Chemical change and conservation of mass
On a cold evening you light a turf fire in the grate. The turf glows, gives off heat, and after an hour only a small pile of ash is left. Where did all the turf go? Did it simply disappear? By the end of this page you will be able to answer that question.
Physical and chemical changes
When you melt ice or dissolve sugar in tea, the substance is still the same substance. Ice and liquid water are both water. This is a physical change: no new substance is made, and it can usually be reversed easily (freeze the water again, or evaporate the tea to get the sugar back).
When wood burns or iron rusts, something different happens. The wood turns into ash and gases. The iron turns into a flaky orange-brown solid that is not iron any more. This is a chemical change (a chemical reaction): one or more new substances are made, and it is usually hard or impossible to reverse.
How can you tell that a chemical change has happened? Look for evidence:
| Evidence | Example |
|---|---|
| Gas given off (bubbles, fizzing) | Baking soda and vinegar |
| Colour change | Iron going rusty |
| A new solid forms in a liquid | A white solid appearing when two clear solutions are mixed |
| Heat or light given off, or a temperature change | Burning a candle |
| Hard to reverse | Cooking an egg |
Classify each change as physical or chemical. a) Ice melting on a window in Sligo b) Wood burning in a fire c) Iron rusting d) Sugar dissolving in tea
Classify each change as physical or chemical.
- a) Ice melting on a window in Sligo
- b) Wood burning in a fire
- c) Iron rusting
- d) Sugar dissolving in tea
The law of conservation of mass
Does mass change when a reaction happens? Careful experiments show that it does not.
Why? In the particle model, a chemical reaction rearranges atoms. The atoms in the reactants are the same atoms that end up in the products, just joined together in new ways. No atoms are added or lost, so the mass cannot change.
Four atoms go in and the same four atoms come out, so the mass is unchanged.
State the law of conservation of mass in your own words.
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. a) What is the total mass of the reactants? b) What mass of…
In a chemical reaction, 12 g of magnesium reacts completely with 8 g of oxygen to form magnesium oxide.
- a) What is the total mass of the reactants?
- b) What mass of magnesium oxide is formed?
Why does the mass sometimes seem to change?
The law is always true, but the reading on a balance sometimes changes. The reason is almost always a gas that you did not count.
Mass seems to go down. Calcium carbonate (chalk) reacts with acid in an open beaker and gives off carbon dioxide. The gas escapes into the air, so the balance reading falls. The missing mass is the mass of the gas.
Mass seems to go up. When magnesium burns in air, it combines with oxygen from the air. The white ash (magnesium oxide) is heavier than the magnesium, because the oxygen is now part of it. You did not count the oxygen as a reactant.
Mass stays the same. If the reaction happens in a closed (sealed) container, no gas can get in or out. The balance reading is constant. A balloon fixed over the neck of a flask does the same job: the gas is trapped in the balloon, which is still on the balance.
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…
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.
- a) Name the gas most likely to have inflated the balloon.
- b) Give two observations that show a chemical change took place.
- 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. a) Explain why the 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.
- a) Explain why the mass reading decreases.
- b) What mass of gas was released?
- 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. a) Calculate the mass of oxygen that combined with the magnesium. b) Explain…
A 2.4 g strip of magnesium is burned in air. The white ash that remains has a mass of 4.0 g.
- a) Calculate the mass of oxygen that combined with the magnesium.
- b) Explain why the ash is heavier than the original magnesium.
- c) A student says that mass was created. Respond to this claim.
Putting it together: comparing changes
Now compare a physical change with a chemical one. When ice melts, the particles stay the same, so the mass is unchanged. When a candle burns, the wax reacts with oxygen to make carbon dioxide and water vapour. These gases drift into the room. A candle of 40 g that has a mass of 34 g after an hour has not lost 6 g of matter. The 6 g is the wax that turned into gases. The oxygen it used came from the air, so the gases weigh more than the 6 g of wax. If you collected everything, reactants and products would have equal masses.
| Melting ice | Burning a candle | |
|---|---|---|
| Type of change | Physical | Chemical |
| New substance? | No | Yes (carbon dioxide, water vapour) |
| Easily reversed? | Yes, freeze it | No |
| Mass conserved? | Yes | Yes, if the gases are counted |
When two solutions are mixed in a sealed flask and a solid forms, the mass reading stays the same: nothing could escape. A new solid appearing from clear liquids is evidence of a chemical change. If you then filter off the solid, the mass of the remaining liquid is the total minus the mass of the solid.
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…
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.
- a) Explain both results using conservation of mass.
- b) Calculate the mass of oxygen gained by the steel wool.
- 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. a) Predict the…
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.
- a) Predict the mass reading after mixing and justify your prediction.
- b) Is the formation of the white solid a physical or chemical change? Give two pieces of evidence.
- 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: a) State whether it is physical or chemical, and whether it is easily reversed. b) Explain whether mass is…
Compare melting ice with burning a candle. For each change:
- a) State whether it is physical or chemical, and whether it is easily reversed.
- b) Explain whether mass is conserved in each change.
- 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.
Back to the turf fire: the turf did not disappear. Most of it combined with oxygen from the air and left up the chimney as gases (carbon dioxide and water vapour). The ash is only what was left behind. Counting the gases, no mass was lost.