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Intro Chemistry · Period 2 · Class code DEM-O01 · Thursday, Jan 1Name: ______________________ Period: ______

Why Does Ice Float?

Today's goal: Explain why ice floats on water by describing what happens to water molecules when they freeze.

  • I can define density and compare the density of ice and liquid water.
  • I can describe how hydrogen bonds arrange water molecules into an open hexagonal pattern.
  • I can explain why lakes freeze from the top down and why that matters for living things.

For the adult in the room: the one-page survival guide is at https://coveredclass.com/class/DEMO01/guide

The hook

Here's something you've seen a thousand times and probably never questioned. Drop an ice cube in a glass of water. It floats. But almost every other substance sinks when it freezes. The solid is denser than the liquid. Water breaks that rule. When it freezes, water expands. The ice ends up about nine percent less dense than the liquid it came from. That happens because of how water molecules grab onto each other. As they slow down and lock into place, they form a wide, open, hexagonal pattern. Lots of empty space. That empty space is why lakes freeze from the top down instead of the bottom up. It's why fish survive the winter.

Mini-lesson

Read on your own. About 8 minutes.

Density is how much stuff is packed into a certain amount of space. Scientists write it as mass divided by volume. A brick and a sponge might be the same size, but the brick has more mass squeezed into that space, so it is denser. When you put two things together, the less dense one floats on top of the denser one. That is why oil floats on water and why a helium balloon rises in air.

For almost every substance, the solid form is denser than the liquid form. When a liquid cools, its molecules slow down and move closer together. They pack in tight, like people squeezing onto a crowded bus. Less space for the same molecules means higher density, so the solid sinks in its own liquid. Solid wax sinks in melted wax. Solid iron sinks in molten iron.

Water is the famous exception, and the reason is a weak connection called a hydrogen bond. A water molecule is one oxygen atom with two hydrogen atoms attached, shaped like a wide V. The oxygen end is slightly negative and the hydrogen ends are slightly positive, so the hydrogens on one molecule are attracted to the oxygen on its neighbors. In liquid water these bonds form and break constantly as the molecules tumble past each other.

When water freezes, the molecules slow down enough that the hydrogen bonds lock in place. Each molecule ends up bonded to four neighbors in a fixed arrangement, and that arrangement is a ring of six molecules: a hexagon. Hexagons stacked in every direction make a lattice with a lot of open space inside the rings. The same molecules now take up more room, so ice is about 9 percent less dense than liquid water. That is also why a water bottle left in the freezer can crack.

Because ice is less dense, it floats. On a lake in winter, the coldest water is at the surface, and when it freezes the ice stays on top. That ice acts like a lid: it slows the rest of the lake from losing heat, so the water underneath stays liquid, usually around 4 degrees Celsius. Fish, frogs, and insects spend the winter down there. If ice sank, lakes would freeze from the bottom up and many could turn solid, killing most of what lives in them.

Activity: Model It: Liquid Water vs. Ice

About 20 minutes.

  1. Get a blank sheet of paper (or use the space on your packet). Draw a box and label it LIQUID WATER. Inside, draw 12 small V shapes to stand for water molecules. Pack them close together, pointing in different directions, with only a little space between them.
  2. Draw a second box the same size and label it ICE. Inside, draw water molecules connected in rings of six, like a honeycomb. Leave clear empty space in the middle of each ring. Count how many molecules fit in the box compared to your first drawing.
  3. Below your drawings, answer the four written prompts on this page in complete sentences. Use the words density, hydrogen bond, and hexagonal at least once each.
  4. If you finish early, check your answers against the mini-lesson, then start the extension at the bottom of the page.

1. Look at your two drawings. Which box has fewer molecules in the same amount of space? Use the word density to explain what that tells you about ice compared to liquid water.

Finish this sentence: The ICE box has ____ molecules in the same space, so ice is ____ dense than liquid water. That means ice will ____ on water.

2. In your own words, explain what a hydrogen bond is and what it does to water molecules when they freeze.

3. Why does the hexagonal pattern in ice take up more room than the loose arrangement in liquid water? Describe what is inside each hexagon.

