Changing Mass
Chemistry: AQA GCSE Higher, topic 3.1.5
Changing Mass
In chemical reactions, substances may seem to lose or gain mass. Normally, 1 of the reactants (or products) is a gas, and gas can escape into the surrounding environment.
Burning of wood
- When wood is burnt, it releases carbon dioxide and water (and often other gases).
- These gases can escape into the atmosphere. These gases can't be weighed easily at the end of a reaction.
Corrosion of metals
- The mass of the oxide produced will be larger than the mass of the starting metals because oxygen atoms from the atmosphere have been added:
- E.g. 2Mg + O2 → 2MgO
- The change in mass will equal the mass of the oxygen atoms which have been added.
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Zinc + hydrochloric acid
- During this reaction, you would expect the mass of the flask and reactants (zinc and hydrochloric acid) to decrease.
- This is because the hydrogen gas produced will escape from the flask.
Test yourself on Changing Mass
When potassium reacts with water to create potassium hydroxide and hydrogen gas, the mass of the products is less than the mass of the reactants. Why?
Examples of reactions in which mass changes
The solid product of magnesium corrosion weighs more than the metal, but the flask and its contents weigh less after zinc reacts with hydrochloric acid. What explains this difference?
A student reacts zinc with hydrochloric acid in a sealed flask, so no gas can escape, instead of using an open flask. How would the total mass of the flask and its contents at the end compare with an open flask?
A student burns a piece of wood inside a sealed container, so no gases can escape, instead of burning it in the open air. How would the total mass of the container and its contents at the end compare with burning in the open air?