Matter on the Scale: Conservation of Mass
Students compare mass measurements before and after heating, cooling, or mixing matter in closed containers to determine whether the total amount of matter changes.
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Predicting Changes in Mass
Before an investigation, make a prediction about what will happen to mass when matter is heated, cooled, or mixed. Mass is the amount of matter in an object or group of objects. Imagine a sealed bag containing 25.4 grams of vinegar and a small cup holding 4.6 grams of baking soda. The substances are inside the same closed bag but have not yet mixed. Their total mass is 30.0 grams. When they mix, bubbles form and the bag expands. You might predict that the mass will increase because the bag becomes larger, or decrease because a gas forms. However, size and mass are not the same. Record your prediction and explain your reasoning. A useful prediction identifies the system being measured and considers whether any matter can enter or leave it.
Measuring a Closed System
A system is everything included in an investigation. In a closed system, matter cannot enter or leave. To test conservation of matter, place the entire closed container and all its contents on a scale. For example, a sealed container of water might have a mass of 142.7 grams before cooling. After the water freezes, dry the outside without opening the container and measure it again on the same scale. The reading should remain about 142.7 grams because the ice is made of the same matter as the liquid water. Use the same container, scale, and units for both measurements. Check that the scale reads zero before beginning. Small differences can result from spilled matter, a leaking container, water on the outside, or limits in the scale’s precision.
Comparing Before-and-After Data
Organize measurements in a table, then use decimal subtraction to compare each before-and-after mass. Suppose a sealed container has a mass of 68.35 grams before heating and 68.34 grams after heating. Calculate 68.35 − 68.34 = 0.01 gram. This tiny difference may come from the precision of the scale rather than a real loss of matter. A second trial might measure 91.20 grams both before and after mixing, for a difference of 0.00 gram. Make a bar graph with paired bars for each trial. Use the same scale on the vertical axis so the bars can be compared fairly. Look across several trials rather than relying on one result. If before-and-after measurements are equal or nearly equal in closed systems, the data support conservation of mass.
Explaining Conservation of Matter
Conservation of matter means that matter is not created or destroyed during ordinary heating, cooling, or mixing. Matter may change state, spread out, or form new substances, but its total mass stays the same in a closed system. When vinegar and baking soda react, they produce carbon dioxide gas along with other substances. In an open cup, the gas escapes into the air, so the measured mass in the cup decreases. In a sealed bag, the gas remains inside, and the mass of the entire bag and its contents stays about the same. The bag may inflate because gas particles spread through more space, but no extra matter has appeared. The system boundary is important: scientists must measure every solid, liquid, and gas inside it when comparing total mass.
Writing an Evidence-Based Conclusion
An evidence-based conclusion includes a claim, evidence, and reasoning. Begin with a clear claim: “The total amount of matter was conserved in the closed containers.” Next, cite measurements from more than one source, such as a data table, graph, and observations. For example, the water container measured 142.7 grams before and after cooling, and the mixing trial measured 91.20 grams both before and after. The graph also showed paired bars of equal height. Then explain the reasoning: heating, cooling, and mixing can change the form or appearance of matter, but matter cannot leave a properly sealed system. Mention unusual results honestly. If one trial differed by 0.01 gram, explain that scale precision or measurement error may account for the difference. End by connecting all evidence to the claim rather than simply repeating the numbers.
