Question: What happens when a substance melts?
Part 1
ONLY ANSWERS WRITTEN IN BLUE COLOR WILL BE READ
1. Observe. Look at the molecular view of water.
A. What do you see? Complete the blanks using words from the vocabulary.
The atoms in each water molecule are held together by covalent bonds . The molecules are held together by
intermolecular forces. This kind of substance is called a molecular solid
B. In the solid state, how would you describe the position of the molecules?
The molecules are stiff/still and stay in its place because there's no space for them to go around like liquid or
gas.
C. Using the Gizmo, determine the melting point and boiling point of water. To find the approximate melting
and boiling points you can heat the sample quickly. Then, run another trial at a slower speed to find the exact
temperatures.
Melting point: 0’C-3’C
Boiling point: 100’C
D. Can you find the sublimation point? Explain
Yes because you just need to go at a slower speed and read the temperature where it's below the melting
point, where it’ll go from ice to gas.
2. Experiment
Record the melting point and boiling point of water in the table below. Then, use the Gizmo to find the
melting and boiling points of hydrogen sulfide and ethanol. In each case, record the range of temperature for
each transition.
Chemical Melting point (°C) Boiling point (°C) Other transition point (°C)
Water 0-3 degrees 100 degrees 50 degrees
Hydrogen sulfide -77 degrees 50 degrees 35 degrees
Ethanol -111 degrees 80 degrees -55 degrees
Carbon dioxide -56 degrees -78 degrees -72 degrees
Now try carbon dioxide. Do you notice anything unusual?
3. Observe (Fill in the blanks). At normal atmospheric pressure, carbon dioxide undergoes sublimation
where the solid transforms directly to a gas. Using the Gizmo, determine the sublimation point of carbon
dioxide and record it in the table under “Other transition point.”
4. Interpret: The melting point and boiling point can be used to measure the strength of intermolecular
forces holding the molecules together. Based on the melting and boiling points, which substance do you think
has the strongest intermolecular forces? Water
The weakest? Ethanol
Explain your conclusions: Water is the strongest because it stays in its phases for the longest duration of time
and Ethanol is the weakest of all because it changes its phase the quickest.
5. Extend your thinking: Ethanol is an alcohol. The boiling points and molecular weights of other alcohols are
in the table (you can fill in ethanol’s boiling point from the last page).
Chemical Molecular weight (u) Boiling point (°C)
Methanol 32 65
Ethanol 46 78
1-Propanol 60 97
1-Butanol 74 117
A. Is there a pattern in the data? Explain. When there's an increase of 14 units in molecular weight, the
boiling point also increases by 37 degrees celsius.
B. Make a prediction about the boiling point of 1-pentanol, an alcohol with a molecular weight of 88 u.
Explain. 125 degrees celsius because when added 37 to 88, its 125 degrees celsius.
6. Analyze. Based on the boiling point data in your data on the previous chart, does the pattern in the alcohol
boiling points always apply? Is molecular weight an important contributor to intermolecular forces? Why or
why not?
The pattern in alcohol boiling point apply and molecular weight is an important contributor because you can
know the boiling point because of it.
