mass heading

Higher and foundation tiers

The conservation of mass in chemical reactions

If you think about what happens to the particles during a chemical reaction you would probably realise that all the particle present in the reactants appear in the products. All that really happens to the particles in a chemical reaction is that they are rearranged as they change from the reactants into products.

Example 1- The reaction of hydrogen with oxygen to make hydrogen oxide (water).

If you study the image below which simply shows hydrogen molecules reacting with oxygen molecules to make water you will see that NO atoms appear or disappear. All the atoms present in the reactants appear in the products - just arranged in a different way! There are 4 hydrogen atoms and 2 oxygen atoms on the reactants side of the equation and the two water molecules on the products side of the equation which contain 4 hydrogen atoms and 2 oxygen atoms - in chemical reactions such as this the number of atoms present in the reactants is always the same as the number of atoms present in the products- it always balances! This is the law of conservation of mass in a nutshell- chemistry is not magic; you cannot make things appear and disappear. The total mass of all the particles in the reactants will be the same as the total mass of all the particles in the products.

conservation of mass using the burning of hydrogen in air as an example.

Example 2- Using relative formula masses to show that mass is conserved in a chemical reaction.

We can show that mass is conserved in chemical reactions by using formula masses. The total mass of all the reactants should be the same as the total mass of all the products. As an example consider the combustion of methane gas to form carbon dioxide and water vapour; word and symbolic equations for this reaction are shown below.

Space filled model and balanced symbolic equation to show the combustion of methane.

By calculating the relative formula masses of all the reactants and products when methane is combusted it is possible to show that no mass is lost of gained during a chemical reaction; nothing appears or disappears; that is mass is conserved during a chemical reaction. The table below shows the total formula mass of all the reactants and products in this reaction and it is obvious that the mass of the reactants and products molecules are the same, that is mass is conserved.

Reactant Relative formula mass (Mr)   Product Relative formula mass (Mr)
methane (CH4) Mr of CH4=16   carbon dioxide (CO2) Mr of CO2=44
oyygen (O2) Mr of O2=32
Mr of O2=32
  Water (H2) Mr of H2O=18
Mr of H2O=18
Total mass of all reactants: 16 + 64= 80   Total mass of all products: 44 + 36= 80

Do gases have mass?

Gases have mass; some people mistakenly believe that gases have no mass. Carbon dioxide (CO2) has a Mr of 44, this means that 1 mole of carbon dioxide gas has a mass of 44g. It's a heavy gas; it is used in fire extinguishers because it is so heavy it surrounds the fire and prevents the air from getting to it.

Gases have mass despite what many student think!

Gase have mass, helium balloons float because they are lighter than air

If you assume that the air is 80% nitrogen (N2) and 20% oxygen (O2). The Ar of nitrogen is 28 and the Ar of oxygen is 32 then the average mass of the gases in the air is about 29. Carbon dioxide gas (CO2) has an Mr of 44; so it's heavier than air. That is why balloons filled with CO2 sink. A balloon filled with hydrogen gas rises rapidly because hydrogen gas; formula H2 has a Mr of 2. So the mass of 1 mole of hydrogen gas is 2g. The gases in the air as mentioned have an average mass of about 29. Balloons filled with helium (Ar=4) will obviously rise very rapidly in air, but not as rapidly as hydrogen balloons.

Law of conservation of mass

Consider the two reactions shown in the image below. In the first example, the contents of one beaker are poured into the other; the balance scale reads 250g before and after the reaction. This is probably what you expect considering what was said above; you cannot make atoms appear or disappear. Everything you start with you end up with- just a bit more mixed up perhaps!

However in the second example; chalk or calcium carbonate is added to hydrochloric acid and this time the balance reading seems to have gone down. The key to explaining why the mass decreases is in the state symbols for the reactants and products. In the first example all the reactants and products are either solutions - state symbol (aq) or solids -state symbol (s). Nothing enters or leaves the beakers. However in the second example one of the products is a gas- carbon dioxide. Carbon dioxide is a heavy gas and in this reaction it leaves the beaker and enters the atmosphere. This means that one of the products is escaping into the air; so the mass of the products left in the beaker will be less than the mass of the reactants. However the atmosphere will be gaining mass; so once again mass is conserved.

conservation of mass using the reaction of hydrochloric acid and chalk as an example.  Here mass is lost fron one beaker since carbon dioxide gas is released.

Gaining mass- metal oxides

Metals react with oxygen to produce metal oxides.

metal(s) + oxygen(g) metal oxide(s)
No doubt at some time in your science lessons you will have held a piece of magnesium ribbon in a Bunsen flame and cautiously observed the bright flash from the burning magnesium.

magnesium burning in air

Burning magnesium in a crucible

Burning magnesium ribbon in a crucible.

You may have carried out an experiment similar to the one shown opposite, here a 5 gram strip of magnesium ribbon was placed in a metal crucible; which has a mass of 30g. The magnesium ribbon inside the crucible was heated strongly with a Bunsen burner for several minutes, during this heating process the lid of the crucible was occasionally and carefully opened slightly to allow in air but was not opened long enough to allow any fumes to escape, the air which enters the crucible air will allow the magnesium ribbon to burn or combust and after a few minutes the shiny metallic magnesium ribbon should have been replaced by a white ash, this white ash is magnesium oxide.

The crucible was then allowed to cool down and its contents examined. If the crucible and its contents were placed on a balance and weighed would you expect the mass to have gone down; increased or stayed the same? Well according to the equation below; which shows the combustion of magnesium ribbon; oxygen gas has been added to the magnesium ribbon. Oxygen gas like any gas has mass so the mass of the crucible and the magnesium inside it should have increased during this combustion reaction since the oxygen gas from the air is being added to it:

magnesium(s) + oxygen(g) metal oxide(s)

Burning copper metal

copper metal burning, the oxidation of copper to form copper oxide. If a square of copper metal is held with a pair of tongs in a hot Bunsen flame for about 30 seconds the shiny bronze coloured copper turns black. No flames or bright flashes are produced. The copper metal is reacting with the oxygen in the air, it is being oxidised. An equation for this reaction is:

copper(s) + oxygen(g) → copper oxide(s)
This combustion reaction is similar to the one above for magnesium. The original copper square is reacting with oxygen gas from the air. The oxygen gas is reacting with the copper metal to form a new compound; copper oxide. The mass of the original copper square will increase as the oxidation reaction takes place.

Key points

Check your understanding - Questions on the conservation of mass