Density, Solutions, and Solubility
Why does a small steel ball sink while a huge log floats? The answer is density — how much mass is packed into a given volume. Density is one of the few chemistry topics with a clean formula, and it comes up often, so it is worth getting comfortable with. This lesson covers density, solutions, and solubility.
The Density Formula
Density is mass divided by volume: \[ \text{density} = \dfrac{\text{mass}}{\text{volume}}. \] If an object has a mass of \(30\) grams and a volume of \(10\) cubic centimeters, its density is \[ \dfrac{30}{10} = 3 \text{ g/cm}^3. \] You can rearrange the formula to find any missing piece: mass is density times volume, and volume is mass divided by density.
Density decides whether something floats. An object floats in water if its density is less than water’s (about \(1\) g/cm\(^3\)) and sinks if it is greater. A log floats because wood is less dense than water; a steel ball sinks because steel is much denser.
Solutions
A solution is a special kind of mixture where one substance dissolves completely in another, like sugar in water. The substance that dissolves is the solute (sugar); the substance it dissolves into is the solvent (water). In a solution, the solute spreads out so evenly that you cannot see it or filter it out — salt water looks clear even though the salt is still there.
Solubility
Solubility is how much of a solute can dissolve in a solvent. There is a limit: add too much sugar to water and the extra sits at the bottom, undissolved. Temperature usually matters — most solids dissolve better in hot water than in cold. That is why more sugar dissolves in hot tea than in iced tea. On the test, if a question asks why more dissolved at a higher temperature, solubility is the reason.
Watch: A Short Video Lesson
Chemistry with Dr Steph walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A Routine for These Questions
- Density = mass divided by volume; rearrange to find any missing value.
- Less dense than water floats; more dense sinks.
- In a solution, the solute dissolves into the solvent.
- Solubility has a limit and usually increases with temperature.
Practice
- Write the formula for density.
- A block has a mass of \(40\) g and a volume of \(8\) cm\(^3\). What is its density?
- Will an object with density \(0.8\) g/cm\(^3\) float or sink in water?
- In salt water, what is the solute and what is the solvent?
- What is solubility?
- Does more sugar usually dissolve in hot or cold water?
Answers
- \(\text{density} = \dfrac{\text{mass}}{\text{volume}}\).
- \(\dfrac{40}{8} = 5 \text{ g/cm}^3\).
- Float (it is less dense than water).
- Salt is the solute; water is the solvent.
- How much of a solute can dissolve in a solvent.
- Hot water.
Where This Fits in Your Science Prep
Density uses the same skills as units and formulas, and solutions build on mixtures. See all topics on the Science Topics Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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