8.5E Evidence of a Chemical Reaction/Law of Conservation of Mass

A student mixes two substances together in a flask. Which observation could indicate that a chemical reaction is taking place in the flask?

• The flask heats up

• Bubbles of gas from in the flask

• The contents change color

• All of these

Which chemical equation correctly demonstrates the law of conservation of mass?

• CH₄ + 2O₂ → CO₂ + H₂O

• H₂O₂ → 2H₂O + O₂

• 2Na + Cl₂ → 2NaCl

• 2NH₄OH → 2NH₃ + H₂O

Which of these activities with eggs involves a chemical reaction?

• Frying a raw egg

• Scrambling eggs and milk together

• Drawing on the shell of a hard-boiled egg

• Breaking a raw egg

The photo show a solid as it drops into water and starts producing bubbles. Which statement correctly explains the changes in the solid?

• A chemical reaction is happening because the solid is dissolving

• A chemical reaction is happening because bubbles are forming

• Since bubbles contain only gas, a chemical reaction is not happening

• Since no new solid is present, a chemical reaction is not happening

A scientist carried out three experiments and recorded observations in a table. During each experiment, the scientist mixed together two substances. In which experiment did a chemical reaction take place?

• Experiment 1

• Experiment 2

• Experiment 3

• All Experiments

All of the following represent chemical reactions except

• nails rusting

• paper burning

• sugar dissolving

• leaves decaying

A student used tongs to hold a strip of gray metal ribbon over a small flame. The burning metal ribbon produced a bright light. When the reaction ended, small pieces of white ash remained. The white ash would not burn. Which observation does NOT necessarily show that a chemical reaction took place?

• The production of a bright light

• The change from one piece of metal to many small pieces

• The formation of the new substance in the ash

• The change in chemical properties of the solid after it burned

Nitrogen gas (N₂) reacts with hydrogen gas (H₂) to form ammonia (NH₃). Which is a balanced equation that correctly represents this reaction?

• N₂ + H₂ → NH₃

• N₂ + H₂ → 2NH₃

• N₂ + 2H₂ → 2NH₂

• N₂ + 3H₂ → 2NH₃

Sodium chloride (NaCl) reacts with sulfuric acid (H₂SO₄). The products are sodium sulfate (Na₂SO₄) and hydrochloric acid (HCl). Four students wrote equations to represent this reaction. Which student wrote an equation that demonstrates the law of conservation of mass?

• 1

• 2

• 3

• 4