Conservation of mass: count the complete system
| English | 中文 | Pinyin |
|---|---|---|
| conservation of mass/ˌkɒnsəˈveɪʃn ɒv mæs/ | 质量守恒 | zhì liàng shǒu héng |
| system boundary | 系统边界 | xì tǒng biān jiè |
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?
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.
Match each technical term to its precise meaning.
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?
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.
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.
The result is 4 g. 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.
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: 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.
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.
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.
The total mass of all reactants equals the total mass of all products in a chemical reaction: write the technical term.
conservation of mass means The total mass of all reactants equals the total mass of all products in a chemical reaction.