The Crystal Clock: Measuring a Change Too Slow to Watch

Investigation question: How fast does a crystal grow when nobody can see it growing, and how long would it take, at that rate, to become a crystal one meter long?

Your role today

• Solution maker: Measures the alum into the cup, receives the hot water from the teacher, stirs until a few grains stay undissolved, and labels the cup. Never carries hot water.
• Measurer: Each day lifts the cup onto the millimeter grid card (or holds the ruler flat against the cup wall), reads the longest edge of the largest crystal to the nearest 0.5 mm with the hand lens, and counts the crystals, without stirring or tilting the solution.
• Recorder and grapher: Writes each day's value and observation in the table, plots the point on the graph the same day, sketches the largest crystal on the first and last day, and keeps the sheet with the cup.

Safety

• Goggles on whenever solutions are being made, poured or emptied. Alum and Epsom salt solutions are eye irritants.
• No tasting, no sipping, no fingers in the cup. Alum is sold as a food additive but in these amounts can cause stomach upset; Epsom salt is a laxative. Label every cup 'Do not drink'. Wash hands after handling crystals.
• Only the teacher handles hot water: water at 60 to 70 °C is poured by the teacher into cups that are already on a tray at the student station; students never carry a cup of hot solution.
• Carry cooled cups with two hands on a tray; mop any spill at once, since a wet floor with dissolved alum is slippery.
• Do not use thin plastic cups or glass that is not heat-tolerant; hot water can soften or crack them.
• Crystals have sharp edges once removed; handle them by the broad faces, not the points.
• Disposal: small volumes of alum or Epsom salt solution can go down the drain with plenty of running water; crystals go in the trash or home in a bag if your school allows.

Procedure

1. Session 1 (day 1). Put on goggles. Label your cup with your group number, 'Do not drink' and your cooling condition (SLOW or FAST). Weigh or measure about 30 g of alum into the cup and record the mass.
2. Set the cup on the tray. The teacher pours 100 mL of hot water (60 to 70 °C) into it. Stir gently for about two minutes until nearly all the alum dissolves. If every grain disappears, add half a teaspoon more and stir; stop when a few grains stay on the bottom. Record the water temperature the teacher reads.
3. SLOW groups: wrap the cup in the towel and set it inside the box with the lid loosely on. FAST groups: stand the cup in the tray of room-temperature water for the rest of the period, then move it to the open shelf. Do not stir again. Record the time you put it away.
4. Session 2 (day 2 or 3). Without tilting or stirring, lift the cup onto the millimeter grid card. Find the largest crystal. Using the hand lens and the grid or a ruler held flat against the outside of the cup, measure the longest edge of that crystal to the nearest 0.5 mm. Record it as the first measurement, with the date, and count how many crystals you can see.
5. Sketch the largest crystal with its shape and the number of flat faces you can see. Return the cup to its shelf and condition.
6. Every school day after that (five minutes at the start of class), repeat the measurement of the same crystal, count the crystals, note anything new (new crystals, cloudiness, a crust at the surface) and plot the day's point on the graph before you sit down.
7. Session 3 (day 7 or 8). Take the final measurement in the solution. Then pour the solution into the waste container, lift out the largest crystal with the spoon, dry it on a paper towel, measure it directly with the ruler, and (if a scale is available) record its mass. Sketch it next to your first sketch, at the same scale.
8. Calculate your growth rate: (final length minus first length) divided by the number of days between them, in millimeters per day. Then calculate how many days, and how many years, a crystal would need to reach 1,000 mm (one meter) at that rate.
9. Post your group's cooling condition, number of crystals and largest-crystal length on the class table. Answer the analysis questions individually, then compare within the group.

Record your observations

Lab 1 recording sheet: The crystal clock

Session 1: fill in the setup table. Session 2 and every school day after: one line in the daily table and one point on the graph. Session 3: the final line, both sketches, the rate calculation and the questions. Never stir or tilt the cup after day 1.

Analysis questions