Lesson 1: Physical & Chemical Properties of Matter and Density Applications
Matter is scientifically defined as everything that has mass and occupies space, possessing a measurable volume. Substances in our surrounding environment exhibit an extensive diversity in physical and chemical properties, enabling scientists and engineers to identify, classify, and utilize them effectively across modern technological applications. Physical properties can be observed or measured without altering the chemical composition of the substance, including color, odor, taste, density, melting point, boiling point, hardness, and thermal and electrical conductivity. Density is the most distinctive physical characteristic of any pure substance, representing the mass per unit volume in g/cm³. This fundamental principle explains why substances with lower densities like wood, cork, and oil float on water, while denser substances like iron sink. It also explains why water cannot extinguish petroleum fires, why celebration balloons are filled with helium or hydrogen gas, and how metals are alloyed according to national curriculum standards.
1. The Law of Density & Physical Applications
Density is defined as the mass of unit volume (1 cm³) of a substance. It is a characteristic physical constant for each pure material:
- Constancy of Density: The density of a pure substance remains constant regardless of changes in mass or volume at a constant temperature.
- Pure Water Density: Equal to 1.0 g/cm³. Substances with density < 1.0 float on water; substances with density > 1.0 sink.
- Petroleum Fires: Water is never used to extinguish petroleum fires because oil density is lower than water density; oil floats on water and keeps burning in contact with atmospheric oxygen.
- Festive Balloons: Filled with helium or hydrogen because their densities are much lighter than air density, causing them to rise upwards automatically.
2. Melting & Boiling Points & Industrial Alloys
Thermal transition thresholds characterize different materials:
| Concept | Scientific Definition | Industrial Application |
|---|---|---|
| Melting Point | Temperature at which matter starts changing from solid to liquid | Smelting metals to shape them and formulate homogeneous alloys |
| Gold-Copper Alloy | Homogeneous metallic blend | Used in making jewelry to enhance the hardness of pure gold |
| Nickel-Chrome Alloy | Superior heat-resistant alloy | Used in heating coils of electric irons, toasters, and room heaters |
| Boiling Point | Temperature at which matter changes from liquid to gas | Fractional distillation of crude petroleum based on differing boiling points |
3. Electrical & Thermal Conductivity & Hardness
Classification based on physical conduction and rigidity:
- Electrical Conductivity:
- Good Conductors: Metals (copper, aluminum, silver), acids, alkalis, and salt solutions (sodium chloride).
- Bad Conductors (Insulators): Wood, plastics, sulfur, phosphorus, sugar solution in water, hydrogen chloride in benzene, and dry gases.
- Thermal Conductivity: Metals conduct heat rapidly (cooking pans are made of aluminum; handles are crafted from wood or plastic).
- Hardness Levels: Soft at room temperature (rubber), soften upon heating (metals: iron, copper), and do not soften upon heating (coal and sulfur).
4. Chemical Activity of Metals
Metals vary substantially in their chemical reactivity when exposed to humid atmospheric oxygen:
- Very Active Metals: React instantaneously with humid oxygen upon exposure (e.g., Potassium K and Sodium Na). Kept submerged under kerosene to prevent oxidation.
- Moderately Active Metals: React slowly over several days forming a layer of rust (e.g., Iron Fe, Aluminum Al, Copper Cu). Bridges and light poles are painted periodically to prevent corrosion.
- Least Active Metals: Exhibit negligible reactivity with oxygen (e.g., Gold Au, Silver Ag, Platinum Pt, Nickel Ni). Extensively used in jewelry making and plating corrosive metals.
Problem 1: Calculating Density
Question: An iron block has a mass of 78 g and a volume of 10 cm³. Calculate its density, and state whether it floats or sinks in water.
• Density (d) = Mass ÷ Volume = 78 ÷ 10 = 7.8 g/cm³.
• Since iron density (7.8 g/cm³) is greater than water density (1.0 g/cm³), the iron block sinks in water.
Problem 2: Irregular Solid Volume by Displacement
Question: When a copper piece of mass 176 g is immersed in a graduated cylinder containing 60 cm³ of water, the water level rises to 80 cm³. Calculate copper density.
• Volume of copper = 80 - 60 = 20 cm³.
• Density = Mass ÷ Volume = 176 ÷ 20 = 8.8 g/cm³.
Problem 3: Calculating Liquid Mass
Question: Find the mass of 50 cm³ of benzene knowing that its density is 0.7 g/cm³.
• From the formula triangle: Mass = Density × Volume.
• Mass = 0.7 × 50 = 35 grams.
Problem 4: Testing Milk Purity
Question: A student measured the mass of 100 cm³ of milk and found it to be 105 g. Is the milk sample pure or adulterated? (Pure milk density = 1.03 g/cm³).
• Density = Mass ÷ Volume = 105 ÷ 100 = 1.05 g/cm³.
• Since the measured density (1.05 g/cm³) differs from pure milk density (1.03 g/cm³), the milk is adulterated.
Problem 5: Finding Solid Volume
Question: Calculate the volume of an aluminum cube having a mass of 54 g, knowing that aluminum density is 2.7 g/cm³.
• Volume = Mass ÷ Density.
• Volume = 54 ÷ 2.7 = 20 cm³.
Problem 6: Floating and Sinking Analysis
Question: Two blocks A and B each have a mass of 100 g. Volume of A is 50 cm³ and B is 200 cm³. Explain which one floats and which sinks in water.
• Density of A = 100 ÷ 50 = 2.0 g/cm³ (> 1.0 ⇐ sinks).
• Density of B = 100 ÷ 200 = 0.5 g/cm³ (< 1.0 ⇐ floats on water surface).
Problem 7: Storing Active Metals
Question: Give reason: Sodium and potassium are kept under kerosene in the science laboratory.
• Because they are very active metals that react vigorously and instantaneously with humid atmospheric oxygen. Kerosene acts as an insulating barrier isolating them from moist air.
Problem 8: Kitchenware Material Design
Question: Give reason: Cooking pots are made of aluminum while their handles are made of plastic or wood.
• Pots are made of aluminum because it is an excellent thermal conductor with a high melting point.
• Handles are made of plastic or wood because they are thermal insulators, preventing heat from burning hands.
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