Q8
What happens when an iron nail is dipped in copper sulphate solution? Write the balanced chemical equation and explain the displacement process. Why does a colour change occur?
When an iron nail is dipped in blue copper sulphate () solution, iron displaces copper from copper sulphate because iron is more reactive than copper. The blue colour of the solution fades, and a brownish coating of copper metal deposits on the iron nail. The reaction is: . The colour change happens because the blue ions are replaced by pale green ions in the solution.
Q9
What is a double displacement reaction? Explain with the example of the reaction between sodium sulphate and barium chloride solutions. What is a precipitation reaction and how is it related to the above example?
A double displacement reaction involves an exchange of ions between two reactant compounds, forming two new compounds. When sodium sulphate solution reacts with barium chloride solution, a white insoluble solid — barium sulphate — precipitates out: . A precipitation reaction is any reaction in which an insoluble solid (precipitate) is formed. The above reaction is also a precipitation reaction because is insoluble in water and appears as a white precipitate.
Q10
Define oxidation and reduction in terms of gain or loss of oxygen. Using the reaction of copper oxide with hydrogen, identify which substance is oxidised and which is reduced. Explain why this reaction is called a redox reaction.
Oxidation is the gain of oxygen (or loss of hydrogen) by a substance. Reduction is the loss of oxygen (or gain of hydrogen) by a substance. In the reaction : copper oxide loses oxygen and is reduced to copper, while hydrogen gains oxygen (forming water) and is oxidised. It is called a redox reaction because both oxidation and reduction occur simultaneously — one reactant is oxidised while the other is reduced.
Q11
What is corrosion? Explain with reference to the rusting of iron. Why are iron articles often painted, and how does it prevent corrosion?
Corrosion is the process in which a metal is attacked and damaged by substances in its environment such as moisture and acids. Rusting of iron is the most common example — when iron is exposed to moist air, it develops a reddish-brown powdery coating called rust. Iron articles are painted to create a protective barrier that prevents the metal surface from coming into direct contact with moisture and air, slowing or preventing corrosion.
Q12
What is rancidity? Explain why food items containing oil and fat are often packaged in an atmosphere of nitrogen gas. List the methods used to prevent rancidity.
Rancidity is the spoilage of oils and fats in food due to oxidation by atmospheric oxygen, leading to an unpleasant smell and taste. Nitrogen gas is used in packaging because it is an inert (unreactive) gas that displaces oxygen, preventing the oxidation of fats and oils. Methods to prevent rancidity include: (i) adding antioxidants (substances that prevent oxidation), (ii) storing food in airtight containers to limit exposure to air, (iii) flushing food packages with nitrogen gas, and (iv) refrigeration to slow down the oxidation rate.
Q13
Differentiate between exothermic and endothermic reactions. Why is respiration considered an exothermic process? Give one example of each type of reaction.
Exothermic reactions release heat energy to the surroundings, making the reaction mixture warm. Endothermic reactions absorb heat energy from the surroundings, making the reaction mixture cool. Respiration is an exothermic process because glucose () combines with oxygen in our body cells to release energy, which is used for life processes: . Example of exothermic reaction: burning of natural gas (). Example of endothermic reaction: thermal decomposition of calcium carbonate ().
Q14
Differentiate between displacement and double displacement reactions with the help of a balanced chemical equation for each type.
In a displacement reaction, a more reactive element displaces a less reactive element from its compound — only one element moves. Example: (iron displaces copper). In a double displacement reaction, ions are exchanged between two compounds, forming two new compounds. Example: (sodium and barium exchange their partner ions). Key difference: displacement involves one element replacing another, while double displacement involves a mutual exchange of ions between two compounds.