Chemical Reactions and Equations
Subject: General Science
Sample Practice Questions
Question 1: The reaction between magnesium and oxygen to form magnesium oxide is an example of a:
- Decomposition reaction
- Displacement reaction
- Combination reaction
- Double displacement reaction
Explanation: In this reaction, two reactants (magnesium and oxygen) combine to form a single product (magnesium oxide). This is the definition of a combination reaction: 'Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.'
Question 2: What is the name of the method used to balance chemical equations in the text?
- Algebraic method
- Electron method
- Hit-and-trial method
- Ion-electron method
Explanation: The text states: 'This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.'
Question 3: The reaction between barium hydroxide and ammonium chloride is:
- Exothermic
- Endothermic
- Combination
- Redox
Explanation: The text describes the activity: 'Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix ... Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?' This reaction absorbs heat from the surroundings, making the test tube feel cold, hence it is endothermic.
Question 4: What is the chemical formula of marble?
- CaO
- Ca(OH)2
- CaCO3
- CaCl2
Explanation: The 'Do You Know?' section states: 'It is interesting to note that the chemical formula for marble is also CaCO3.' Marble is a form of calcium carbonate.
Question 5: Which of the following gases is evolved when dilute hydrochloric acid is added to iron fillings?
- Chlorine gas
- Oxygen gas
- Hydrogen gas
- Carbon dioxide gas
Explanation: The reaction is: Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g). Iron reacts with dilute hydrochloric acid to produce iron chloride and hydrogen gas. This is confirmed by the question in the text: 'What happens when dilute hydrochloric acid is added to iron fillings?' and the correct option is hydrogen gas and iron chloride are produced.
Question 6: What is the colour of the iron nail after it has been dipped in copper sulphate solution for some time?
- Black
- Brownish
- Greenish
- Remains the same
Explanation: The text states: 'Why does the iron nail become brownish in colour and the blue colour of copper sulphate solution fades?' The brownish colour is due to the deposition of copper metal on the surface of the iron nail.
Question 7: The reaction between barium chloride and sodium sulphate is an example of:
- Combination reaction
- Displacement reaction
- Double displacement reaction
- Redox reaction
Explanation: The reaction is: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq). There is an exchange of ions between the reactants (Ba2+ with SO42- and Na+ with Cl-), forming a precipitate. The text calls this a double displacement reaction (and a precipitation reaction).
Question 8: What is the common name of calcium hydroxide?
- Quick lime
- Slaked lime
- Lime water
- Limestone
Explanation: The reaction is: CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat. (Quick lime) (Slaked lime). Calcium hydroxide is commonly known as slaked lime. Its solution is called lime water.
Question 9: What is the correct balanced chemical equation for the reaction of iron with water (steam)?
- Fe + H2O → FeO + H2
- 2Fe + 3H2O → Fe2O3 + 3H2
- 3Fe + 4H2O → Fe3O4 + 4H2
- Fe + 2H2O → Fe(OH)2 + H2
Explanation: The text details the step-by-step balancing of this reaction, concluding with the balanced equation: 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g). This equation satisfies the law of conservation of mass, with equal numbers of each atom on both sides.
Question 10: What is the colour of ferrous sulphate crystals before heating?
- Blue
- White
- Green
- Grey
Explanation: The activity states: 'Note the colour of the ferrous sulphate crystals. ... Have you noticed that the green colour of the ferrous sulphate crystals has changed?' The green crystals (FeSO4·7H2O) lose water and change colour upon heating.