Snowflakes

INTRODUCTION: This lesson expands on the idea of crystals and what they tell us about how atoms fit together. It also lays more groundwork for the differences between solids, liquids and gases and connects with maths on the subject of symmetrical shapes.

PREPARATION Cut the straws to about 20cm in length, removing any ‘bendy’ joints. Cut off the points

of the skewers and trim the remainder to be a bit smaller than the straws, roughly 15cm in length. Bend the straws in half at an angle, as shown in the figure below. While the exact straw lengths don’t matter too much, the two halves of the bend should be equal and the exact angle of the bend is important (110 degrees): you can hold the straws up to the figure below to use as a template. Once the angle is right, wrap some tape around each straw to secure the shape.

Relevant KS2 Links: SC1 1a, 1b, 2b, 2h, 2i, 2j, 2k, 2l; SC3 2b, 2c, 2d; MA3 1h, 2b, 2c, 2d, 3b; MA4 1a, 1g, 2a

Suggested Year Group:

3-4

EQUIPMENT • 10 plastic straws; • 10 bamboo skewers; • Some tape; • A piece of cardboard, A4 or larger; • The accompanying picture of a water molecule; • The accompanying worksheet on water crystals; • Photographs of snowflakes (or link to an appropriate website);

Assumes Intro to Atoms

Base Concepts Conveyed: Crystals are a regular stacking of atoms or molecules. Snowflakes are crystals of water molecules.

TEACHING: Review once more the idea of atoms, how everything we see is made of combinations of different types of atoms, and that the combination of a specific number of atoms that are held together tightly is called a molecule, like water. Also recall how atoms and molecules can sometimes be carefully stacked together in a regular pattern to make crystals, like with table salt (sodium chloride). In fact, even water can be made into crystals! Does anyone remember the atoms that make up a water molecule? That’s right, H2O, which means two atoms of hydrogen and one atom of oxygen. So, what does that look like? Show the picture of a water molecule. Obviously this is much, much, much, much, much bigger than real water molecules, which are much to small to see even with the best magnifying lens! (They also don’t really have these colours or big letters on them!) However, if you could see them, they would look a bit like fuzzy balls. They are actually made from other particles (protons, neutrons and electrons) that we’ll talk about another time. See how the two hydrogen atoms are stuck on the oxygen atom? What sticks them? We’ll get to that in another lesson, but basically it has to do with things called electric “charges,” that act a bit like magnets.

What does a molecule of water look like?

Notice how the atoms make a sort of an angle (trace lines between the centres of the atoms with your finger). Hold one of the bent straws up to the picture to show that this represents the same angle. Describe how water molecules can actually stick to each other a bit. Invite a volunteer up and explain that each of the bent straws represents a molecule of water and show how they can be joined together by inserting one end of a bamboo stick into the end of one of the bent straws and the other end of the stick into the end of another bent straw. Ask your helper to try this, joining up as many as it takes to make a complete loop of water molecules. They should lay this on the piece of cardboard so it doesn’t fall apart. When this is done, they should end up with a 6-sided hexagon, which you should show to the rest of the class. Note: the interior angles of a true hexagon are, of course, 120 degrees, not the 110 degrees of water molecules. However, water molecules can actually ‘twist’ a bit to accommodate hexagonal structures… and that’s what this construction also allows, so the demonstration is relatively true to life. Next explain that crystals can be built up by stacking these hexagons together. Does anyone know what these crystals are called? Snowflakes!

ACTIVITY & PLENARY: Pass out the accompanying worksheets, in which each student is asked to construct their own symmetric snowflake-like pattern by colouring in adjacent hexagons of water molecules. When they are finished, ask them to work out how many ways their figure can be divided into symmetric halves (i.e. the number of symmetry axes). This is tantamount to counting the number of “sides.” You should find that this number is always 6 if they’ve made their figures as symmetric as possible. Now look at some photographs of actual snowflakes. An excellent internet site for this is snowcrystals.com , which also provides descriptions of the science behind snowflakes. As such, this could also be made the basis of an ICT exercise. What do you notice about actual snowflakes? They all have 6 symmetry axes! So, why do snowflakes have 6 sides? Because they are all made from 6-sided hexagons of water molecules. And why do water molecules form hexagons in crystals? Because of the angle that naturally exists between the hydrogen and oxygen atoms in H2O. So, when you look at the pretty shapes of snowflakes, you are actually seeing something about the shapes of the molecules that make them up! Of course, “normal” blocks of ice are not crystalline… that is, the water molecules do not generally form a regular pattern. Think about how you turn a snowball to ice by scrunching it and packing it down. This twists and breaks the regular structure of the flakes to make something more compact, with the water molecules facing in lots of different directions. This is what it’s more generally like when larger amounts of water freeze (like ice cubes). Heating snow turns it back into regular water, right? What does that mean must happen to the crystals? The heat must break them up into individual molecules again. So “heat” must have something to do with making atoms and molecules move around more… which is absolutely right!

What shapes are naturally made if you carefully fit water molecules together?

What is heat?

What do snowflakes look like?

Why do snowflakes have 6 sides?

Why isn’t all ice crystalline?

Quick Reference

o Review atoms, molecules and crystals and the differences between them. o Water molecules can also be stacked into crystals. o What does water look like? Show picture. o Note angle of hydrogen atoms – compare with bent straw. o Have volunteer try to connect similar bent straws into complete loop. o Hexagons are basic unit of water crystal (snowflakes!) o Have students make snowflake patterns using worksheet. o Discuss the symmetries that emerge and why they are 6-fold. o Compare with photographs of actual snowflakes (possible ICT exercise). o Why isn’t all ice crystalline? (Think about packed snowball). o What does it mean when snow melts? What is “heat?”

Snowflake Patterns Join up rings of H2O molecules in an interesting pattern by colouring some of the ones below. Try to make all your coloured rings connect and make the pattern as symmetric as possible!