Chemical Reactions

Chemical Reactions

Table of Contents

Introduction of Chemical Reactions

Chemical reactions are processes where substances undergo transformations to form new substances.

Importance of chemical reactions in everyday life:

  • Energy Production: Reactions like combustion produce energy for heating, cooking, and transportation.
  • Metabolism: In our bodies, chemical reactions break down food and produce energy.
  • Medicine: Chemical reactions are crucial in the synthesis of medications and treatments.
  • Manufacturing: They’re essential in producing everyday items like plastics, cosmetics, and cleaning agents.
  • Environment: Natural processes like photosynthesis involve chemical reactions that maintain our ecosystem.

Types of Chemical Reactions

Combination (Synthesis) Reactions:
  • Explanation: Two or more substances combine to form a single new compound.
  • Example: (Hydrogen gas combines with oxygen gas to form water.)

Synthesis Reaction | Formula, Types & Examples - Lesson |

Decomposition Reactions:
  • Explanation: A single compound breaks down into two or more simpler substances.
  • Example: (Water breaks down into hydrogen and oxygen gases.)

Decomposition Reaction: Definition, Types, Examples, Uses & FAQs

Single Displacement (Replacement) Reactions:
  • Explanation: An element reacts with a compound, displacing another element from it.
  • Example: (Zinc reacts with copper sulfate, displacing copper.)

Displacement Reactions - Chemistry Steps

Double Displacement (Metathesis) Reactions:
  • Explanation: Two compounds exchange ions or bonds to form two new compounds.
  • Example: (Silver nitrate reacts with sodium chloride to form silver chloride and sodium nitrate.)

What is a double displacement reaction?

Reactants and Products

Reactants and Products


  • Definition: Reactants are the starting substances in a chemical reaction.
  • Role: They undergo chemical changes and get transformed into new substances during the reaction.

How to identify reactants and products in a chemical reaction - Quora


  • Definition: Products are the substances formed as a result of a chemical reaction.
  • Role: They are the end result of the reaction and are produced from the reactants.
Identify Reactants and Products

Chemical Equation Format: A chemical equation represents a chemical reaction using symbols and formulas for reactants and products.


  • Position: Reactants are typically written on the left side of a chemical equation, separated by a plus sign or an arrow.
  • Symbols: They are represented by chemical formulas or element symbols.


  • Position: Products are written on the right side of a chemical equation, separated by an arrow.
  • Symbols: Like reactants, products are also represented by chemical formulas or element symbols.

Consider the combustion of methane () in oxygen () to produce carbon dioxide () and water ():

  • Reactants: and are the reactants as they are on the left side of the equation.
  • Products: and are the products as they are on the right side of the equation.

Balancing Chemical Equations

Balancing Chemical Equations | Definition, Process & Examples - Lesson |

  • Conservation of Mass: Balancing ensures that the number of atoms of each element is the same on both sides of the equation, maintaining the principle of conservation of mass.
  • Predicting Products: A balanced equation provides a clear understanding of the reactants and products involved in a chemical reaction.
  • Chemical Understanding: It reflects the actual proportions in which substances react and combine.

How to Balance a Chemical Equation?

  1. Write Down the Unbalanced Equation: Start by writing the chemical equation for the given reaction.
  2. Count the Atoms: Count the number of each type of atom on both the reactant and product sides.
  3. Start with Complex Compounds: Balance elements that appear only once on each side, except for oxygen and hydrogen. For example, if there’s one carbon atom on both sides, balance it first.
  4. Balance Hydrogen and Oxygen Last: Typically, these elements are balanced last. Balance hydrogen atoms by adding coefficients (numbers in front of molecules) to the appropriate molecules. Balance oxygen atoms similarly.
  5. Check and Adjust: After balancing, recount the atoms to ensure they are balanced. Adjust coefficients as needed to achieve balance.
  6. Reducing Coefficients: If possible, reduce the coefficients to their simplest whole-number ratio.

Endothermic and Exothermic Reactions

Endothermic Reactions


  • Endothermic reactions are chemical reactions that absorb heat from their surroundings.
  • In these reactions, energy is taken in from the environment, usually resulting in a temperature decrease of the surroundings.

4,100+ Endothermic Reaction Stock Photos, Pictures & Royalty-Free Images -  iStock


Melting of Ice:

  • + Heat
  • Here, heat is absorbed to melt ice, making it an endothermic process.


  • Plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.
Exothermic Reactions


  • Exothermic reactions are chemical reactions that release heat to their surroundings.
  • In these reactions, energy is given off to the environment, usually resulting in a temperature increase of the surroundings.

