Grade 7
  1. Introduction to Chemistry  1.1. What is Chemistry?  1.2. Importance of chemistry in daily life  1.3. Branches of chemistry  1.4. Scientific Method in Chemistry  1.5. Laboratory Safety Rules and Precautions  1.6. Common Laboratory Equipment and Their Uses  1.7. Units of measurement in chemistry
  2. Matter and its properties  2.1. Definition of Matter  2.2. Classification of matter  2.3. Properties of matter  2.3.1. Physical properties  2.3.2. Chemical properties  2.4. States of matter  2.4.1. Solid state  2.4.2. Fluid state  2.4.3. Gaseous state  2.4.4. Plasma and Bose–Einstein condensates  2.5. Changes in the states of matter  2.5.1. Melting and freezing  2.5.2. Evaporation and Condensation  2.5.3. Sublimation and deposition  2.6. Density and its applications  2.7. Diffusion and its importance
  3. Elements, Compounds, and Mixtures  3.1. Definition of the element  3.2. Classification of elements  3.3. Metals, Nonmetals and Metalloids  3.4. Noble gases and their properties  3.5. Introduction to the Periodic Table  3.6. Definition of Compound  3.7. Properties of Compounds  3.8. Introduction to Chemical Formulas  3.9. Definition of Mixture  3.10. Types of mixtures  3.10.1. Homogeneous mixture  3.10.2. Heterogeneous mixtures  3.11. Difference Between Compounds and Mixtures  3.12. Solutions, Colloids and Suspensions
  4. Separation of mixtures  4.1. Need for separation of mixtures  4.2. Separation methods  4.2.1. Filtration  4.2.2. Sedimentation and decantation  4.2.3. Evaporation  4.2.4. Crystallization  4.2.5. Distillation  4.2.6. Chromatography  4.2.7. Magnetic Separation  4.2.8. Centrifugation
  5. Atomic Structure  5.1. Introduction to Atoms  5.2. Structure of the atom  5.2.1. Nucleus and electron cloud  5.3. Subatomic particles  5.3.1. Proton  5.3.2. Neutron  5.3.3. Electrons  5.4. Atomic number and mass number  5.5. Isotopes and their uses  5.6. Ions and ion formation
  6. Periodic table  6.1. Introduction to the Periodic Table  6.2. Groups and periods  6.3. Trends in the Periodic Table  6.3.1. Atomic Size  6.3.2. Metallic and non-metallic character  6.3.3. Electronegativity  6.3.4. Reactivity of the elements  6.4. Alkali metals and their properties
  7. Chemical bond  7.1. Why do atoms form bonds?  7.2. Types of chemical bonds  7.2.1. Ionic bond  7.2.2. Covalent bond  7.2.3. Metal Bond  7.3. Properties of Ionic and Covalent Compounds  7.4. Intermolecular forces
  8. Chemical reactions  8.1. Introduction to Chemical Reactions  8.2. Signs of a chemical reaction  8.3. Types of Chemical Reactions  8.3.1. Combination Reactions  8.3.2. Decomposition reactions  8.3.3. Displacement reactions  8.3.4. Double displacement reactions  8.3.5. Combustion reactions  8.4. Law of conservation of mass  8.5. Balancing simple chemical equations
  9. Acids, Bases and Salts  9.1. Definition of Acid and Base  9.2. Properties of Acids  9.3. Properties of bases  9.4. pH Scale and Indicators  9.5. Neutralization reactions  9.6. Common Acids and Bases in Everyday Life  9.7. Preparation and use of salts  9.8. Strong and weak acids and bases
  10. Solutions and Solubility  10.1. Definition of Solution  10.2. Components of the solution  10.2.1. Solvent  10.2.2. solute  10.3. Factors Affecting Solubility  10.4. Concentration of solutions  10.5. Saturated and unsaturated solutions  10.6. Colloids and suspensions  10.7. Solubility curve and its interpretation
  11. Air and atmosphere  11.1. Composition of air  11.2. Properties of gases in the air  11.3. Importance of Oxygen and Carbon Dioxide  11.4. Air pollution and its effects  11.5. Greenhouse effect and global warming  11.6. Ways to reduce air pollution  11.7. Ozone layer and its importance
  12. Water and its importance  12.1. Properties of Water  12.2. Water as the universal solvent  12.3. Importance of water for life  12.4. Water pollution and its effects  12.5. Water Purification  12.5.1. Filtration  12.5.2. boil  12.5.3. Chlorination in water purification  12.5.4. reverse osmosis  12.6. Hard and soft water  12.7. Water cycle and its importance
  13. Fuel and energy  13.1. Types of fuel  13.1.1. Fossil fuels  13.1.2. Renewable energy sources  13.2. Combustion and energy release  13.3. Global warming and its effects  13.4. Alternative sources of energy  13.5. Ways to conserve energy
  14. Metals and Nonmetals  14.1. Physical and chemical properties of metals  14.2. Physical and Chemical Properties of Non-Metals  14.3. Difference Between Metals and Nonmetals  14.4. Uses of metals and nonmetals  14.5. Corrosion of metals and its prevention  14.6. Alloys and their importance
  15. Plastics and polymers  15.1. Natural and synthetic polymers  15.2. Properties of plastics  15.3. Biodegradable and Non-Biodegradable Plastics  15.4. Environmental impact of plastic  15.5. Recycling and waste management in plastics and polymers  15.6. Daily Uses of Polymers
  16. Introduction to Organic Chemistry  16.1. Basic definitions of organic chemistry  16.2. Carbon Compounds in Daily Life  16.3. Hydrocarbons  16.4. Simple functional groups (alcohols and carboxylic acids)  16.5. Importance of organic compounds in industry

