Core Practical: Investigating Changes of State
Experimental Setup: Fill a beaker halfway with ice cubes and insert a thermometer. Support the beaker on a tripod and gauze over a Bunsen burner to ensure even and controlled heating.
Data Collection Protocol: Take initial temperature readings before heating begins. Start the stopwatch and record the temperature at regular intervals (typically every 60 seconds) to ensure a sufficient number of data points for plotting.
Observation and Continuity: Continue heating and recording data until the substance has completely changed state and the temperature begins to rise again, ensuring the entire transition is captured.
Resolution and Precision: Utilize a thermometer with a resolution of at least and a stopwatch with resolution to minimize measurement uncertainty.
| Feature | Temperature Increase | Change of State (Melting) |
|---|---|---|
| Effect on Particles | Increases average kinetic energy | Overcomes intermolecular forces (potential energy) |
| Graph Feature | Sloped line | Horizontal plateau (flat region) |
| Thermal Energy Use | Raises the substance's temperature | Breaks bonds between particles |
| State of Matter | Single phase (solid or liquid) | Two phases present (solid and liquid) |
Plateau vs. Slope: In a heating curve, a sloped region represents the heating of a single state, while a flat region specifically represents the energy being used for a phase transition without a temperature change.
Evaporation vs. Melting: While both involve bond weakening, melting occurs throughout the solid at a fixed temperature, whereas evaporation typically occurs only at the surface of a liquid over a range of temperatures.
Graph Analysis: Examiners frequently ask students to identify the melting point from a graph. Always look for the first horizontal plateau; the temperature coordinate of this line is the melting point.
Axes Labeling: When drawing or interpreting graphs, ensure the y-axis is labeled 'Temperature ()' and the x-axis is labeled 'Time (s)'. Failure to include units is a common source of lost marks.
Energy Explanations: When asked why the temperature doesn't rise during melting, explicitly state that energy is 'used to break bonds' or 'weaken intermolecular forces' rather than 'increasing kinetic energy'.
Safety Protocols: Always mention safety measures such as wearing goggles to protect from splashes and using a heatproof mat to prevent surface damage from the Bunsen burner.
Parallax Error: Reading the thermometer at an angle rather than at eye level leads to inaccurate temperature data. Always state that the thermometer must be read vertically at eye level to avoid this systematic error.
Uneven Heating: If the ice is not stirred or the heat is applied too quickly, the temperature readings may not represent the average state of the beaker. Slow heating and ensuring the thermometer is surrounded by ice are crucial.
Temperature Start: Beginning the experiment when the ice is already at might miss the initial solid heating phase. Ideally, start with ice taken directly from a freezer at a sub-zero temperature.
Beaker Contact: If the thermometer bulb touches the bottom of the beaker, it measures the temperature of the glass being heated directly rather than the ice. Ensure the bulb is fully submerged in the ice cubes only.
Specific Latent Heat: This practical provides the basis for calculating the specific latent heat of fusion, which is the amount of energy required to change of a substance from solid to liquid without changing its temperature.
Internal Energy Systems: The experiment links directly to the concept of internal energy, showing that the sum of kinetic and potential energies in a system can increase even when one component (kinetic) stays constant.
Purity of Substances: A flat plateau at indicates the water is pure; impurities would cause the ice to melt over a range of temperatures or at a lower fixed point.