Why Does Ice Float? Density, States of Matter, and the Mystery of Water
Shared by A STEM Educator
Students will be able to (1) identify and describe the three states of matter by their physical properties and particle arrangement; (2) calculate density using D = m/v; (3) use density data as evidence to explain and predict whether an object will sink or float in water; and (4) construct a molecular-level explanation for why ice is less dense than liquid water — aligned to NGSS MS-PS1-4 and MS-PS1-1.
Lesson Overview
Students anchor their learning in the puzzling observation that ice floats on liquid water — an unusual property that defies what we expect from most solids. Through hands-on investigation, molecular modeling, and data analysis, students build an evidence-based explanation connecting particle arrangement, mass, volume, and density to predict whether objects sink or float.
Materials
- Clear plastic cups or beakers (1 per group)
- Ice cubes (pre-made, 1–2 per group)
- Room-temperature water
- Vegetable oil (small amount per group)
- Rubbing alcohol (small amount per group — teacher-supervised)
- Triple beam balance or digital scale (1 per group or per station)
- Graduated cylinders, 100 mL (1 per group)
- Rulers (1 per group)
- Small objects for sink/float testing: a grape, a small wooden block, a metal washer, a plastic bottle cap, a cork
- Sticky notes or whiteboards for student predictions
- Student investigation handout (printed, 1 per student)
- Colored pencils (for molecular diagrams)
- Chart paper and markers (for class data sharing)
- Projector/screen for phenomenon introduction video or image
- Calculators (1 per student or shared)
Scaffolded Task Progression
(a) Identify the state of matter in each cup.(b) Describe what you can observe about each sample: Does it have a definite shape? Definite volume? Can you compress it?(c) In the boxes below, draw a particle model diagram for each state of matter. Use dots to represent water molecules. Show how closely packed and how organized they are. [ Interactive graph / diagram — available in the full lesson ](d) Based on your particle diagrams, which state do you predict has the most mass packed into a given space? Which has the least?
Density is a measure of how much mass is packed into a given volume.
Formula: D = m ÷ v (Density = Mass ÷ Volume)
Units: g/cm³ or g/mL
The density of liquid water is 1.0 g/mL. This is our reference point.
- If an object's density is LESS than 1.0 g/mL → it FLOATS
- If an object's density is GREATER than 1.0 g/mL → it SINKS
Use the data table below to practice:
| Object | Mass (g) | Volume (mL or cm³) | Density (g/cm³) | Sink or Float? |
|---|---|---|---|---|
| Wooden block | 18 g | 30 cm³ | ? | ? |
| Metal washer | 55 g | 7 cm³ | ? | ? |
| Plastic bottle cap | 4 g | 8 cm³ | ? | ? |
| Cork | 3 g | 10 cm³ | ? | ? |
| Grape | 8 g | 7 cm³ | ? | ? |
(a) Calculate the density of each object. Show your work.(b) Predict whether each object will sink or float in water.(c) Explain in one sentence: What is the relationship between density and whether an object floats?
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