Exercises: Chemical change and conservation of mass

Classify each change as physical or chemical.

1. a) Ice melting on a window in Sligo

1. b) Wood burning in a fire

1. c) Iron rusting

1. d) Sugar dissolving in tea

State the law of conservation of mass in your own words.

In a chemical reaction, 12 g of magnesium reacts completely with 8 g of oxygen to form magnesium oxide.

1. a) What is the total mass of the reactants?

1. b) What mass of magnesium oxide is formed?

A student places baking soda and vinegar in a flask and puts a balloon over the neck. The total mass before mixing is 150.0 g. After the reaction, the balloon is inflated with gas and the mass reads 150.0 g.

1. a) Name the gas most likely to have inflated the balloon.

1. b) Give two observations that show a chemical change took place.

1. c) Explain how the reading is consistent with conservation of mass.

Calcium carbonate (5.0 g) reacts with an excess of acid in an open beaker on a balance. The mass reading falls by 2.2 g during the reaction.

1. a) Explain why the mass reading decreases.

1. b) What mass of gas was released?

1. c) How could the experiment be changed so that the reading stays constant?

A 2.4 g strip of magnesium is burned in air. The white ash that remains has a mass of 4.0 g.

1. a) Calculate the mass of oxygen that combined with the magnesium.

1. b) Explain why the ash is heavier than the original magnesium.

1. c) A student says that mass was created. Respond to this claim.

A student heats 10.0 g of steel wool in air and finds that its mass increases to 12.9 g. In a second trial, she burns 10.0 g of a wooden splint and the ash has a mass of 0.5 g.

1. a) Explain both results using conservation of mass.

1. b) Calculate the mass of oxygen gained by the steel wool.

1. c) Describe how the second experiment could be carried out so that the total mass could be shown to be conserved.

Silver nitrate solution (50.0 g) is mixed with sodium chloride solution (40.0 g) in a sealed flask. A white solid forms and the flask is weighed again.

1. a) Predict the mass reading after mixing and justify your prediction.

1. b) Is the formation of the white solid a physical or chemical change? Give two pieces of evidence.

1. c) The solid is filtered off and has a mass of 14.3 g. What can you say about the mass of the remaining liquid?

Compare melting ice with burning a candle. For each change:

1. a) State whether it is physical or chemical, and whether it is easily reversed.

1. b) Explain whether mass is conserved in each change.

1. c) A candle of mass 40 g has a mass of 34 g after burning for an hour in an open room. Explain where the missing 6 g has gone and why this does not break the law of conservation of mass.