Test your understanding of chemical reactions with this “TNPSC Chemistry Chemical Reactions quiz”. This free online test covers important basics such as combination, decomposition, displacement, neutralization, oxidation, reduction, redox and combustion reactions, along with reactants, products and the law of conservation of mass. Attempt all 20 questions and use the explanations to strengthen your TNPSC General Science preparation.
Note: The detailed explanations provided below the test can help you clarify difficult points, revise the topic, and improve your overall preparation.
TNPSC Chemistry Chemical Reactions Quiz 1 – Essential Points for Revision
The explanations below help you understand the key ideas behind each question, rather than simply checking the correct answer. You will revise how chemical reactions take place, how to identify combination, decomposition, displacement and neutralisation reactions, and how oxidation, reduction and redox reactions are related. The explanations also cover important examples such as rusting, burning of magnesium, electrolysis of water and combustion, along with reactants, products, precipitates and the law of conservation of mass.
1. A chemical reaction is a process in which:
Answer: B. New substances are formed
Explanation:
A chemical reaction is a process in which one or more substances, called reactants, undergo a chemical change to form one or more new substances, called products. The new substances have different chemical properties from the original substances.
During a chemical reaction, the atoms present in the reactants are rearranged to form new substances. Chemical reactions may be accompanied by observable changes such as a change in colour, formation of a gas, formation of a precipitate, change in temperature, or emission of light.
For example, when iron reacts with oxygen and moisture, it forms iron oxide (rust). Rust has properties different from those of iron, showing that a new substance has been formed.
Real-Life Example:
When magnesium ribbon burns in oxygen, it forms magnesium oxide, a new substance. The formation of magnesium oxide shows that a chemical reaction has taken place.
2. Which of the following indicates that a chemical reaction has occurred?
Answer: D. All of the above
Explanation:
A chemical reaction often produces observable changes that indicate the formation of new substances. Common signs include a change in colour, formation of a gas, and change in temperature. Another important indication is the formation of a precipitate, which is an insoluble solid formed when two solutions react. These changes occur because the original substances undergo a chemical transformation and form products with different properties.
A colour change may occur when new substances with different colours are formed. Gas formation can be observed as bubbles or effervescence, while a temperature change may occur when heat is released or absorbed during the reaction. However, these observations are indicators rather than the definition of a chemical reaction; the key feature is the formation of new substances.
Real-Life Example:
When vinegar reacts with baking soda, bubbles of carbon dioxide gas are produced and the mixture becomes cooler. These observable changes indicate that a chemical reaction has occurred.
3. When iron reacts with oxygen to form rust, the reaction is called:
Answer: A. Combination reaction
Explanation:
A combination reaction is a chemical reaction in which two or more substances combine to form a single product. When iron reacts with oxygen in the presence of moisture, iron oxide (rust) is formed. Since iron and oxygen combine to form an iron oxide product, rusting is commonly classified as a combination reaction at the school level.
Rusting is also an example of corrosion, in which iron gradually reacts with oxygen and moisture from the surroundings. The rust formed has properties different from those of iron.
Real-Life Example:
An iron gate or bicycle left exposed to air and moisture gradually develops a reddish-brown layer of rust because iron reacts with oxygen in the presence of moisture.
4. A reaction in which a single substance breaks down into simpler substances is called:
Answer: B. Decomposition reaction
Explanation:
A decomposition reaction is a chemical reaction in which one compound breaks down into two or more simpler substances. It is essentially the opposite of a combination reaction. Decomposition can occur when a compound is supplied with heat, light, or electricity, depending on the substance involved.
For example, when calcium carbonate (CaCO₃) is strongly heated, it breaks down into calcium oxide (CaO) and carbon dioxide (CO₂).
CaCO₃ → CaO + CO₂
This reaction is a thermal decomposition reaction because heat is required for the compound to break down.
Real-Life Example:
Baking soda decomposes on heating to produce sodium carbonate, carbon dioxide, and water vapour. This decomposition is useful in baking because the carbon dioxide gas helps baked foods rise.
5. When zinc reacts with hydrochloric acid, hydrogen gas is produced. This is an example of:
Answer: A. Displacement reaction
Explanation:
A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound. Zinc is more reactive than hydrogen, so when zinc reacts with hydrochloric acid, it displaces hydrogen from hydrochloric acid. As a result, zinc chloride and hydrogen gas are formed.
Zn + 2HCl → ZnCl₂ + H₂
The release of hydrogen gas can be observed as bubbles during the reaction. This reaction is also useful for understanding the reactivity series of metals, where zinc is placed above hydrogen.
Real-Life Example:
When zinc granules are added to dilute hydrochloric acid in a laboratory, bubbles of hydrogen gas are produced as zinc gradually reacts with the acid.