Exothermic and endothermic reactions - Labster


Burning of Natural Gas (Methane):

  • Natural gas burns in the presence of oxygen, releasing heat, making it an exothermic reaction.

Neutralization of Hydrochloric Acid with Sodium Hydroxide:

  • When an acid reacts with a base, the reaction is exothermic, releasing heat.

Applications of Chemical Reactions


  • Real-life example: Plants use photosynthesis to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and release oxygen (O₂) using sunlight.
  • Importance: It’s vital for the survival of plants and contributes to the production of oxygen in the atmosphere, which is essential for all living organisms.


  • Real-life example: Burning of natural gas (CH₄) in stoves or gasoline (C₈H₁₈) in car engines.
  • Importance: Provides energy for heating, transportation, and electricity generation. However, incomplete combustion can produce pollutants harmful to the environment.

Rusting (Oxidation):

  • Real-life example: Iron or steel objects exposed to moisture and oxygen over time develop rust (Fe₂O₃).
  • Importance: Can lead to degradation and weakening of metal structures, emphasizing the need for rust prevention methods.

Importance of Chemical Reactions

  • Pharmaceutical Industry: Understanding chemical reactions is crucial for drug synthesis, ensuring effective and safe medications for various diseases and conditions.
  • Agriculture: Chemical reactions guide the development of fertilizers, pesticides, and herbicides, enhancing crop yield and quality.
  • Energy Sector: Chemical reactions drive processes in power generation, such as combustion in fossil fuel power plants or reactions in nuclear power generation.
  • Food Industry: Understanding reactions involved in food processing and preservation ensures the production of safe and nutritious food products for consumption.
  • Environmental Science: Chemical reactions help in understanding pollution sources, developing remediation strategies, and predicting environmental impacts.
  • Material Science: Understanding reactions is essential for designing and producing materials with desired properties, such as strength, durability, and conductivity.


  • Fundamental ProcessesChemical reactions are fundamental processes that govern natural phenomena, including the production of oxygen through photosynthesis and the degradation of materials like rust.
  • Energy and Industry: They play a pivotal role in energy production, industrial processes, and technological advancements across various sectors, from pharmaceuticals to agriculture.
  • Environmental Impact: Understanding and managing chemical reactions are crucial for addressing environmental concerns, pollution control, and sustainable development.

In summary, chemical reactions are integral to both the natural world and human activities, influencing everything from our environment and health to technology and industry.


A chemical reaction is a process where substances (called reactants) undergo a change in their chemical composition and structure, forming new substances (called products) with different properties. During this process, the atoms in the reactants rearrange to create the products.

  1. Burning: When wood burns, it reacts with oxygen in the air to produce carbon dioxide, water vapor, and heat.
  2. Rusting: Iron reacts with oxygen and moisture in the air to form iron oxide (rust).
  3. Food Spoilage: Bacteria break down the complex molecules in food, causing it to rot.
  4. Cooking: Applying heat to food causes chemical changes, like egg proteins solidifying or starches breaking down.
  5. Digestion: Enzymes in our body break down food molecules into smaller molecules for absorption.
  6. Photosynthesis: Plants use sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen.
  7. Soap Making: Fats react with an alkali (like lye) to form soap and glycerol.
  8. Vinegar Production: Alcoholic beverages ferment (react with yeast) to produce vinegar (acetic acid).
  9. Neutralization: An acid reacts with a base to produce a salt and water.
  10. Fireworks: Chemical reactions between different compounds create the colorful bursts and explosions.

There are five main types of chemical reactions:

  1. Combination reactions: Two or more reactants combine to form a single product. (Example: Hydrogen + Oxygen -> Water)
  2. Decomposition reactions: A single reactant breaks down into two or more products. (Example: Calcium Carbonate -> Calcium Oxide + Carbon Dioxide)
  3. Combustion reactions: A fuel reacts with oxygen, releasing heat and often light. (Example: Methane + Oxygen -> Carbon Dioxide + Water Vapor + Heat)
  4. Double displacement reactions: Two ionic compounds exchange ions to form two new ionic compounds. (Example: Sodium Chloride + Silver Nitrate -> Silver Chloride + Sodium Nitrate)
  5. Acid-base reactions: An acid and a base react to produce a salt and water. (Example: Hydrochloric Acid + Sodium Hydroxide -> Sodium Chloride + Water)
  1. Burning wood
  2. Rusting of iron
  3. Cooking an egg
  4. Spoilage of food
  5. Fermentation of grapes into wine
  6. Curdling of milk
  7. Dissolving sugar in water
  8. Leaves changing color in fall
  9. Paper burning
  10. Mixing baking soda and vinegar (creates bubbles due to CO2 gas formation)
  11. Food browning during frying
  12. Hair turning gray
  13. Photosynthesis in plants
  14. Igloo melting into water
  15. Chopping wood into smaller pieces (physical change, but alters how it reacts)
  16. Crushing a can (physical change, but alters how it reacts)
  17. Tearing paper (physical change)
  18. Mixing sand and water (physical change)
  19. Melting chocolate (physical change)
  20. Sublimation of dry ice (solid CO2 changing to gas without liquid phase)