4. Prediction: Imagine ice were denser than water and sank instead of floated. Describe what would happen to a pond during a long, cold winter, and what that would mean for the fish and other living things in it.

Check what you learned

Name: ______________________ Period: ______

On your own. About 10 minutes. Circle one answer for each multiple-choice question.

1. Density is best described as:

  • AHow heavy something feels when you lift it
  • BHow much mass is packed into a certain amount of space
  • CHow fast molecules are moving
  • DThe temperature at which a substance freezes

2. Compared to liquid water, ice is:

  • AAbout 9 percent more dense
  • BExactly the same density
  • CAbout 9 percent less dense
  • DAbout half as dense

3. What holds water molecules in an open hexagonal pattern when they freeze?

  • AHydrogen bonds between neighboring molecules
  • BGravity pulling the molecules downward
  • CAir bubbles trapped in the ice
  • DSalt dissolved in the water

4. Why do lakes freeze from the top down?

  • ASunlight warms the bottom of the lake
  • BIce is less dense than liquid water, so it stays at the surface
  • CFish keep the bottom of the lake moving
  • DCold air can only reach the top few inches of water

5. In one sentence: what is the shape of the pattern water molecules form when they freeze, and why does that shape make ice float?

Finished early?

Water is not the only substance that expands when it freezes; silicon, gallium, and bismuth do too, but they are rare. Write a short paragraph answering this: Earth's oceans hold most of the planet's water. If ice sank, what might the deep ocean look like after thousands of winters? What would that mean for ocean life and for the climate at the surface? Use at least two ideas from today's lesson to support your thinking.

Before you leave: Submit your answers on this page before the bell, and leave your two drawings in the tray on the teacher's desk with your name on them.

Teacher copy — do not hand out

Answer key · Why Does Ice Float?

Check what you learned — answers

  1. Density is best described as:
    B. How much mass is packed into a certain amount of space
    Density is mass divided by volume: how much stuff is packed into a certain amount of space. Weight and speed are different ideas.
  2. Compared to liquid water, ice is:
    C. About 9 percent less dense
    When water freezes its molecules lock into an open hexagonal pattern, so the same molecules take up more room and ice is about 9 percent less dense.
  3. What holds water molecules in an open hexagonal pattern when they freeze?
    A. Hydrogen bonds between neighboring molecules
    Hydrogen bonds between the slightly positive hydrogen of one molecule and the slightly negative oxygen of its neighbor hold the rings of six in place.
  4. Why do lakes freeze from the top down?
    B. Ice is less dense than liquid water, so it stays at the surface
    Ice is less dense than liquid water, so it floats and forms a lid at the surface; the water underneath stays liquid.
  5. In one sentence: what is the shape of the pattern water molecules form when they freeze, and why does that shape make ice float?
    Model answer: Water molecules lock into a hexagonal (six-sided) pattern with empty space inside each ring, so ice is less dense than liquid water and floats.
    Accept any answer mentioning: hexagon, hexagonal, six, empty space, open space, less dense
    A good answer names the hexagonal (six-sided) pattern and says the empty space inside each ring makes ice less dense, so it floats.

Standards (please verify)

  • NGSS MS-PS1-4 — Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
  • NGSS MS-PS1-1 — Develop models to describe the atomic composition of simple molecules and extended structures.

Aligned to NGSS MS-PS1-4 (particle models and state changes) — please verify against your district's pacing guide.

Activity prompts (what to look for)

  1. Look at your two drawings. Which box has fewer molecules in the same amount of space? Use the word density to explain what that tells you about ice compared to liquid water.
    Scaffold given: Finish this sentence: The ICE box has ____ molecules in the same space, so ice is ____ dense than liquid water. That means ice will ____ on water.
  2. In your own words, explain what a hydrogen bond is and what it does to water molecules when they freeze.
  3. Why does the hexagonal pattern in ice take up more room than the loose arrangement in liquid water? Describe what is inside each hexagon.
  4. Prediction: Imagine ice were denser than water and sank instead of floated. Describe what would happen to a pond during a long, cold winter, and what that would mean for the fish and other living things in it.

Online submissions for class DEM-O01 are scored automatically; this key is for paper packets.