Elements and compounds
Grade 8 Science Worksheet - Challenge Me
Circle the best answer, or write your answer on the line.
Water holds hydrogen and oxygen in a fixed ratio by mass, about 1 g to 8 g. How many grams of water form from 2 g of hydrogen and plenty of oxygen, Ari?
Water is about 1 g of hydrogen for every 8 g of oxygen. Esme's sample of water was made from 5 g of hydrogen. What is the sample's mass in grams?
Read this first
Use this
(particle model).
Which statement does the particle model support about the atoms, Jun?
- Atoms were only rearranged: 8 H and 4 O before and after
- Four new O atoms formed to make the water
- The H and O are still separate in the after box, like a mixture
- The hydrogen was used up, so 8 atoms are gone
How do the kinds of atoms and the kinds of substances change from before to after, Kira?
Use the picture in question 3.
- Two kinds of atoms before and three after the reaction
- Two kinds of atoms before and one after, since one substance remains
- Nothing changes: it is still two substances after
- Two kinds of atoms before and after; elements became a compound
Luz says the after box is a mixture of hydrogen and oxygen. Which evidence from the model answers that best?
Use the picture in question 3.
- A mixture always has exactly two substances in it
- Water is common, so it cannot be a mixture of gases
- Every O is joined to two H, forming one kind of molecule
- The after box has fewer molecules than before, so atoms were lost
Test results (example data)
sample separated by a magnet split by electricity X yes (two solids) no Y no yes (two gases) Z no no Which sample is most likely a compound, Milo?
- Z, since it resisted both tests, so it must be joined tightly
- X, since a magnet pulled two different solids out of it
- Y: a magnet cannot separate it, but electricity splits it
- Both Y and Z, since neither was separated by a magnet
Noah says sample Z must be an element, since it did not split in either test. How strong is that evidence?
- Wrong: Z must be a mixture, since nothing came out
- Strong: Z passed the magnet test, which shows it is an element
- Strong: any pure substance that does not split is an element
- Weak: Z may be a compound that these two tests cannot split
Two compounds (example data)
compound atoms in one molecule what it does water (H₂O) 2 H and 1 O dissolves salt and is safe to drink hydrogen peroxide (H₂O₂) 2 H and 2 O used to bleach hair and clean cuts Ren says H₂O₂ is just water, since both hold only hydrogen and oxygen. What does the table show?
- The same two elements make the same compound, in any ratio
- A different ratio of the same elements makes a different compound
- H₂O₂ is a mixture of water with some extra oxygen mixed in
- Adding oxygen to water just makes more of the same water
Heating sugar (example data)
observation result formula of sugar C₁₂H₂₂O₁₁ before heating white crystals that dissolve in water after strong heating black solid (carbon) and water vapor Tala says sugar is a mixture of carbon and water, since both come out when it is heated. Which reply is best supported?
- Disagree: sugar is a compound; heating rearranges its joined atoms
- Agree: sugar looks like water and charcoal stirred together
- Agree: whatever comes out of it must have been in it as a mixture
- Disagree: sugar is an element, since it has a formula of its own
Zee tests a pure white solid. Which result is the strongest evidence that it is a compound?
- It is white, hard and brittle like chalk
- It breaks into two different new substances
- It melts when heated, then hardens again
- It dissolves in water and the water turns clear