# Chapter 3 - Key Concepts

```Chemistry – Properties and Interactions of Matter
Chapter 3: Key Concepts
Lesson 3.1: What is Density?
 Density is a characteristic property of a substance.
 The density of a substance is the relationship between the mass of the substance and how much
space it takes up (volume).
 The mass of atoms, their size, and how they are arranged determine the density of a substance.
 Density equals the mass of the substance divided by its volume; D = m/v.
 Objects with the same volume but different mass have different densities.
Lesson 3.2: Finding Volume: The Water Displacement Method
 A submerged object displaces a volume of liquid equal to the volume of the object.
 One milliliter (1 mL) of water has a volume of 1 cubic centimeter (1cm3).
 Different atoms have different sizes and masses.
 Atoms on the periodic table are arranged in order based on the number of protons in the nucleus.
 Even though an atom may be smaller than another atom, it might have more mass.
 The mass of atoms, their size, and how they are arranged determine the density of a substance.
 Density equals the mass of the object divided by its volume; D = m/v.
 Objects with the same mass but different volume have different densities.
Lesson 3.3: Density of Water
 Just like solids, liquids also have their own characteristic density.
 The volume of a liquid can be measured directly with a graduated cylinder.
 The molecules of different liquids have different size and mass.
 The mass and size of the molecules in a liquid and how closely they are packed together
determine the density of the liquid.
 As in a solid, the density of a liquid equals the mass of the liquid divided by its volume; D = m/v.
 The density of water is 1 gram per cubic centimeter.
 The density of a substance is the same regardless of the size of the sample.
Lesson 3.4: Density: Sink and Float for Solids
 The density of an object determines whether it will float or sink in another substance.
 An object will float if it is less dense than the liquid it is placed in.
 An object will sink if it is more dense than the liquid it is placed in.
Lesson 3.5: Density: Sink and Float for Liquids
 Since density is a characteristic property of a substance, each liquid has its own characteristic
density.
 The density of a liquid determines whether it will float on or sink in another liquid.
 A liquid will float if it is less dense than the liquid it is placed in.
 A liquid will sink if it is more dense than the liquid it is placed in.
Lesson 3.6: Temperature Affects Density
 Heating a substance causes molecules to speed up and spread slightly further apart, occupying a
larger volume that results in a decrease in density.
 Cooling a substance causes molecules to slow down and get slightly closer together, occupying a
smaller volume that results in an increase in density.
 Hot water is less dense and will float on room-temperature water.
 Cold water is more dense and will sink in room-temperature water.
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