IMSO 2015 Science International - Experiment

Editorial safety note: This is a teacher-supervised laboratory exercise. Do not taste unknowns, use test samples for cooking, or apply these reagents to skin. Use teacher-prepared dilute chemicals and eye protection. Record real observations only.

EXPERIMENT I : What are those white powders? Today is mother’s birthday. You want to cook an easy meal for her. It is your first time venture into her kitchen territory. Opening the seasoning shelf, you found five unlabeled jars which have different lid color. Each jar contains white or colorless solid. You kind of have an idea that they could be salt, sugar, baking soda, starch, and monosodium glutamate (MSG). Before start cooking anything, you must first figure out the identity of the solid in each jar. You do not dare to taste the unknown samples because you know that monosodium glutamate can trigger allergic symptoms. Luckily, there are some household chemicals, such as vinegar, Condy’s crystals solution and Tincture of iodine, that can help you solve the puzzle. Can you correctly identify the mystery powder?

Hints: 1. Vinegar is a common acid in the kitchen. Its reaction with base gives some bubbles. 2. Condy’s crystals solution (potassium permanganate, KMnO4) is used for wound cleansing. It turns from purple to brown when reacts with sugar, starch, and monosodium glutamate. 3. Tincture of iodine is an antiseptic. Iodine reacts with starch. The color changes from brown to purple color. 4. Solubility of MSG in water is better than that in vinegar.

Materials: 2 measuring spoons; 2 plastic droppers; 2 clear plastic plates; vinegar; iodine solution; Condy’s crystals solution (KMnO₄); water.

Procedures

1. Place one spoonful of each powder in each well of clear plastic plates.

2. Take the dropper and place 4 mL of water on each individual powder. Examine what happens? Do the powders dissolve? Write your observations in the result table.

3. Add 4 mL of water to another empty well as a blank for color comparison. Add 2 drops of Condy’s crystals solution to each unknown solution and the blank solution. Record your observations in the “Color test with KMnO₄” column.

4. Repeat steps 1–2. Then, add 2 drops of iodine solution to each unknown solution and a blank solution. Record your observations in the “Iodine test” column.

5. Repeat steps 1–2, but change from 4 mL of water to 4 mL of vinegar. Record what happens? Do the powders dissolve? Write your observations in the result table.

Caution: Iodine solution and Condy’s crystals solution should be handled with care.

EXPERIMENT I ANSWER SHEET

Write your observations in the result table. (15 points)

Indicate the solubility as the following: /// = all dissolved, // = partly dissolved, / = hardly dissolved.

| Unknown | Solubility in water | Solubility in vinegar | Color test with KMnO₄ | Iodine test | Identity |
| --- | --- | --- | --- | --- | --- |
| A | … | … | … | … | … |
| B | … | … | … | … | … |
| C | … | … | … | … | … |
| D | … | … | … | … | … |
| E | … | … | … | … | … |

Questions 1. Which powder undergoes a chemical reaction? Explain what supports your answer. (3 points)

Questions 2. What kind of gas in the reaction of vinegar with base? (1 point)

Questions 3. If you don’t have vinegar in the kitchen, what else can be used instead? Why? (1 point)

Editorial safety note: Work with teacher supervision, use only the supplied liquid, avoid splashes, and keep small metal balls away from mouths. All measurement cells remain blank until measured.

EXPERIMENT II Falling ball inside the liquid

Introduction

When a metal ball is dropped into a liquid, a viscous force will drag it. Naturally, this force is not constant but depends on the ball’s speed. The greater the speed, the greater the viscous force. In this experiment, a ball will be released just above the liquid surface, and after a while its speed will become constant called “terminal speed”. By intuition, one would expect that this terminal speed must be greater for a bigger ball because it is heavier. In this experiment, one wants to know exactly whether the terminal speed is proportional to (i) surface area of the ball ($4\pi r^2$) or (ii) the volume of the ball ($4\pi r^3/3$). To prove the proportionality, we need to plot a graph between two quantities on x-axis and y-axis respectively. When the plot of data has a linear tendency, i.e. fitted nicely with a linear line, one can say that they are proportional to one another.

Equipment and accessories 1 plastic bottle with unknown liquid 18 metal balls (6 sizes, 3 balls each) 1 stopwatch 1 magnet 2 rubber bands 1 ruler 2 graph papers Tissue paper

Experimental procedure

1. Wrap the plastic bottle with 2 rubber bands and set their position as the start and finish lines. Keep the start line about 60-70 mm below the liquid surface and the finish line as low as possible. Measure the distance between them with 1 mm precision.

2. Drop the ball less than 10 mm above the liquid surface and measure the time of falling between the start and finish lines. Leave the balls in liquid. Record your observation in the table.

3. Repeat this one more time and for all other provided ball sizes.

4. Calculate the terminal speed with two digit decimal in mm/s unit.

5. Plot your data on graph paper so that one can make conclusion whether the terminal speed is proportional to the surface area or the volume of the ball.

6. Answer questions 1–3 in your answer sheet.

Distance between the liquid surface and the start rubber band is … mm.

Distance between two rubber bands is … mm.

| No. | Surface area A (mm²) | Volume V (mm³) | Time trial 1 (s) | Time trial 2 (s) | Average time (s) | Terminal velocity (mm/s) |
| --- | --- | --- | --- | --- | --- | --- |
| 1 | 12.6 | 4.2 | … | … | … | … |
| 2 | 17.8 | 7.1 | … | … | … | … |
| 3 | 28.3 | 14.1 | … | … | … | … |
| 4 | 31.7 | 16.8 | … | … | … | … |
| 5 | 50.3 | 33.5 | … | … | … | … |
| 6 | 71.2 | 56.5 | … | … | … | … |

Plot your data on graph paper so that one can make conclusion whether the terminal speed is proportional to the surface area or the volume of the ball.

Question 1. Is the terminal speed proportional to the surface area or the volume of the ball? How do you make that conclusion? (1 point)

Question 2. From the information provided in the introduction, explain clearly why does the speed finally become constant? (2 points)

Question 3. If a student mistakenly set the start rubber band at the liquid surface, the speed will be too large or too small? Explain it. (2 points)