Temperature During Boiling
In Experiment 3-2 you probably noted that within experimental error the melting point was the same as the freezing point. This transition temperature is a landmark that can be passed in either direction. Such transitions as freezing point and melting point are uniquely located for each substance so are known as characteristic properties. These properties can be used to identify a mysterious substance as having the same characteristic property as a known substance. Chemists use such properties to identify substances. There are many characteristic properties. A few of the more useful will be studied later in this chapter and those which follow.
Water undergoes another transition called boiling from its liquid state to gaseous at a higher temperature.
In this experiment we want to compare the pattern in temperature as water boils with the features of melting.
- Place 50 to 100 cm3 pure water with a thermometer into a container that can be heated without damage.
- Steadily heat the container, water and thermometer. (Caution: Hot water and steam can cause flesh burns.)
- Record the temperature at a set interval, perhaps every 20 or 30 seconds until at least half of the water has boiled to steam.
- Construct a line graph of the temperature with time as independent variable.
- Does the slope of the line for water warming indicate that hot and cold water require about the same amount of energy to warm? (Your graph won't provide precise evidence on this due to uneven heat losses.)
- Is there a boiling point plateau similar to what was found from freezing and melting?
- Compare the melting point from Experiment 3-2 with this graph. How do the two temperatures compare?
- Estimate the time needed to boiling all the water. Compare that time with the time needed to warm the water 10°C warmer.
- Compare the apparent amount of energy (compared to 10°C warming) with similar measures for melting and freezing. If one is bigger, how many times bigger?