Chemical and Physical Changes — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Fireworks exploding in the sky are a physical change.
Corrected: Fireworks exploding in the sky are a chemical change.
Fireworks involve combustion, a chemical change that produces light, heat, sound, and brand new gas substances that did not exist before the explosion.
2. Fix the sentence:
Mixing vinegar and baking soda fizz because of a physical change.
Corrected: Mixing vinegar and baking soda fizzes because of a chemical change.
The fizz is carbon dioxide gas, a new substance, so this is a chemical change. The singular gerund subject requires the verb 'fizzes'.
3. Fix the sentence:
Plants doing photosynthesis is just moving sunlight around physically.
Corrected: Plants doing photosynthesis make new sugar through a chemical change.
Photosynthesis is a chemical change because plants combine carbon dioxide and water to form glucose and oxygen, two products different from the reactants.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. When food is digested in your stomach, it goes through a chemical change that creates new substances.
Digestion is a chemical change because stomach acid and enzymes turn food into new substances like simple sugars and amino acids.
2. The fizzing bubbles when vinegar meets baking soda are made of carbon dioxide gas.
Vinegar and baking soda react chemically to form carbon dioxide gas, water, and sodium acetate, signaling a chemical change.
3. Plants use sunlight to combine water and carbon dioxide into glucose, a sugar that fuels their growth.
Photosynthesis is a chemical change that produces glucose and oxygen, two new substances different from the original reactants.
4. The combustion of a firework releases gases, light, heat, and sound all at once.
Combustion is a chemical change because burning materials react with oxygen to form new gas products and release energy as light, heat, and sound.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Why is rust forming on an old bicycle a chemical change instead of a physical change?
Sample answer: Rust is a chemical change because iron in the bicycle reacts with oxygen and water in the air to form a new substance called iron oxide. Rust has a different color, texture, and properties from the original iron, and you cannot easily turn it back into shiny metal again.
Rust forms when iron combines with oxygen, creating iron oxide. The new substance has different properties from iron, which is the key sign of a chemical change.
2. Explain why baking a cake is a chemical change, even though it starts with simple ingredients.
Sample answer: Baking a cake is a chemical change because heat causes the flour, eggs, sugar, and baking soda to react and form new substances. The fluffy cake has different taste, texture, and smell from the runny batter, and you cannot turn the cake back into batter, showing new substances formed.
Baking causes proteins to set and gases to form, creating new substances with new properties. Because the change is not reversible, it is chemical, not physical.