Pure Substances
What is a substance called when it has a constant composition and consistent properties throughout?
Pure substance
What are the smallest building blocks that make up matter?
Atoms.
What do we call a substance that occurs naturally in the environment?
A natural substance.
What happens to atoms during a chemical reaction?
Atoms rearrange to form new substances.
Is color a physical property or a chemical property?
Physical property.
A container has only pure water in it. Would you expect the water molecules to have the same composition throughout the container?
Yes. Pure water has the same composition throughout
How many hydrogen atoms are in one molecule of H₂O?
2 hydrogen atoms.
Is a plastic water bottle an example of a natural or synthetic material?
Synthetic.
A substance changes color and produces a gas. What might these observations indicate?
They may indicate a chemical reaction.
What property describes how much mass is contained in a given volume?
Density.
Two samples look identical, but one contains only iron and the other contains iron mixed with sand. Are they both pure substances? Explain.
No. Iron mixed with sand is a mixture; the iron alone is a pure substance
A compound contains two carbon atoms and four hydrogen atoms. Write its chemical formula.
C₂H₄.
Scientists change a naturally occurring substance through a chemical process to create a new material. What type of material might they produce?
A synthetic material.
A student mixes two materials. The container becomes warmer and begins to inflate. What evidence suggests a chemical reaction occurred?
Gas production and a temperature change are evidence.
Two metal samples have different sizes but the same density. What does this tell you about the samples?
They may be the same substance because density does not depend on sample size.
A student says two materials must be the same substance because they have the same color. Why is this not enough evidence?
Different substances can have the same color but different compositions and properties.
Why can two substances made of the same elements have different properties?
They may have different ratios or arrangements of atoms.
Why can a synthetic material be useful to society but also cause environmental or health concerns?
It can provide useful products but may also harm human health or the environment.
Baking soda dissolves in water in one experiment. In another experiment, it is heated and forms different substances. Which process is a physical change and which is a chemical change?
Dissolving is a physical change; heating to form new substances is a chemical change.
A student wants to identify an unknown metal. Which would be more useful: its mass alone or its density and melting point? Explain.
Density and melting point; these properties can help identify a substance.
A scientist tests an unknown substance. Its composition and properties remain consistent throughout the sample. What conclusion can the scientist make, and what evidence supports it?
It is likely a pure substance because it has constant composition and consistent properties.
A student creates a model of a compound. What two things must the student represent correctly to make the model scientifically accurate?
The correct types and numbers of atoms.
Explain how a naturally occurring substance can be used to create a synthetic material. Include the role of a chemical change in your answer.
A natural substance can be chemically changed to produce a new synthetic substance.
A sealed container originally contains two gases. After they interact, a new solid forms. Explain why this is evidence that a chemical reaction occurred.
The formation of a new solid with different properties indicates that a new substance formed.
A student claims that two metal samples are the same substance. Describe how the student could use physical properties and evidence to support the claim.
Compare measurable physical properties such as density and melting point. Similar results support the claim.
Scientists place two unknown substances into a sealed container. After some time, a new substance forms with different properties from the original materials. Write a scientific explanation describing how the observations provide evidence that a chemical reaction occurred. Include a claim, two pieces of evidence, and reasoning.
The claim should state that a chemical reaction occurred. Evidence should include the formation of a new substance and its different properties. The reasoning should explain that a chemical reaction rearranges atoms to form substances with new properties