Is mass lost in a reaction?
Weigh a flask of vinegar with baking soda before and after they react, once with the flask open and once closed with a balloon. Find out whether mass is really lost.
Core concept
In a chemical reaction atoms are only rearranged, never made or destroyed, so the total mass stays the same (the law of conservation of mass); mass only seems lost when a gas escapes from an open container
Topic: Chemical reaction
When vinegar (a solution: A mixture where one substance has dissolved completely in a liquid, like salt water. All key words of acetic acid: A substance that tastes sour, like lemon juice or vinegar. It turns blue litmus red. All key words) meets baking soda (sodium bicarbonate), it fizzes: a chemical reaction makes sodium acetate, water and carbon dioxide: A gas we breathe out. Plants take it in to make their food. All key words gas. NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. Does the mass: How much matter something has, measured in grams or kilograms. It is the same everywhere. All key words change? In 1789 the French chemist Antoine Lavoisier weighed reactions in sealed vessels and found that the total mass before and after was always the same. Atoms are not made or destroyed in a reaction; they are only rearranged into new substances. This is the law of conservation of mass, and it is why a chemical equation must be balanced. In this lab the flask holds 100 g of vinegar (the flask itself is 120 g), and the baking soda is put either in a balloon over the neck or in a dish on the pan.
What you will learn
- State the law of conservation of mass.
- Explain why mass seems to be lost when a reaction gives off a gas in an open container.
- Show that mass is conserved in a closed container.
- Connect the law to balanced chemical equations.
Before you start
- Know the difference between a physical and a chemical change.
Materials
- Vinegar, baking soda, a balloon
Equipment
- A conical flask and a digital balance
Procedure
- With the balloon, record the mass before mixing and after mixing.
- With the flask open, record the mass before mixing and after mixing.
- With the flask open, add 6 g of soda and record after mixing.
How this simulation is simplified
- The vinegar has more than enough acid, so all the soda reacts: 84 g of soda gives 44 g of carbon dioxide.
- The balloon weighs 2 g; the upthrust of the air on the blown-up balloon is left out.