Required Practical: Determining Density

Purpose of the Practical

The main aim of this practical is to measure the density of different solids, both regular and irregular in shape. Density is a fundamental property of matter that relates the mass of a substance to the volume it occupies. Understanding density helps us apply the particle model of matter, which explains how particles are arranged and packed in solids, liquids, and gases.

The key formula used is:

Density = mass  volume

By accurately measuring mass and volume, you can calculate density and compare your results to known values to identify materials or check for purity.

Measuring Mass

Mass is measured using a balance, which gives a precise reading in grams (g). Before starting, ensure the balance is calibrated correctly (zeroed) to avoid errors. Typically, balances can measure mass to the nearest gram or to decimal places (e.g., 0.01 g) for greater precision.

• Place the object gently on the balance pan.
• Record the mass to the nearest gram or decimal place if using a digital balance.
• Repeat the measurement to check consistency.

For example, if a cube of metal has a mass reading of 120 g on the balance, this value will be used in the density calculation.

Measuring Volume

Regular solids (e.g. cubes, cuboids, spheres) have volumes calculated from their dimensions:

• Measure length, width, and height using a ruler or vernier caliper.
• Calculate volume using the appropriate formula, e.g. for a cuboid:

$$V = l \times w \times h$$

For example, a cuboid with length 5 cm, width 3 cm, and height 2 cm has a volume:

$5 \times 3 \times 2 = 30 \text{ cm}^3$

Irregular solids (e.g. stones, metal lumps) cannot have their volume calculated from dimensions easily. Instead, use the displacement method:

• Fill a measuring cylinder with water and record the initial volume.
• Carefully submerge the object in the water without spilling.
• Record the new volume reading.
• The volume of the object is the difference between the new and initial water volumes.

For example, if the water level rises from 50 ml to 65 ml, the volume of the object is:

$65 - 50 = 15 \text{ cm}^3$ (since 1 ml = 1 cm8)

Calculating Density

Once you have both mass and volume, calculate density using:

$\text{Density} = \frac{\text{mass}}{\text{volume}}$

Make sure mass is in grams (g) and volume in cubic centimetres (cm8) or millilitres (ml) for solids and liquids. The density unit is grams per cubic centimetre (g/cm8). Note that 1 g/cm8 is equivalent to 1000 kg/m9, so unit conversions may be necessary depending on context.

Compare your calculated density to known densities of materials to identify the substance or check its purity.

Example: For instance, if a metal cube has a mass of 120 g and a volume of 30 cm8, its density is:

$$\text{Density} = \frac{120}{30} = 4 \text{ g/cm}^3$$

Safety and Accuracy

Safety is important when handling equipment and materials:

• Handle the balance and measuring cylinder carefully to avoid damage.
• Be cautious when using water to avoid spills that could cause slips or damage equipment.
• Use a flat, stable surface for measurements to improve accuracy.

To improve accuracy and reliability:

• Repeat measurements several times and calculate an average.
• Take care to read the measuring cylinder at eye level to avoid parallax error.
• Use appropriate equipment for the size of the object (e.g., a larger measuring cylinder for bigger objects).