All liquids can be defined by their density. Density is defined by dividing the mass of a liquid by its volume (d = m/v). Just like all measurements in science, liquids are compared to a standard. This standard is pure water, which is one gram/cm cubed. Where gram is the mass of water and “cm cubed” is the volume of water. Another way to look at the density of water is that the density of water is one gram per milliliter. Mass is measured in grams and liquid is measured in milliliters.
This all seems straight forward until you consider that liquids are subject to changes in temperature and atmospheric pressure. For the standard of one gram per milliliter for water to be true, the temperature must be 4 degrees Celsius at one atmospheric pressure. One atmospheric pressure is equal to a barometric pressure of 29.92 inHg (inches of Mercury).
- Liquid density decreases and increases due to changes in temperature. For example water with a temperature of 80 degrees Celsius has a density of .98 g/cm cubed, which is less than the standard of one.
- Atmospheric pressure increases or decreases due to changes in altitude, along with low and high pressure weather. For example air pressure increases at lower altitudes and with a high pressure weather. Increases in pressure also increases temperature.
How to Determine the Density of Liquids
This investigation is designed for gifted and talented students in grades four or five. Students apply the scientific method and problem-based learning as they complete this investigation. Additionally this investigation can be used in a middle school physical science course (typically seventh or eighth grade).
Materials (per student or group)
- 4 – 100 mL graduated cylinders labeled A, B, C, and D
- 1 – Triple beam balance
- 4 – 100 mL beakers with unknown liquids labeled A, B, C, and D
- 1 - Thermometer
- Liquid A – Distilled water
- Liquid B – Sea water (use sea salt/water solution – not table salt)
- Liquid C – Isopropyl alcohol
- Liquid D – Cooking oil
- Safety goggles
Procedures
Students:
- Measure the mass of each empty graduated cylinder and record their measurements
- Measure the temperature of the unknown liquids and record their measurements
- Pour 50 mL of each liquid in the beakers into the corresponding lettered graduated cylinder
- Measure the mass of each graduated cylinder and record their measurements
- Calculate the density of each unknown liquid and record their answers
Student Questions
- Which liquid is less dense?
- Which liquid has the greatest density?
- In which liquid would wood float higher in the liquid? Why?
- What would the density be if the temperature of the liquids dropped to 0 degrees Celsius? Temperature is increase to 100 degrees Celsius?
- If a fast moving low or high pressure area moved past the school during the investigation, would the density of the unknown liquids change?
- What would happen to the liquids if they were all placed in the same beaker?
- Is there any relationship between this investigation and a Galileo thermometer?
Additional questions are developed as students use their critical thinking skills to answer these questions.
Extensions
Allow students to apply concepts in this lesson by researching other liquids using the Internet and other sources. Then have them develop a multimedia presentation about their findings.
Making Connections
As students internalize about the science concepts developed in this investigation, they can transfer these concepts to the density of objects other than liquids. Determining the density of liquids leads to other science concepts such as buoyancy, barometric pressure, and properties of matter. The problem solving strategies students used in this investigation involved the use of proportional reasoning and logical-deductive thinking.
David R. Wetzel - Dr. David Wetzel's experience includes more than 25 years in continuing, adult, and teacher education.
Join the Conversation