# 4.1 Energy changes and conservation of matter  (Page 3/3)

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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 $\to$ 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 $\to$ 2NH 3

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

## Summary

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

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 $\to$ 2NH 3 + CO 2 + H 2 O
2. 4Fe + 3O 2 $\to$ 2Fe 2 O 3
3. N 2 (g) + 3H 2 (g) $\to$ 2NH 3
4. CaCO 3 (s) $\to$ CaO + CO 2
3. For the following equation: CaCO 3 $\to$ CO 2 + CaO show that the 'law of conservation of mass' applies.

where we get a research paper on Nano chemistry....?
what are the products of Nano chemistry?
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
I only see partial conversation and what's the question here!
what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
how did you get the value of 2000N.What calculations are needed to arrive at it
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