The law of conservation of mass explains why the total mass of a chemical reaction remains constant when all matter is accounted for. This Grade 8 Science worksheet explores conservation of mass in chemical reactions through closed-system measurements, escaping gases, atom rearrangement, and the balanced chemical equation for photosynthesis.

Students investigate an important distinction: a measured decrease in the mass of materials does not necessarily mean matter has disappeared. When gas escapes from an open container, the remaining materials may weigh less because some matter is no longer included in the measurement. In a closed system, matter cannot enter or leave, so the total mass should remain constant when measurements are accurate.

From reaction evidence to conserved atoms

The reading passage introduces chemical reactions as processes in which atoms rearrange as bonds break and new bonds form. Students connect this idea to a sealed reaction bag weighed before and after a reaction, considering why bubbles or colour changes do not automatically indicate a change in total mass.

The worksheet also uses photosynthesis to show how chemical equations represent conserved atoms. Students identify carbon dioxide and water as reactants and glucose and oxygen as products, then consider how a balanced equation represents equal numbers of each type of atom on both sides. This connects observable mass measurements with the particle-level explanation of chemical change.

Questions and activities

• Multiple-choice questions apply conservation of mass to sealed containers, escaping gas, reaction measurements, balanced equations, and photosynthesis.
• True-or-false statements check misconceptions about closed systems, atom conservation, and changes in measured mass.
• Short-answer questions ask students to explain why sealed containers help test conservation, interpret mass loss from an open system, and connect photosynthesis with conserved atoms.
• A labeled diagram activity supports visual work with the lesson concepts.
• A categorization activity gives students an opportunity to organize relevant information.
• A short-note task provides an additional way to record key learning.

Files and teacher support

• The printable student worksheet includes Student Notes lines at the bottom for recording important ideas.
• A separate answer key PDF supports checking responses and reviewing student work.
• The Answer Key PDF also contains a Teacher Guide with Learning Objectives, Teaching Tips, Discussion Questions, and Common Mistakes.

Ways to use this resource

Use this chemical reactions worksheet after introducing atoms and chemical change, as guided practice during a lesson on matter, or as independent review before an assessment. The questions help reveal whether students can distinguish total mass from the mass of only the remaining contents in an open container. Written explanations also give teachers a way to check whether students can support their reasoning with evidence rather than simply repeat the conservation rule.

Designed for Grade 8 Science in a Texas Science 8 lesson context, the resource names TEKS as its framework. It brings together reaction evidence, system boundaries, atom rearrangement, and photosynthesis in one focused practice resource, with a separate answer key and teacher guidance for convenient lesson preparation and review.