PAG: Determining Density

Section 1 of 7Density and its units

Definition

• Definition Density is the mass per unit volume of a substance, calculated from ρ = m ÷ V.

Units

• Units Mass in kg and volume in m³ give density in kg/m³; mass in g and volume in cm³ give g/cm³.
• 1 g/cm³ = 1000 kg/m³.

Aim

• Aim To find the density of different materials by measuring mass and volume separately.

Section 2 of 7Regular solid

1

Measure the mass of the block on a balance, taring it first so it reads zero.

2

Measure the length, width and height with a ruler or vernier calipers; repeat and take a mean.

3

Calculate volume by multiplying the three lengths together.

4

Calculate density using ρ = m ÷ V.

Section 3 of 7Irregular solid

1

Find the mass of the object on the balance.

2

Half fill a measuring cylinder with water and record the volume.

3

Lower the object gently in using a thread and read the new volume; the object must be fully under water.

4

The difference between the two readings is the volume of the object; or use a eureka can and collect the displaced water.

5

Divide mass by volume to find the density.

Section 4 of 7Liquid

1

Place an empty measuring cylinder on the balance and set it to zero.

2

Pour in a known volume of liquid, reading the bottom of the meniscus at eye level.

3

Record the mass shown on the balance, which is the mass of the liquid alone.

4

Calculate density, and repeat for different volumes to calculate a mean.

Section 5 of 7Improving the result

Source of error	How to reduce it
Reading the meniscus from the wrong angle	Read with the eye level with the liquid surface
Air bubbles clinging to the object	Tap the object or use a thread to remove them
Small objects with a small volume change	Use a narrower cylinder or a larger object
Balance not zeroed	Tare with the empty container before adding the substance
Single measurement only	Repeat and calculate a mean density

Section 6 of 7Key terms

Term	Meaning
Density	Mass per unit volume of a material.
Displacement	The method of finding the volume of an irregular object by the water it pushes aside.
Meniscus	The curved upper surface of a liquid in a narrow tube.
Tare	To reset a balance to zero with the empty container on it.
Eureka can	A container with a spout, used to collect water displaced by an object.

Section 7 of 7Exam technique

1

Mixing units, for example using grams with m³, so the density is wrong by a factor of 1000.

2

Reading the volume from the top of the meniscus instead of the bottom for water.

3

Letting the object touch the sides of the cylinder or float, so the displaced volume is not measured correctly.

4

Taking the volume of water plus the object as the volume of the object, instead of subtracting the starting volume.