Changing Mass
Chemistry: Edexcel IGCSE, topic 5.1.17
Changing Mass
In chemical reactions, substances may seem to lose or gain mass. Normally, one of the reactants (or products) is a gas. 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.
Calculating mass
- 2Na + Cl2 → 2NaCl.
- If 2.30g of sodium reacted in excess chlorine and 5.85g of NaCl was produced, we can calculate the mass of chlorine reacted.
- 5.85g - 2.30g = 3.55g of chlorine reacted.
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