Conservation of mass: count the complete system
Введено| English | Значение |
|---|---|
| conservation of mass/ˌkɒnsəˈveɪʃn ɒv mæs/ | закон сохранения массы |
| system boundary/ˈsɪstəm ˈbaʊndəri/ | граница системы |
What would explain this observation?
- A sealed reaction vessel weighs the same before and after a chemical reaction even when a gas appears. The atoms have been rearranged inside the weighed system.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- The law of conservation of mass 质量守恒 states that atoms are neither made nor lost in a chemical reaction. The total mass of products equals the total mass of reactants when every substance is included. A balanced symbol equation has the same number of atoms of each element on both sides. It does not require the same number of separate molecules on each side.
- conservation of mass: The total mass of all reactants equals the total mass of all products in a chemical reaction; system boundary 系统边界: The defined limit separating the material being considered from its surroundings.
What does the 2 before H₂O in a balanced equation mean?
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The coefficient 2 counts whole water molecules: two H2O. The subscript 2 inside the formula is part of water's identity, and H2O2 would be hydrogen peroxide - a different substance.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For 2H₂ + O₂ → 2H₂O, the coefficient 2 multiplies an entire formula. The subscript 2 belongs to the preceding element, so one water molecule contains two H atoms and one O atom. Both sides contain four H atoms and two O atoms. Using supplied relative atomic masses H=1 and O=16 gives reactant totals 2×2 + 32 =36 and product total 2×18=36. The coefficients 2:1:2 are not gram ratios.
- Make an atom-count table before doing a mass calculation. Identify the physical boundary of the weighed system: vessel, liquid, solid and any contained gas. For an actual supervised demonstration, use an approved arrangement that accommodates any pressure change; do not improvise a heated sealed vessel. Compare readings only after correcting for anything added or removed from the balance.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: an enclosed model reaction starts with 4.0 g A and 7.0 g B, and has no other reactants. The combined products weigh 11.0 g. If one product weighs 8.5 g, the other weighs 11.0−8.5=2.5 g. This mass accounting uses the complete stated system and does not identify either substance chemically.
A closed model reaction starts with 6 g and 9 g of reactants. One product weighs 11 g. Find the other product mass. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A closed model reaction starts with 6 g and 9 g of reactants. One product weighs 11 g. Find the other product mass.
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The result is 4 g. Known: reactants total 6 + 9 = 15 g in a closed system; one product is 11 g. By conservation, other product = 15 - 11 = 4 g. Mass is neither created nor destroyed in the reaction.
Check the conclusion and its limits
- Conservation does not mean that every individual substance keeps its own mass. Reactants are converted into products. Altering a subscript to balance an equation changes the substance, whereas changing a coefficient changes the number of particles. Include material crossing the system boundary before calling a result a failure of conservation.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Balanced coefficients always give the ratio of masses in grams. This claim is false: Coefficients count PARTICLES (or moles of them). Because each substance has a different molar mass, a 2:1 particle ratio is not a 2:1 mass ratio - convert through moles before comparing masses.
Conservation of mass: count the complete system: For 2H₂ + O₂ → 2H₂O, the coefficient 2 multiplies an entire formula. The subscript 2 belongs to the preceding element, so one water molecule contains two H atoms and one O atom. Both sides contain four H atoms and two O atoms. Using supplied relative atomic masses H=1 and O=16 gives reactant totals 2×2 + 32 =36 and product total 2×18=36. The coefficients 2:1:2 are not gram ratios.
Balanced coefficients always give the ratio of masses in grams.
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Coefficients count PARTICLES (or moles of them). Because each substance has a different molar mass, a 2:1 particle ratio is not a 2:1 mass ratio - convert through moles before comparing masses.
The total mass of all reactants equals the total mass of all products in a chemical reaction: write the technical term.
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conservation of mass means The total mass of all reactants equals the total mass of all products in a chemical reaction.