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Compare the physical characteristics of substances based on their bonding types.
Why does a sprinkle of salt survive a 500°F oven easily, while a sprinkle of sugar turns into a liquid brown puddle in seconds? The secret lies in the invisible 'glue' holding their atoms together.
The physical state of a substance depends on how much energy is needed to break the bonds between its particles. Ionic compounds are held together by intense electrostatic attraction in a giant crystal lattice. To melt an ionic solid like , you must break these strong bonds, resulting in extremely high melting points (often above ). In contrast, covalent compounds exist as individual molecules. While the bonds inside the molecule are strong, the forces between the molecules are weak. This means they require much less energy to pull apart, leading to relatively low melting and boiling points.
1. Consider Table Salt (), an ionic compound. Its melting point is approximately (). 2. Consider Table Sugar (), a covalent compound. Its melting point is approximately (). 3. Because the ionic lattice of salt is so stable, it remains a solid in a standard oven, while the weak intermolecular forces in sugar break down quickly.
Quick Check
Which type of compound generally requires more thermal energy to change from a solid to a liquid?
Answer
Ionic compounds, because of the strong electrostatic forces within their crystal lattice.
Electricity is the flow of charged particles. For a substance to conduct electricity, it must have mobile ions or electrons. Ionic compounds are insulators when solid because their ions are locked in place. However, when melted or dissolved in water (), the lattice breaks, and the ions (, ) are free to move and carry a current. These are called electrolytes. Most covalent compounds (like sugar or oil) are non-electrolytes; they do not have charges even when dissolved, so they cannot conduct electricity.
Imagine a circuit with a lightbulb and two beakers of distilled water: 1. Add (Ionic) to Beaker A: The salt dissolves into and ions. The lightbulb glows brightly. 2. Add (Ethanol/Covalent) to Beaker B: The molecules stay neutral and whole. The lightbulb stays dark.
Quick Check
Why does solid salt not conduct electricity, but salt water does?
Answer
In solid form, ions are locked in a lattice; in water, the ions are free to move and carry the electrical charge.
Solubility follows the rule: 'Like Dissolves Like.' Water is a polar molecule, meaning it has a slight positive and negative end. This allows water to pull apart the ions in ionic compounds, making most of them highly soluble. Many covalent compounds are non-polar (like oil or wax). Because they lack charges, water cannot grab onto them, so they do not dissolve. However, some covalent compounds are polar (like sugar) and will dissolve in water, though they won't conduct electricity.
You are given 'Substance Z' with the following data: 1. Melting Point: (Low). 2. Solubility: Does not dissolve in water. 3. Conductivity: Does not conduct in any state. Conclusion: Substance Z must be a non-polar covalent compound (like a fat or oil) because of its low melting point and lack of interaction with polar water.
A substance has a melting point of . What is its most likely bonding type?
Which of these will conduct electricity when dissolved in water?
All covalent compounds are insoluble in water.
Review Tomorrow
In 24 hours, try to explain to a friend why salt water conducts electricity but sugar water does not.
Practice Activity
Look at the ingredients on a food label. Identify one ionic compound (like Sodium Phosphate) and one covalent compound (like Glucose) and predict which would melt first if heated.