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 → Matter and its properties → Changes in the states of matter ↓


Melting and freezing


In the fascinating world of chemistry, the transition of matter from one state to another is a fundamental concept. These transitions include the processes of melting and freezing. These processes are all around us and play an important role in a variety of everyday phenomena.

Understanding matter and its states

All matter is made up of tiny particles called atoms and molecules, which are in constant motion. The way these particles are arranged and move determines the state of matter. The three most common states are solid, liquid, and gas.

  • Solids: The particles in solids are packed together in a regular pattern and vibrate in place. Solids have a definite shape and volume.
  • Liquid: The particles in a liquid are close together but not in a fixed position. They can move past each other, giving the liquid a definite volume but not a definite shape.
  • Gas: Particles in gases are far apart and move around freely. Gases do not have a definite volume or shape.
 H 2 O (solid) → H 2 O (liquid)

This equation shows the melting of ice to form liquid water.

Melting: Change from a solid to a liquid

Melting is the process by which a substance changes from a solid state to a liquid state. It occurs when the substance absorbs so much heat that the forces holding the particles together in a rigid structure are broken.

How does melting occur?

When a solid substance is heated, energy in the form of heat causes the particles to vibrate more vigorously. As the temperature rises, these vibrations become strong enough to overcome the forces holding the particles in place, causing them to slide past one another. This is when the substance turns into a liquid.

The temperature at which this occurs is called the melting point. This temperature can be different for different substances. For example, the melting point of ice (solid water) is 0°C (32°F).

Imagine a piece of butter taken out of the refrigerator. As it warms to room temperature, it begins to soften and melts into a liquid.

Solid Liquid Absorb heat

Freezing: The change from liquid to solid

Freezing is the opposite of melting. It is the process in which a liquid turns into a solid when it loses heat. During freezing, the motion of the particles slows down as they lose energy and become fixed in a fixed position, forming a solid structure.

How does ice formation occur?

As the temperature of the liquid falls, the particles move slower and the attraction between them increases. Eventually, the temperature drops to a point where the particles no longer have the energy to move past one another, resulting in the formation of a solid.

The temperature at which this process occurs is called the freezing point. The freezing point for water is the same as its melting point: 0°C (32°F).

Pond water often freezes into ice during the winter. As the air temperature drops, heat energy from the water is released into the surrounding atmosphere, causing the water molecules to become solid ice.

Liquid Solid Losing heat

Factors affecting melting and freezing

Both melting points and freezing points can be affected by a variety of factors, including pressure and the presence of impurities.

Effect of pressure

Pressure can have a noticeable effect on melting and freezing points. An increase in pressure generally raises the melting point of most substances, making them more difficult to melt. Conversely, boiling points can vary significantly with changes in pressure. For example, water boils at 100°C (212°F) at sea level, but boils at a lower temperature at higher altitudes, where the pressure is lower.

Effect of impurities

Impurities in a substance can lower its melting point or raise its freezing point. This phenomenon is called freezing point depression and boiling point elevation. For example, adding salt to snow lowers its melting point, which is why salt is often used to melt ice on icy roads.

Applications of melting and freezing

Melting and freezing processes have many practical applications, ranging from cooking to food preservation and industrial manufacturing.

Melting in everyday life

  • Cooking: Melting chocolate is an important process in cooking. For example, chocolate is melted to make desserts and coatings.
  • Welding: Welding is the process by melting metals together to form a strong joint.

Cold in daily life

  • Preservation: Freezing is widely used to store perishable foods, slowing the growth of spoilage bacteria and enzymes.
  • Medicine: Cryopreservation is a technique that involves freezing biological samples, such as cells and tissues, to preserve them for future use.

Conclusion

Melting and freezing are fundamental processes that remind us of the dynamic nature of matter. By understanding these processes, we gain insight into how substances behave under different conditions and can apply this knowledge to a variety of fields.


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Melting and freezing

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