7th grade science test questions on elements and compounds.

📚 Quick Study Guide

• ⚛️ Elements: Pure substances that cannot be broken down into simpler substances by chemical means. Examples include Hydrogen (H), Oxygen (O), and Gold (Au).
• 🤝 Compounds: Substances formed when two or more elements are chemically bonded together in a fixed ratio. Examples include Water ($H_2O$) and Carbon Dioxide ($CO_2$).
• ➕ Chemical Formula: A way of representing compounds using symbols and numbers. For example, $H_2O$ indicates two hydrogen atoms and one oxygen atom.
• ⚖️ Law of Definite Proportions: A given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation.
• ➗ Mixtures vs. Compounds: Mixtures are combinations of substances that are physically combined, not chemically bonded (e.g., salad, air). Compounds are chemically bonded.
• 🔥 Chemical Reactions: Processes that involve the rearrangement of atoms and molecules to form new substances.
• 🌡️ States of Matter: Elements and compounds can exist as solids, liquids, or gases, depending on temperature and pressure.

🧪 Practice Quiz

1. Which of the following is an element?
   1. Water ($H_2O$)
   2. Carbon Dioxide ($CO_2$)
   3. Gold (Au)
   4. Salt (NaCl)
2. What is a compound?
   1. A pure substance that cannot be broken down
   2. A substance made of two or more elements chemically bonded
   3. A mixture of different elements
   4. A type of energy
3. Which chemical formula represents water?
   1. $CO_2$
   2. $NaCl$
   3. $H_2O$
   4. $O_2$
4. What is the Law of Definite Proportions?
   1. Elements combine in any ratio to form compounds.
   2. A compound always contains its elements in a fixed ratio.
   3. Elements can only exist as solids.
   4. Compounds can be broken down into mixtures.
5. Which of the following is a mixture?
   1. Water
   2. Salt
   3. Air
   4. Sugar
6. What happens during a chemical reaction?
   1. Atoms are physically mixed.
   2. Atoms are rearranged to form new substances.
   3. The state of matter remains the same.
   4. No new substances are formed.
7. Which state of matter does oxygen primarily exist in at room temperature?
   1. Solid
   2. Liquid
   3. Gas
   4. Plasma