Chemical Reactions Practice Questions
19 free Chemical Reactions practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following is an example of a 'Combination Reaction'?
- A Heating calcium carbonate to form calcium oxide and carbon dioxide
- B Dipping an iron nail into copper sulfate solution
- C Neutralising an acid with a base
- D Burning magnesium ribbon in air to form magnesium oxide
Correct answer: Burning magnesium ribbon in air to form magnesium oxide
A combination reaction joins two or more reactants into a single product. Burning magnesium in air combines the element magnesium with oxygen to give one compound, magnesium oxide. Heating calcium carbonate is decomposition, iron in copper sulfate is displacement, and acid with base is neutralisation.
What type of reaction occurs when silver chloride turns grey in the presence of sunlight?
- A Thermal decomposition
- B Photolytic decomposition
- C Electrolytic decomposition
- D Displacement reaction
Correct answer: Photolytic decomposition
Silver chloride decomposes into silver metal and chlorine gas when exposed to light. Since light energy is used to break the chemical bonds, it is specifically called photolytic decomposition.
In the reaction: $CuO + H_2 \rightarrow Cu + H_2O$, which substance is being reduced?
- A $H_2$
- B $H_2O$
- C $CuO$
- D $Cu$
Correct answer: $CuO$
Reduction is the loss of oxygen from a substance. Here $CuO$ loses its oxygen and becomes copper metal, so copper(II) oxide is the substance reduced, while hydrogen is oxidised to water.
Which of the following observations is NOT necessarily an indicator that a chemical reaction has taken place?
- A Change of state from solid to liquid
- B Evolution of a gas from the mixture
- C Rise in temperature of the mixture
- D Formation of an insoluble precipitate
Correct answer: Change of state from solid to liquid
Melting is a physical change, not a chemical reaction, as the chemical identity of the substance remains the same. Chemical reactions are typically characterized by the formation of new substances, often accompanied by gas evolution, temperature changes, or color shifts.
What happens when dilute hydrochloric acid is added to iron filings?
- A Hydrogen gas and iron chloride are produced.
- B Chlorine gas and iron hydroxide are produced.
- C No reaction takes place.
- D Iron salt and water are produced.
Correct answer: Hydrogen gas and iron chloride are produced.
Iron is more reactive than hydrogen and displaces it from the acid. This single displacement reaction produces iron(II) chloride and evolves hydrogen gas, which can be tested with a popping sound.
What is the chemical formula for 'rust' formed on iron objects?
- A $Fe(OH)_2 \cdot xH_2O$
- B $Fe_2O_3$
- C $Fe_3O_4$
- D $Fe_2O_3 \cdot xH_2O$
Correct answer: $Fe_2O_3 \cdot xH_2O$
Rust is chemically known as hydrated ferric oxide. It forms when iron reacts with oxygen in the presence of moisture (water), resulting in a flaky brown coating.
Which gas is used to flush bags of potato chips to prevent the oil from becoming rancid?
- A Oxygen
- B Hydrogen
- C Nitrogen
- D Carbon dioxide
Correct answer: Nitrogen
Rancidity is caused by the oxidation of fats and oils. Nitrogen is an inert gas that displaces oxygen in the packaging, preventing the oxidation process and keeping the chips fresh for longer.
When aqueous sodium sulfate reacts with aqueous barium chloride, a white precipitate is formed. This is an example of a:
- A Single displacement reaction
- B Double displacement reaction
- C Combination reaction
- D Redox reaction
Correct answer: Double displacement reaction
In this reaction, the ions of the two reactants exchange partners to form barium sulfate and sodium chloride. Since an insoluble solid (barium sulfate) is formed, it is also known as a precipitation reaction.
What is the primary function of a catalyst in a chemical reaction?
- A To increase the yield of the product
- B To change the physical state of the reactants
- C To speed up the reaction without being used up
- D To supply the activation energy needed for the reaction
Correct answer: To speed up the reaction without being used up
A catalyst works by providing an alternative pathway with a lower activation energy for the reaction. It speeds up the chemical process but remains chemically unchanged at the end of the reaction.
