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).
How do the kinds of atoms and the kinds of substances change from before to after, Kira?
- Two kinds of atoms before and after; elements became a compound
- Nothing changes: it is still two substances after
- Two kinds of atoms before and three after the reaction
- Two kinds of atoms before and one after, since one substance remains
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.
- Every O is joined to two H, forming one kind of molecule
- Water is common, so it cannot be a mixture of gases
- A mixture always has exactly two substances in it
- 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?
- Both Y and Z, since neither was separated by a magnet
- X, since a magnet pulled two different solids out of it
- Z, since it resisted both tests, so it must be joined tightly
- Y: a magnet cannot separate it, but electricity splits it
Noah says sample Z must be an element, since it did not split in either test. How strong is that evidence?
- Strong: any pure substance that does not split is an element
- Weak: Z may be a compound that these two tests cannot split
- Wrong: Z must be a mixture, since nothing came out
- Strong: Z passed the magnet test, which shows it is an element
Two salt samples (example data)
sample source sodium (% by mass) chlorine (% by mass) A sea water 39.3 60.7 B a mine 39.3 60.7 What do the two rows show about sodium chloride, Pia?
- Salt from different places has different amounts of sodium
- Each sample is a mixture, since percentages are shown for two parts
- A compound has a fixed ratio of elements, wherever it comes from
- Chlorine is the larger part, so the salt is mostly chlorine gas
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?
- H₂O₂ is a mixture of water with some extra oxygen mixed in
- Adding oxygen to water just makes more of the same water
- The same two elements make the same compound, in any ratio
- A different ratio of the same elements makes a different compound
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?
- Agree: sugar looks like water and charcoal stirred together
- Disagree: sugar is a compound; heating rearranges its joined atoms
- Disagree: sugar is an element, since it has a formula of its own
- Agree: whatever comes out of it must have been in it as a mixture
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 dissolves in water and the water turns clear
- It melts when heated, then hardens again
- It breaks into two different new substances