Grade 7 → Chemical reactions ↓


Signs of a chemical reaction


Chemistry is the study of matter and its changes. Understanding chemical reactions is a fundamental part of chemistry. Chemical reactions occur all around us and are important in everyday life. How can we tell when a chemical reaction has occurred? In this lesson, we will explore the signs that indicate a chemical reaction has occurred.

What is a chemical reaction?

A chemical reaction is a process in which substances, called reactants, change into different substances, called products. The bonds between atoms in the reactants are broken, and new bonds are formed to form products. Chemical reactions are usually accompanied by changes that indicate a reaction has occurred.

Signs of a chemical reaction

There are several main signs that a chemical reaction is taking place. These signs include:

  • Change in color
  • Changes in temperature
  • Gas formation
  • Formation of precipitate
  • Changes in smell
  • Changes in light or sound

Change in color

One of the most common signs of a chemical reaction is a change in color. When substances react, the new substances formed may have a different color than the original reactants. For example, when a piece of iron is exposed to air and moisture, it reacts to form iron oxide, or rust, which is reddish-brown in color.

A more controlled example is the reaction between phenolphthalein, an indicator, and a base such as sodium hydroxide (NaOH). Phenolphthalein is colorless in acidic solution but turns pink in basic solution.

Changes in temperature

Some chemical reactions release energy as heat, while others absorb energy and result in a decrease in temperature. These are called exothermic and endothermic reactions, respectively.

Exothermic Reaction: Burning of wood is an example of this. 
C6H10O5 + 6O2 → 6CO2 + 5H2O + heat
Endothermic Reaction: Dissolving ammonium nitrate in water is another example of this. 
NH4NO3 (s) + H2O (l) → NH4NO3 (aq)

Gas formation

The formation of a gas is a sign that a chemical reaction has occurred. It can often be seen as bubbles or smoke. For example, when vinegar (acetic acid) is mixed with baking soda (sodium bicarbonate), carbon dioxide gas is formed:

CH3COOH + NaHCO3 → CO2 + H2O + NaCH3COO

Formation of precipitate

A precipitate is a solid that forms when two solutions are mixed together. This solid does not dissolve in the solution and often sinks to the bottom. For example, when solutions of silver nitrate (AgNO3) and sodium chloride (NaCl) are mixed, a white precipitate of silver chloride (AgCl) forms:

AgNO3 + NaCl → AgCl(S) + NaNO3

Changes in smell

Some chemical reactions result in a change in odor. This often indicates that new substances with different properties have formed. For example, when food spoils, it often has a bad odor because of the chemical reactions that occur during the food's decomposition.

Changes in light or sound

Some chemical reactions produce light or sound. For example, chemical reactions are used in fireworks to produce light and sound. Another example is the reaction between hydrogen and oxygen in rocket engines, which releases a lot of energy in the form of sound and light.

Understanding and predicting reactions

Studying chemical reactions helps us predict the products that will form. Chemists use balanced chemical equations to represent these reactions. This helps them ensure that the number of atoms is the same before and after the reaction, obeying the law of conservation of mass.

For example, in the reaction between hydrogen and oxygen to form water, the balanced chemical equation looks like this:

2H2 + O2 → 2H2O

Conclusion

Chemical reactions are an important part of the natural world and our daily lives. They allow us to transform substances, generate energy, and create new materials. By understanding the signs of chemical reactions, we can identify when a reaction occurs and learn more about the substances involved. Whether it's iron rusting or baking soda and vinegar fizzing, chemical reactions happen everywhere, making our understanding of these processes even more important.


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Introduction to Chemical Reactions
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Signs of a chemical reaction

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