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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

how can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit 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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