9.8 Summary & Keep the Curiosity Alive

Key Points

  • A solution is a uniform mixture. The solute dissolves in the solvent; for a solid-in-liquid, the solid is the solute. For two liquids, the one in smaller amount is the solute.
  • An unsaturated solution can dissolve more solute; a saturated one cannot (at that temperature). Solubility is the maximum solute a fixed solvent dissolves; concentration describes dilute vs concentrated.
  • Raising the temperature increases the solubility of most solids but decreases the solubility of gases — so cold water holds more dissolved oxygen for aquatic life.
  • Density = mass ÷ volume (g/cm³ or kg/m³). Water ≈ 1 g/cm³. An object floats if less dense than water and sinks if denser; relative density compares it to water.
  • Mass is measured with a balance, volume with a measuring cylinder (read at the meniscus) or by displacement for irregular solids; a regular cuboid’s volume is l \times w \times h.
  • Heating lowers density (volume rises) — hot air rises and balloons float. Pressure affects gases most. Ice floats because water expands on freezing, keeping lakes liquid below.

A Closing Challenge: Keep the Curiosity Alive

Think It Through

  1. True or false? (i) Oxygen is more soluble in hot water than cold. (ii) A sand-and-water mixture is a solution. (iii) An unsaturated solution holds more solute than a saturated one.
  2. Calculate. An object has mass 400 g and volume 40 cm³ — find its density. Will it float or sink?
  3. The two oranges. An unpeeled orange floats, but a peeled one sinks. Explain using density. (Hint: the spongy peel traps air.)
  4. Make the egg float. A raw egg sinks in tap water. What can you add to the water to make it float, and why does that work?
  5. Find the volume. A block of iron has mass 600 g and density 7.9 g/cm³. What is its volume?

That completes Part III — The Material World: from the particles that make up matter, to elements, compounds and mixtures, to the solutions and densities we meet every day. In Part IV, we turn our gaze outward — to light, the night sky, the web of life, and the planet we call home.