6. The reaction between an acid and a base to form salt and water is called:
Answer: D. Neutralization reaction
Explanation:
A neutralization reaction occurs when an acid reacts with a base to form salt and water. In this reaction, the acidic and basic properties are neutralized. The hydrogen ions (H⁺) from the acid combine with hydroxide ions (OH⁻) from the base to form water, while the remaining ions form the salt.
For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride (salt) and water:
HCl + NaOH → NaCl + H₂O
Neutralization reactions are important because they help reduce excessive acidity or alkalinity in different situations.
Real-Life Example:
Antacids contain basic substances that react with excess hydrochloric acid in the stomach, helping to neutralize the acid and relieve acidity.
7. In oxidation reactions:
Answer: C. Oxygen is gained
Explanation:
Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen. In terms of electron transfer, oxidation also involves the loss of electrons. The gain of oxygen is one of the simplest ways to identify oxidation at the school level.
For example, when magnesium reacts with oxygen, it gains oxygen and forms magnesium oxide:
2Mg + O₂ → 2MgO
Here, magnesium undergoes oxidation because it combines with oxygen.
Real-Life Example:
The rusting of iron is an oxidation process because iron reacts with oxygen in the presence of moisture to form iron oxide. This is why exposed iron objects gradually develop a reddish-brown layer of rust.
8. In reduction reactions:
Answer: A. Oxygen is lost
Explanation:
Reduction is a chemical process in which a substance loses oxygen or gains hydrogen. In terms of electron transfer, reduction involves the gain of electrons. Loss of oxygen is a simple way to identify reduction in many chemical reactions.
For example, when copper oxide reacts with hydrogen, copper oxide loses oxygen and forms copper:
CuO + H₂ → Cu + H₂O
Here, copper oxide undergoes reduction because it loses oxygen.
Real-Life Example:
During the extraction of metals from their ores, metal oxides can be reduced by removing oxygen from them. For example, iron oxide can be reduced to obtain iron during the extraction of iron from its ore.
9. Oxidation and reduction reactions occur together and are called:
Answer: B. Redox reactions
Explanation:
A redox reaction is a chemical reaction in which oxidation and reduction occur simultaneously. The term redox is formed from the words reduction and oxidation. When one substance loses electrons through oxidation, another substance gains those electrons through reduction. Therefore, oxidation and reduction always occur together in a redox reaction.
For example, when zinc reacts with copper sulphate, zinc loses electrons and undergoes oxidation, while copper ions gain electrons and undergo reduction:
Zn + CuSO₄ → ZnSO₄ + Cu
Here, zinc is oxidized and copper ions are reduced.
Real-Life Example:
Rusting of iron involves oxidation of iron, while oxygen is reduced during the overall reaction. This makes rusting an example of a redox process.
10. Rusting of iron is an example of:
Answer: D. Oxidation
Explanation:
Rusting is a slow chemical process in which iron reacts with oxygen in the presence of moisture to form hydrated iron(III) oxide, commonly called rust. It is considered an oxidation process because iron combines with oxygen during the reaction.
Rusting requires both oxygen and moisture. The presence of salts or acids can speed up the corrosion process. The reddish-brown rust formed is different from the original iron, showing that a new substance has been produced.
Real-Life Example:
An iron gate exposed to rain and air gradually develops a reddish-brown coating. This happens because iron reacts with oxygen and moisture over time, causing rusting.
11. Burning of magnesium ribbon in air produces:
Answer: A. Magnesium oxide
Explanation:
When a magnesium ribbon burns in air, it reacts rapidly with the oxygen present in the air and forms magnesium oxide (MgO). Magnesium burns with a bright white flame and produces a white powder of magnesium oxide. This is a combination reaction because magnesium and oxygen combine to form a single product.
2Mg + O₂ → 2MgO
The reaction also involves oxidation, because magnesium gains oxygen during the reaction.
Real-Life Example:
In a laboratory, a magnesium ribbon is cleaned with sandpaper before burning to remove the thin layer of magnesium oxide already present on its surface. When ignited, it burns with a dazzling white flame.
12. A reaction in which two substances combine to form a single product is called:
Answer: B. Combination reaction
Explanation:
A combination reaction is a chemical reaction in which two or more reactants combine to form a single product. The general form of a combination reaction can be represented as A + B → AB. The substances that combine may be elements, compounds, or a combination of both.
For example, calcium oxide reacts with water to form a single product, calcium hydroxide:
CaO + H₂O → Ca(OH)₂
Since two reactants combine to produce one product, this is a combination reaction.
Real-Life Example:
When quicklime (calcium oxide) is mixed with water, it forms slaked lime (calcium hydroxide). This reaction is used in processes such as construction and whitewashing.
13. A reaction in which one element replaces another in a compound is called:
Answer: C. Displacement reaction
Explanation:
A displacement reaction occurs when a more reactive element replaces a less reactive element from its compound. The ability of an element to displace another element depends on its position in the reactivity series. A more reactive metal can displace a less reactive metal from its salt solution.
