Reading structure from properties

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

Both a metal and an ionic compound conduct electricity. Which test separates them?
- weigh the sample and compare it with water
- see which one dissolves in water first
- check which one is shinier to look at
- test the solid before melting anything
Only a metal conducts electricity while it is still solid.
Circle one: True False
A sample conducts electricity as a solid and can be drawn out into a wire. How many of the four structure types fit?
Answer: ______________
A sample melts at -7 degrees. How many of the four structure types does that number rule out?
Answer: ______________
Sodium chloride melts at 801 degrees and magnesium oxide at 2852. How many degrees higher is magnesium oxide?
Answer: ______________
Mercury is a liquid at room temperature and conducts electricity. What structure is it?
- simple molecular, since only small molecules melt this low
- metallic, since molecules never conduct in any state
- ionic, since it is liquid and liquids conduct
- giant covalent, since mercury is a single element
A sample conducts as a solid. How many of the four structure types does that rule out completely?
Answer: ______________
Told that a sample is simple molecular, you can predict it will have a low melting point.
Circle one: True False
You are told a sample is ionic before any test is run. Which card would you predict?
- low melting point, conducts as a solid, insoluble
- very high melting point, never conducts, insoluble
- melts in the hundreds, conducts as a solid, dissolves easily
- melts high, conducts only once it is molten, dissolves
Setting graphite aside, how many of the four structure types conduct electricity when molten?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading structure from properties W1-mt_yCA66GjWRw-s1

test the solid before melting anything · A metal conducts as it is, because its electrons are already free. An ionic compound conducts only after melting or dissolving frees its ions.
False · Graphite does it too, using the spare electron on each carbon atom. That is why the solid test narrows the answer to two rather than one.
1 · Conducting as a solid leaves a metal or graphite. Being drawn into a wire rules graphite out, since only a weak pull holds its sheets together and the piece crumbles. Only metallic fits.
2 · Ionic lattices and giant covalent networks both need hundreds or thousands of degrees, so both are out. It does not rule out a metal, since mercury melts near -39 and is still a metal.
2051 · 2852 - 801 = 2051. Both are ionic, so the gap comes from the size of the charges rather than from a different structure type.
metallic, since molecules never conduct in any state · The melting point suggests molecular, but no molecular substance conducts in any state. Conduction is the harder evidence, so mercury is a metal with an unusually low melting point.
2 · Simple molecular and ionic are both ruled out, since neither has a free charge carrier in the solid. Metallic fits, and giant covalent survives only through graphite.
True · The argument runs both ways. Simple molecular means whole molecules only have to be pulled apart from each other, and that pull is weak.
melts high, conducts only once it is molten, dissolves · A lattice of ions needs a lot of heat, holds its ions still until it melts, and often comes apart in water. That is the ionic card, predicted rather than measured.
2 · Metals conduct in every state and ionic compounds conduct once melted, so two. Simple molecular has nothing charged, and a giant covalent network has nothing free to move.