Kinetic Molecular Theory: Alka-Seltzer Diffusion and Dissolution Rate Investigation
Science Lab · Grade 7
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science_lab
Pre Lab
Background: Have you ever dropped an Alka-Seltzer tablet into a glass of water and watched it fizz? That fizzing is actually a window into one of the most important ideas in science: **Kinetic Molecular Theory (KMT)**. KMT tells us that all matter is made up of tiny **particles** — atoms and molecules — that are constantly moving. The key word is *constantly*. Even the desk you are sitting at is made of particles that vibrate in place. KMT states that the higher the temperature of a substance, the faster its particles move. This is because temperature is actually a measure of the average **kinetic energy** (energy of motion) of the particles in a substance.
When an Alka-Seltzer tablet is placed in water, it undergoes **dissolution** — the process by which a solid breaks apart and its particles mix with the particles of a liquid to form a solution. The tablet contains citric acid and sodium bicarbonate. When these substances dissolve, they react and produce carbon dioxide gas (CO₂), which is what causes the bubbling. The speed at which this happens is called the **dissolution rate**. According to KMT, water molecules at higher temperatures are moving faster and colliding with the tablet's surface more frequently and with more energy. This means they can break apart the tablet's particles more quickly. The CO₂ gas produced then spreads through the water and into the air through a process called **diffusion** — the movement of particles from an area of higher concentration to an area of lower concentration.
This lab is the culminating investigation for your unit on Matter, Molecules, and Mixtures. You will use what you know about KMT, **phase changes**, **evaporation**, and **conservation of mass** to design your own experiment. Remember: **conservation of mass** tells us that matter is never created or destroyed — the mass of the tablet plus the water should equal the mass of the final mixture plus any gas that escapes. As you investigate, think like a scientist: What do you predict will happen, and *why* does KMT predict that outcome?
Vocabulary
Term: Kinetic Molecular Theory (KMT)
Definition: A scientific model stating that all matter is made of tiny particles that are in constant, random motion, and that the speed of that motion increases as temperature increases.
Term: Diffusion
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Post Lab
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