For example, when zinc is added to copper sulphate solution, zinc displaces copper because zinc is more reactive than copper:
Zn + CuSO₄ → ZnSO₄ + Cu
Here, zinc displaces copper from copper sulphate, producing zinc sulphate and copper.
Real-Life Example:
When a zinc strip is placed in copper sulphate solution, the blue colour of the solution gradually changes because zinc displaces copper from copper sulphate. Copper is deposited on the zinc surface.
14. The substances that take part in a chemical reaction are called:
Answer: B. Reactants
Explanation:
Reactants are the starting substances that take part in a chemical reaction. During the reaction, the atoms in the reactants are rearranged to form new substances called products. Reactants are written on the left side of a chemical equation, while the products are written on the right side.
For example, in the reaction 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are the reactants, while water is the product.
Real-Life Example:
When hydrogen and oxygen combine to form water, hydrogen and oxygen are the reactants because they are the substances that participate in the reaction.
15. The substances formed after a chemical reaction are called:
Answer: C. Products
Explanation:
Products are the new substances formed as a result of a chemical reaction. They usually have chemical properties that are different from those of the reactants. In a chemical equation, products are written on the right side of the arrow.
For example, in 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are the reactants, while water (H₂O) is the product formed by the reaction.
Real-Life Example:
When magnesium burns in oxygen, magnesium oxide is formed. Magnesium oxide is the product because it is the new substance produced during the reaction.
16. Which of the following is an example of a decomposition reaction?
Answer: A. Electrolysis of water
Explanation:
A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. During the electrolysis of water, electrical energy causes water to decompose into hydrogen gas and oxygen gas.
2H₂O → 2H₂ + O₂
Here, water is the single reactant, while hydrogen and oxygen are the products. Since one compound breaks down into simpler substances, electrolysis of water is a decomposition reaction. More specifically, it is an electrolytic decomposition reaction because electricity is used to bring about the decomposition.
Real-Life Example:
In a laboratory, passing electric current through water can produce bubbles of hydrogen and oxygen gases at the electrodes. This demonstrates the decomposition of water.
17. A precipitate is:
Answer: B. A solid formed during a reaction
Explanation:
A precipitate is an insoluble solid that forms when two solutions containing dissolved substances react chemically. Since the newly formed substance does not dissolve in the solution, it separates out as a solid. The formation of a precipitate is one of the observable signs of a chemical reaction.
For example, when silver nitrate solution reacts with sodium chloride solution, insoluble silver chloride is formed as a white precipitate.
AgNO₃ + NaCl → AgCl↓ + NaNO₃
The downward arrow (↓) indicates the formation of a precipitate.
Real-Life Example:
When certain water-treatment chemicals are added to water, unwanted dissolved substances can form insoluble solids that settle down. This process helps remove impurities from the water.
18. The law of conservation of mass states that:
Answer: C. Mass remains constant in a chemical reaction
Explanation:
The law of conservation of mass states that mass can neither be created nor destroyed during a chemical reaction. The total mass of the reactants is equal to the total mass of the products when the reaction is carried out in a closed system.
This happens because chemical reactions involve the rearrangement of atoms. The atoms are not created or destroyed; they are simply rearranged to form new substances. Therefore, the number and types of atoms remain the same before and after the reaction.
Real-Life Example:
If a chemical reaction is carried out in a sealed container, the mass measured before the reaction will be equal to the mass measured after the reaction, even if gases are produced inside the container.
19. Which of the following is an example of a combustion reaction?
Answer: A. Burning of wood
Explanation:
A combustion reaction is a chemical reaction in which a substance reacts rapidly with oxygen, usually releasing heat and light. Burning of wood is a combustion reaction because the substances present in wood react with oxygen and produce heat, light, carbon dioxide, water vapour, and other products.
Combustion is generally an exothermic reaction, meaning that energy is released to the surroundings in the form of heat.
Real-Life Example:
When wood burns in a fireplace, it reacts with oxygen in the air and releases heat and light, which is why combustion reactions are commonly used as sources of energy.
20. Chemical reactions involve:
Answer: D. Rearrangement of atoms
Explanation:
Chemical reactions involve the rearrangement of atoms to form new substances. The atoms present in the reactants are not created or destroyed; instead, their bonds are broken and new bonds are formed, producing substances with different chemical properties.
For example, when hydrogen reacts with oxygen to form water, the hydrogen and oxygen atoms are rearranged into water molecules:
2H₂ + O₂ → 2H₂O
The same hydrogen and oxygen atoms are present before and after the reaction, but they are arranged differently in the product.
Real-Life Example:
When food is digested, complex substances in food are chemically broken down into simpler substances. The atoms are rearranged into new molecules that the body can absorb and use.
Further Reference:
Want to learn more about chemical reactions and equations? You can read the NCERT Science chapter to understand combination, decomposition, displacement, oxidation and reduction reactions with simple explanations and examples. This is also useful for revising important TNPSC General Science topics and strengthening your basics in chemistry.
Read the NCERT chapter on Chemical Reactions and Equations
