An open vessel: explain mass entering or escaping as gas
| English | 中文 | Pinyin |
|---|---|---|
| open system | 开放系统 | kāi fàng xì tǒng |
| thermal decomposition/ˈθɜːml ˌdiːkɒmpəˈzɪʃn/ | 热分解 | rè fēn jiě |
What would explain this observation?
- A heated metal can gain mass, while a heated carbonate can leave a lighter solid. Neither observation contradicts conservation when gas transfer is counted.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In an open system 开放系统, gas may enter from or escape to the surrounding air. Magnesium reacts with oxygen as 2Mg + O₂ → 2MgO. The oxide contains the original magnesium plus oxygen from outside, so it weighs more than the original metal. Calcium carbonate decomposes as CaCO₃ → CaO + CO₂. The remaining solid is lighter because carbon dioxide has escaped, not because atoms have vanished.
- open system: A system that can exchange material with its surroundings; thermal decomposition 热分解: Breaking down a compound into simpler substances by heating.
Why can magnesium oxide weigh more than the starting magnesium?
Identify which material is on the balance at each reading. In the metal example, oxygen joins the weighed solid; in the carbonate example, gas leaves the weighed apparatus. In particle terms the same atoms are redistributed among substances, with some crossing the selected system boundary. A sealed-system total includes all products; an open-residue measurement does not.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Identify which material is on the balance at each reading. In the metal example, oxygen joins the weighed solid; in the carbonate example, gas leaves the weighed apparatus. In particle terms the same atoms are redistributed among substances, with some crossing the selected system boundary. A sealed-system total includes all products; an open-residue measurement does not.
- For a teacher-approved school investigation, record the clean dry vessel mass, sample mass and final cooled vessel-plus-solid mass. Use appropriate heat protection, ventilation and an approved method. Loss of solid through spitting or incomplete reaction can change the reading too. Do not seal heated gas-producing apparatus merely to demonstrate conservation, and do not assume every measured difference is the intended gas transfer.
Which two habits make the investigation or model in this case more defensible?
For a teacher-approved school investigation, record the clean dry vessel mass, sample mass and final cooled vessel-plus-solid mass. Use appropriate heat protection, ventilation and an approved method. Loss of solid through spitting or incomplete reaction can change the reading too. Do not seal heated gas-producing apparatus merely to demonstrate conservation, and do not assume every measured difference is the intended gas transfer.
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: 10.0 g of a model carbonate leaves 5.6 g solid oxide after complete decomposition with no other losses. Escaped gas mass=10.0−5.6=4.4 g. The 5.6 g oxide plus 4.4 g gas equals the original 10.0 g. For 2.4 g Mg forming 4.0 g MgO, oxygen gained=1.6 g.
A carbonate sample starts at 25 g and leaves 14 g oxide. Assuming gas is the only lost material, find its mass. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A carbonate sample starts at 25 g and leaves 14 g oxide. Assuming gas is the only lost material, find its mass.
The result is 11 g. Known: 10.0 g of a model carbonate leaves 5.6 g solid oxide after complete decomposition with no other losses. Escaped gas mass=10.0−5.6=4.4 g. The 5.6 g oxide plus 4.4 g gas equals the original 10.0 g. For 2.4 g Mg forming 4.0 g MgO, oxygen gained=1.6 g.
Check the conclusion and its limits
- A gas has mass even if it is invisible. Mass gain does not imply extra magnesium atoms were created. Cooling before weighing reduces temperature-related weighing problems. If apparatus mass changes or solid is spilled, the simple gas-only explanation needs revision rather than being forced to fit the result.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Escaping gas is massless and therefore cannot explain a lower residue mass. This claim is false: A gas has mass even if it is invisible. Mass gain does not imply extra magnesium atoms were created. Cooling before weighing reduces temperature-related weighing problems. If apparatus mass changes or solid is spilled, the simple gas-only explanation needs revision rather than being forced to fit the result.
An open vessel: explain mass entering or escaping as gas: Identify which material is on the balance at each reading. In the metal example, oxygen joins the weighed solid; in the carbonate example, gas leaves the weighed apparatus. In particle terms the same atoms are redistributed among substances, with some crossing the selected system boundary. A sealed-system total includes all products; an open-residue measurement does not.
Escaping gas is massless and therefore cannot explain a lower residue mass.
A gas has mass even if it is invisible. Mass gain does not imply extra magnesium atoms were created. Cooling before weighing reduces temperature-related weighing problems. If apparatus mass changes or solid is spilled, the simple gas-only explanation needs revision rather than being forced to fit the result.
A system that can exchange material with its surroundings: write the technical term.
open system means A system that can exchange material with its surroundings.