Chemical purity: use melting and boiling evidence
| English | 中文 | Pinyin |
|---|---|---|
| melting interval | 熔化温度区间 | róng huà wēn dù qū jiān |
| chemical purity | 化学纯度 | huà xué chún dù |
What would explain this observation?
- A bottle labelled pure milk still contains water, proteins, fats and other substances. Chemically pure and nothing added describe different things.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In chemistry a pure substance is one element or one compound, not mixed with another substance. Pure oxygen and pure water satisfy that definition despite water containing two elements chemically combined. Milk is a mixture even if it has no additives. Pure elements and compounds melt and boil at specific temperatures for the stated conditions. A mixture commonly melts over a range; an impurity often lowers and broadens the melting interval 熔化温度区间 relative to a pure solid.
- chemical purity 化学纯度: The extent to which a sample consists of one element or compound rather than a mixture; melting interval: The measured temperature range from the start to the completion of melting in the stated test.
Which sample can be chemically pure?
Compare the measured transition with a reference at the same pressure and with calibrated apparatus. A sharp matching melting point supports purity and identity, while a broadened or shifted interval can indicate an impurity. Boiling temperature depends on pressure and the mixture; do not claim every impurity must shift every boiling point in the same direction. A single matching temperature is supporting evidence rather than unique proof of identity, because different substances can have similar values.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Compare the measured transition with a reference at the same pressure and with calibrated apparatus. A sharp matching melting point supports purity and identity, while a broadened or shifted interval can indicate an impurity. Boiling temperature depends on pressure and the mixture; do not claim every impurity must shift every boiling point in the same direction. A single matching temperature is supporting evidence rather than unique proof of identity, because different substances can have similar values.
- Read the temperature at which melting begins and the temperature at which the last solid disappears. Record an interval rather than selecting its midpoint to hide a broad transition. Compare repeat measurements and instrument resolution. Any actual school melting-point or boiling demonstration uses approved samples and controlled heating; never heat an unknown flammable material or close a heated vessel to obtain a more convenient reading.
Which two habits make the investigation or model in this case more defensible?
Read the temperature at which melting begins and the temperature at which the last solid disappears. Record an interval rather than selecting its midpoint to hide a broad transition. Compare repeat measurements and instrument resolution. Any actual school melting-point or boiling demonstration uses approved samples and controlled heating; never heat an unknown flammable material or close a heated vessel to obtain a more convenient reading.
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: pure reference X melts at 80 °C under the stated conditions. Sample A melts from 79.5 to 80.5 °C; sample B from 72 to 77 °C. Interval widths are 1.0 and 5 °C. B has a broader, lower transition and is more consistent with impure X. The measurement resolution and possibility of another material still need consideration.
A sample begins melting at 63 °C and finishes at 69 °C. Find its melting-interval width. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A sample begins melting at 63 °C and finishes at 69 °C. Find its melting-interval width.
The result is 6 °C. Known: pure reference X melts at 80 °C under the stated conditions. Sample A melts from 79.5 to 80.5 °C; sample B from 72 to 77 °C. Interval widths are 1.0 and 5 °C. B has a broader, lower transition and is more consistent with impure X. The measurement resolution and possibility of another material still need consideration.
Check the conclusion and its limits
- Pure does not mean natural, harmless, colourless or a single type of atom. A compound can be pure. Do not identify an impurity solely by the size of the melting-point change or ignore pressure when comparing boiling data. This section applies on both tiers.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A compound containing two elements can never be a pure substance. This claim is false: Pure does not mean natural, harmless, colourless or a single type of atom. A compound can be pure. Do not identify an impurity solely by the size of the melting-point change or ignore pressure when comparing boiling data. This section applies on both tiers.
Chemical purity: use melting and boiling evidence: Compare the measured transition with a reference at the same pressure and with calibrated apparatus. A sharp matching melting point supports purity and identity, while a broadened or shifted interval can indicate an impurity. Boiling temperature depends on pressure and the mixture; do not claim every impurity must shift every boiling point in the same direction. A single matching temperature is supporting evidence rather than unique proof of identity, because different substances can have similar values.
A compound containing two elements can never be a pure substance.
Pure does not mean natural, harmless, colourless or a single type of atom. A compound can be pure. Do not identify an impurity solely by the size of the melting-point change or ignore pressure when comparing boiling data. This section applies on both tiers.
The extent to which a sample consists of one element or compound rather than a mixture: write the technical term.
chemical purity means The extent to which a sample consists of one element or compound rather than a mixture.