Burning of natural gas (methane) is a/an:
- A Oxidation reaction
- B Reduction reaction
- C Endothermic reaction
- D Decomposition reaction
Correct answer: Oxidation reaction
Combustion is a high-temperature redox reaction between a fuel and an oxidant (usually atmospheric oxygen). During the burning of methane, carbon is oxidized to carbon dioxide.
What is the color of the precipitate formed when lead nitrate reacts with potassium iodide?
- A White
- B Blue
- C Yellow
- D Reddish brown
Correct answer: Yellow
The reaction between lead nitrate and potassium iodide produces lead iodide and potassium nitrate. Lead iodide is an insoluble solid that appears as a brilliant yellow precipitate.
The decomposition of vegetable matter into compost is an example of:
- A Endothermic reaction
- B Exothermic reaction
- C Reversible reaction
- D Combination reaction
Correct answer: Exothermic reaction
The breakdown of organic matter by microbes involves the release of energy in the form of heat. Therefore, composting is classified as an exothermic decomposition process.
Why is it necessary to balance a chemical equation?
- A To satisfy the law of conservation of mass
- B To ensure the reaction goes to completion
- C To satisfy the law of constant proportions
- D To make the equation look organized
Correct answer: To satisfy the law of conservation of mass
According to the law of conservation of mass, matter can neither be created nor destroyed in a chemical reaction. Thus, the total number of atoms of each element must be the same on both the reactant and product sides.
Which of the following is a redox reaction?
- A $CaCO_3 \rightarrow CaO + CO_2$
- B $NaOH + HCl \rightarrow NaCl + H_2O$
- C $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$
- D $AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$
Correct answer: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$
In the zinc and copper sulfate reaction, zinc is oxidised as it loses electrons to form $Zn^{2+}$, while copper ions are reduced to copper metal by gaining those electrons. The other three are thermal decomposition, neutralisation, and precipitation reactions, in which no element changes its oxidation state.
What happens when quicklime ($CaO$) is added to water?
- A It dissolves silently to form a neutral solution.
- B It reacts vigorously to produce slaked lime and heat.
- C It slowly reacts to form calcium metal and oxygen gas.
- D No reaction occurs unless the water is boiling.
Correct answer: It reacts vigorously to produce slaked lime and heat.
Calcium oxide reacts vigorously with water to produce calcium hydroxide (slaked lime). This is a highly exothermic combination reaction that produces a large amount of heat.
In the electrolysis of water, what is the ratio of hydrogen to oxygen gas collected by volume?
- A 1:1
- B 1:2
- C 2:1
- D 8:1
Correct answer: 2:1
Water ($H_2O$) consists of two parts hydrogen for every one part oxygen. During electrolysis, the decomposition yields 2 volumes of hydrogen gas at the cathode for every 1 volume of oxygen gas at the anode.
Which of the following prevents the corrosion of iron by 'Galvanisation'?
- A Coating with tin
- B Coating with zinc
- C Coating with chromium
- D Coating with nickel
Correct answer: Coating with zinc
Galvanisation is the process of applying a protective zinc coating to iron or steel. Zinc is more reactive than iron and acts as a sacrificial anode, corroding in place of the iron even if the coating is scratched.
What is the common name for the thermal decomposition of lead nitrate?
- A Smelting
- B Calcination
- C Cracking
- D Pyrolysis
Correct answer: Pyrolysis
Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere. Heating lead nitrate crystals until they decompose into lead oxide, nitrogen dioxide, and oxygen is a classic laboratory example.
Which type of reaction is represented by: $A + BC \rightarrow AC + B$?
- A Combination
- B Decomposition
- C Double Displacement
- D Displacement
Correct answer: Displacement
A displacement reaction involves a more reactive element taking the place of a less reactive element in a compound. In this general equation, element A has displaced element B from the compound BC.