Water is put into a beaker and heated with a Bunsen burner. The temperature of the water, initially at \(25^{\circ} \mathrm{C}\), is monitored. Explain what happens to the temperature as a function of time. Make a sketch of how the temperature might change with time. (Assume that the Bunsen burner is hot enough to heat the water to its boiling point.)

When you boil water, it undergoes what is known as a phase change. This is a transition from one state of matter to another; in this case, from liquid to gas. As you heat water, the molecules within it move faster, and when they gain enough energy, they can escape the liquid phase and enter the gas phase, a process known as vaporization. During the phase change, an important characteristic is observed: the temperature remains steady despite continuous heat application. This happens because the added energy is used for breaking the intermolecular forces that hold the water molecules together in the liquid phase, rather than increasing the temperature. This plateau in temperature is an identifying feature of a phase transition on a heating curve.

Of course, not just boiling exhibits a phase change; freezing water to ice or melting ice into water are also examples of phase transitions, each with its own unique properties and behaviors. In all cases, the phase change signifies a type of equilibrium where the system is absorbing energy for the physical change, without altering temperature.

A heating curve is a graphical representation of how a substance's temperature changes in response to added heat, displayed as a temperature-time graph. For water, the initial part of the curve shows a steady temperature rise as you heat it from room temperature. This slope represents the increasing kinetic energy of the water molecules as they heat up, with the steepness of the incline depending on the heat's intensity.

Yet, when the curve flattens, that's when you've hit a phase change, like the boiling point of water at standard atmospheric pressure. Here, the curve plateaus because we're no longer increasing temperature, but breaking molecular bonds. Once all the water has transitioned into steam, the curve will resume its climb, now representing the rising temperature of the water vapor. In educational terms, the heating curve effectively demonstrates the correlation between heat application and temperature, as well as illustrating phase transitions.

The boiling point of water is typically referenced at 100 degrees Celsius (212 degrees Fahrenheit) at sea level. This is the temperature at which water undergoes a phase change from liquid to gas. It's important to keep in mind that the boiling point can be influenced by external factors such as atmospheric pressure; for example, water boils at lower temperatures on top of a mountain due to lower air pressure.

At the boiling point, bubbles form within the water, which are pockets of water vapor. As these bubbles rise and escape into the air, they carry heat away with them. Despite continuous heat application during this process, the temperature of the water remains constant until the phase transition from liquid to gas is complete.

A water phase transition is a fascinating phenomenon to observe. Starting with liquid water and applying heat, we reach a phase transition as the water begins to boil. What's happening here is that energy is being used to overcome the forces that keep water in its liquid state. When these forces are broken, the water molecules are free to move into a gaseous state, at which point we say the water has boiled.

Now, in reverse, if we cool steam, it can condense back into liquid water, another phase transition. Similarly, cooling liquid water will transition it into solid ice. Each transition involves a change in energy state, and during each phase change, the temperature stops changing until the transition is complete.

A temperature-time graph allows us to visualize the relationship between how long we heat something and its temperature. In the exercise, such a graph for water would start at room temperature (25°C). As we heat water, its temperature rises, and we see an upwards slope signifying an increase in temperature over time, reflecting the water's increasing kinetic energy. When water hits its boiling point, the curve flattens, indicating a phase change is in progress. The graph remains at 100°C for the boiling period, showing that temperature is constant, but energy is still being absorbed. Once all the water has transformed into steam, the line will once again slope upwards, this time denoting the increased temperature of the steam.

Understanding the temperature-time graph is crucial for grasping how heat affects a substance at various stages, including before, during, and after a phase change. It's an essential learning tool in thermodynamics and helps clarify the sometimes counterintuitive idea that temperature is not always a direct indicator of heat input.