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This does not mean that hydrogen and oxygen always combine in a 2:1 ratio to form H 2 O. Multiple proportions are possible. For example, hydrogen and oxygen may combine in different proportions to form H 2 O 2 rather than H 2 O. In H 2 O 2 , the H:O ratio is 1:1 and the mass ratio of hydrogen to oxygen is 1:16. This will be the same for any molecule of hydrogen peroxide.

Volume relationships in gases

In a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure. This relationship is also known as Gay-Lussac's Law .

For example, in the reaction between hydrogen and oxygen to produce water, two volumes of H 2 react with 1 volume of O 2 to produce 2 volumes of H 2 O.

2H 2 + O 2 2H 2 O

In the reaction to produce ammonia, one volume of nitrogen gas reacts with three volumes of hydrogen gas to produce two volumes of ammonia gas.

N 2 + 3H 2 2NH 3

This relationship will also be true for all other chemical reactions.


The following video provides a summary of the concepts covered in this chapter.

Physical and chemical change

  1. Matter does not stay the same. It may undergo physical or chemical changes.
  2. A physical change means that the form of matter may change, but not its identity. For example, when water evaporates, the energy and the arrangement of water molecules will change, but not the structure of the water molecules themselves.
  3. During a physical change, the arrangement of particles may change but the mass, number of atoms and number of molecules will stay the same.
  4. Physical changes involve small changes in energy and are easily reversible.
  5. A chemical change occurs when one or more substances change into other materials. A chemical reaction involves the formation of new substances with different properties . For example, magnesium and oxygen react to form magnesium oxide (MgO)
  6. A chemical change may involve a decomposition or synthesis reaction. During chemical change, the mass and number of atoms is conserved, but the number of molecules is not always the same.
  7. Chemical reactions involve larger changes in energy. During a reaction, energy is needed to break bonds in the reactants and energy is released when new products form. If the energy released is greater than the energy absorbed, then the reaction is exothermic. If the energy released is less than the energy absorbed, then the reaction is endothermic. Chemical reactions are not easily reversible.
  8. Decomposition reactions are usually endothermic and synthesis reactions are usually exothermic .
  9. The law of conservation of mass states that the total mass of all the substances taking part in a chemical reaction is conserved and the number of atoms of each element in the reaction does not change when a new product is formed.
  10. The conservation of energy principle states that energy cannot be created or destroyed, it can only change from one form to another.
  11. The law of constant composition states that in any particular compound, all samples of that compound will be made up of the same elements in the same proportion or ratio.
  12. Gay-Lussac's Law states that in a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure.

Summary exercise

  1. Complete the following table by saying whether each of the descriptions is an example of a physical or chemical change:
    Description Physical or chemical
    hot and cold water mix together
    milk turns sour
    a car starts to rust
    food digests in the stomach
    alcohol disappears when it is placed on your skin
    warming food in a microwave
    separating sand and gravel
    fireworks exploding
  2. For each of the following reactions, say whether it is an example of a synthesis or decomposition reaction:
    1. (NH 4 ) 2 CO 3 2NH 3 + CO 2 + H 2 O
    2. 4Fe + 3O 2 2Fe 2 O 3
    3. N 2 (g) + 3H 2 (g) 2NH 3
    4. CaCO 3 (s) CaO + CO 2
  3. For the following equation: CaCO 3 CO 2 + CaO show that the 'law of conservation of mass' applies.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
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Source:  OpenStax, Siyavula textbooks: grade 10 physical science. OpenStax CNX. Aug 29, 2011 Download for free at http://cnx.org/content/col11245/1.3
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