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.
Summary
The following video provides a summary of the concepts covered in this chapter.
Physical and chemical change
Matter does not stay the same. It may undergo physical or chemical changes.
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.
During a physical change, the
arrangement of particles may change but the mass, number of atoms and number of molecules will stay the same.
Physical changes involve small changes in
energy and are easily reversible.
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)
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.
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.
Decomposition reactions are usually
endothermic and synthesis reactions are usually
exothermic .
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.
The
conservation of energy principle states that energy cannot be created or destroyed, it can only change from one form to another.
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.
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
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
For each of the following reactions, say whether it is an example of a synthesis or decomposition reaction:
(NH
4 )
2 CO
3
2NH
3 + CO
2 + H
2 O
4Fe + 3O
2
2Fe
2 O
3
N
2 (g) + 3H
2 (g)
2NH
3
CaCO
3 (s)
CaO + CO
2
For the following equation:
CaCO
3
CO
2 + CaO
show that the 'law of conservation of mass' applies.
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
