Chemical and Physical Changes — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Mia wants to test if heating sugar causes a chemical change. Which result would best prove a chemical change happened?
A) The sugar melts into a clear liquid that hardens again
B) The sugar turns dark brown and gives off a burnt smell
C) The sugar feels warm to the touch after heating
D) The sugar dissolves quickly in water afterward
Brown color and burnt smell indicate new substances formed, which is a chemical change. Simple melting alone would only be a physical change.
2. To test whether mixing two substances is a chemical change, which observation would be the strongest evidence?
A) The mixture takes up less space than before
B) The mixture feels colder when stirred
C) Bubbles of a new gas form steadily as they mix
D) The mixture appears slightly cloudy at first
A steady stream of new gas bubbles is strong evidence of a chemical change because gases are new substances. Cloudiness alone could just be a physical mixture.
3. Which experiment best tests whether dissolving salt in water is a physical or chemical change?
A) Taste the water before and after adding the salt
B) Evaporate the water and check if salt is recovered
C) Measure how cloudy the salt water becomes over time
D) Stir the mixture longer to dissolve all the salt
Evaporating the water lets you see if salt is recovered unchanged. If the original salt comes back, the change is physical; if not, it would be chemical.
4. Leo wants fair evidence about whether a reaction conserves mass. What is the most important step in his experiment?
A) Use a stopwatch to time how long the reaction takes
B) Use the largest possible amount of each reactant
C) Seal the container so no gas products can escape
D) Stir the mixture quickly with a metal spoon
Sealing the container keeps all reactants and products inside, so the balance shows the same total mass before and after, proving conservation of mass.
Part B: Fill in the Blank
Write the correct answer on each line.
1. In a fair experiment, you should change only one variable at a time so you know what caused the result.
Changing only one variable allows scientists to identify cause and effect. If multiple variables change, you cannot tell which caused the result.
2. To prove a chemical change happened, look for clues like new color, gas bubbles, smell, or new substance formed.
All chemical changes create at least one new substance with new properties. These signs help scientists distinguish chemical changes from physical ones.
3. To check conservation of mass, weigh the reactants before mixing and weigh the products after they react.
Comparing total masses of reactants and products tests conservation of mass. In a sealed system, both totals will be exactly equal.
4. Before doing an experiment, scientists make a hypothesis, which is a testable prediction about what will happen.
A hypothesis is a testable prediction. It guides the experiment design and is supported or refuted by the data collected during the test.
5. When testing reactions, using a sealed bag prevents new gas products from escaping into the air.
Sealed bags trap gas products so their mass is included in measurements. Without sealing, escaping gases would make the reaction appear to lose mass.