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Alka Seltzer and Water Chemical Reaction

Instructor: Amanda Robb
In this experiment, you'll be learning about how temperature can affect the speed of acid base reactions. To do this, we'll be studying the reaction of Alka-Seltzer with water.

Introduction

Research Question: How does temperature affect reaction rate?
Age: Middle school and up
Safety concerns: None
Time: 1 hour
Independent variable: Temperature
Dependent variable: Rate of reaction
Control variables: Amount of water, amount of Alka-Seltzer

One night you're boiling water on the stove for some late night pasta. In your stupor, you forget the stove was on and you come back to find an empty pan and a dangerously hot stove. Where did the water go? When heat was added, the water underwent a phase change from a liquid to a gas. Although it's clear you need to increase temperature to make this happen, what's going on with the molecules? How does the water simply evaporate into something we can't see?

When temperature increases, the heat energy breaks the bonds between the water molecules. The water molecules start to move faster and spread out, forming a gas. That gas escapes the pan and over time your water is gone. Today, we're going to look at how changing temperature, and thus molecular motion, affects a common chemical reaction - Alka-Seltzer and water.

Alka-Seltzer is a tablet used to treat indigestion. It's made of two chemicals, an acid and a base. Acids are molecules that can donate small charged particles called hydrogen ions. Bases are the opposite, they accept hydrogen ions. When acids and bases come in contact with each other, they create a chemical reaction. However, in the Alka-Seltzer, the two chemicals are in a solid form, they can't react because the molecules don't move. When they enter water, the chemicals are released and can react.

In Alka-Seltzer, the citric acid mixes with the base, bicarbonate, to form carbon dioxide bubbles. How quickly these bubbles form indicates how fast the reaction is proceeding, or reaction rate.

Carbon dioxide forms when Alka-Seltzer mixes with water
Alka-Seltzer

Before you get started, think about what will happen when you increase temperature. Will the reaction proceed faster or slower?

Materials

  • 3 Alka-Seltzer tablets
  • 3 cups of water
  • Access to hot water and a refrigerator to make cold water
  • Marker and tape to label your cups
  • Timer
  • Thermometer
  • Data table:

Water Temperature Time for reaction to complete Observations
Cold
Room temperature
Hot

Steps

1. First, get your water to the right temperature. Take three identical cups and label one 'hot', one 'room temperature' and one 'cold'.

2. Start by putting one cup of water in the 'room temperature' cup and place it on the surface you'll do the experiment on to let it equilibrate with the room temperature.

3. Next, add one cup of water to the 'cold' cup and place it in the fridge. Wait 30 minutes for your cold water to get cold.

4. Now, get your timer ready. Add the Alka-Seltzer tablet to the cold water and record how long it takes for the reaction to finish. When no more bubbles appear after 10 seconds, the reaction is done. Record any observations about the reaction also. Was there a lot of bubbles, or very few?

5. Repeat step 4 with the room temperature water.

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