1. What type of reaction is represented by the equation: ?

  • A) Decomposition
  • B) Combination
  • C) Single displacement
  • D) Double displacement

Answer: B) Combination

2. In which reaction does a compound break down into simpler substances?

  • A) Combination
  • B) Decomposition
  • C) Single displacement
  • D) Neutralization

Answer: B) Decomposition

3. What is the product of the combustion of methane (CH₄)?

  • A) CO₂ + H₂O
  • B) H₂O + O₂
  • C) CO + H₂O
  • D) CH₃OH

Answer: A) CO₂ + H₂O

4. What is the rusting of iron an example of?

  • A) Combination
  • B) Decomposition
  • C) Single displacement
  • D) Redox

Answer: D) Redox

5. What type of reaction is ?

  • A) Combination
  • B) Decomposition
  • C) Single displacement
  • D) Double displacement

Answer: D) Double displacement

6. What happens in a neutralization reaction?

  • A) Acid reacts with a base to produce a salt and water.
  • B) Acid reacts with a metal to produce hydrogen gas.
  • C) Metal reacts with an acid to produce a salt.
  • D) Acid reacts with oxygen to produce a metal oxide.

Answer: A) Acid reacts with a base to produce a salt and water.

7. In which reaction is oxygen always a reactant?

  • A) Combustion
  • B) Decomposition
  • C) Neutralization
  • D) Single displacement

Answer: A) Combustion

8. What is the process by which plants produce glucose and oxygen using sunlight?

  • A) Respiration
  • B) Fermentation
  • C) Photosynthesis
  • D) Digestion

Answer: C) Photosynthesis

9. Which substance gains electrons in a redox reaction?

  • A) Oxidized
  • B) Reduced
  • C) Catalyst
  • D) Inhibitor

Answer: B) Reduced

10. What is the primary purpose of understanding chemical reactions in the pharmaceutical industry?

  • A) Energy production
  • B) Drug synthesis
  • C) Food preservation
  • D) Rust prevention

Answer: B) Drug synthesis

11. Which reaction involves the transfer of protons (H⁺)?

  • A) Combustion
  • B) Redox
  • C) Acid-base
  • D) Photosynthesis

Answer: C) Acid-base

12. What does rusting of iron indicate?

  • A) Strengthens the iron
  • B) Purifies the iron
  • C) Degradation of iron
  • D) Turns iron into gold

Answer: C) Degradation of iron

13. What type of reaction is ?

  • A) Combination
  • B) Decomposition
  • C) Single displacement
  • D) Neutralization

Answer: B) Decomposition

14. Which reaction involves the exchange of ions between two compounds?

  • A) Combination
  • B) Decomposition
  • C) Single displacement
  • D) Double displacement

Answer: D) Double displacement

15. What does the term ‘oxidation’ refer to in redox reactions?

  • A) Gain of electrons
  • B) Loss of electrons
  • C) Gain of protons
  • D) Loss of protons

Answer: B) Loss of electrons

16. Which reaction involves the burning of a substance in the presence of oxygen?

  • A) Decomposition
  • B) Neutralization
  • C) Combustion
  • D) Double displacement

Answer: C) Combustion

17. What is the product of the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH)?

  • A) NaCl + H₂O
  • B) NaOH + H₂O
  • C) NaCl + O₂
  • D) HCl + NaO

Answer: A) NaCl + H₂O

18. Which reaction is essential for the production of fertilizers?

  • A) Combustion
  • B) Decomposition
  • C) Neutralization
  • D) Photosynthesis

Answer: C) Neutralization

19. What is the main product of the reaction between zinc (Zn) and hydrochloric acid (HCl)?

  • A) ZnO
  • B) ZnCl₂
  • C) H₂
  • D) Zn(OH)₂

Answer: B) ZnCl₂

20. Which reaction involves the synthesis of a new compound from simpler substances?

  • A) Decomposition
  • B) Combination
  • C) Single displacement
  • D) Neutralization

Answer: B) Combination

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