How to calculate density by water displacement

Written by david chandler
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How to calculate density by water displacement
The graduated cylinder is an important tool in measuring volume. (cylinder image by Hubert from Fotolia.com)

Density, the measure of the relationship between the volume and the mass of a substance, is defined by mass divided by volume. For example, water has a density of 1 gram per cubic centimetre at 3.89 degrees Celsius (-15.6 degrees C Celsius). This means 1 gram of water occupies a volume of 1 cubic centimetre, 2 grams of water occupy a volume of 2 cubic centimetres, and so on. .

Finding the mass of a substance is easily accomplished using a balance; finding its volume requires measuring its physical dimensions. The water displacement method is an effective technique for finding the volume of an insoluble, irregular solid and its subsequent density.

Skill level:
Easy

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Things you need

  • Insoluble, irregularly shaped sollid
  • Balance
  • Graduated cylinder
  • Water

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Instructions

  1. 1

    Find the mass of the substance using a balance. Record the mass.

  2. 2

    Fill a graduated cylinder large enough to hold the insoluble, irregular solid with water to a measured level. Record the volume of water.

  3. 3

    Submerge the solid in the graduated cylinder's water. Tap the cylinder gently to allow trapped air bubbles to escape. Record the volume of the water and the submerged object.

  4. 4

    Subtract the volume of the water from the volume of the water with the submerged object. For example, if the water volume initially was 6 millilitres and, after the object was submerged, measured 8 millilitres, then the volume of the object is 2 millilitres.

  5. 5

    Divide the initial mass measurement by the volume to determine the density of the object. For example, if the mass of the object measured 4 grams and the volume measured 2 millilitres, then the density is 2 grams per millilitre or 2g/ml.

Tips and warnings

  • Remember to tare (zero) the balance with the empty container for the substance to gain a proper reading of the mass of the substance.
  • Solubility is a special concern and water soluble substances may require a substitute liquid.
  • Remember that 1 millilitre equals 1 cubic centimetre.
  • Some objects may float in the water and may need to be physically submerged. However, the object should be pushed just below the surface using a small rod to minimise the error introduced by the submerged portion of the rod.

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