Why does water expand when it freezes?

Have you ever wondered why you use antifreeze for filling the car radiator?

There are two main reason for that: One is to protect your engine from corrosion, and the second is to avoid water freezing. We need to avoid water freezing because it will cause engine overheating when you start it in the morning, and the most important it is because water expand when it freezes and this will cause cracks in the engine cooling system causing a loot of damages to your engine.

And what the heck! I remember from chemistry classes that liquids and other materials shrink when temperatures fall!

Ya! But this is not the case of water, because water have a type of chemical bound that is called the hydrogen bounding. Hydrogen bonding usually occur between molecules (intermolecular bonding). It is an electrostatic attraction between two polar groups that occurs when a hydrogen atom, covalently bound to a highly electronegative atom such as oxygen (in case of water), experiences the electrostatic field of another highly electronegative oxygen atom nearby.

When water molecules are in the liquid state, hydrogen bonds are continuously being formed and reformed in a disordered manner. But when it come to freezing, water molecules lose energy and do not vibrate or move around as powerfully. This allows more stable hydrogen-bonds to form between molecules, as there is less energy to break the bonds. Hydrogen bonds form a crystalline structure that causes density decreasing because each water molecule is held away from its neighbours at a distance equal to the length of the hydrogen bonds. Thus water expands as it freezes, and as we know ice float on water because it is less denser than the liquid water.

Alkanes and Alkane Isomers

Before beginning a systematic study of the different functional groups, let’s look
first at the simplest family of molecules—the alkanes—to develop some general
ideas that apply to all families. We saw  that the carbon–carbon
single bond in ethane results from σ (head-on) overlap of carbon sp3 hybrid
orbitals. If we imagine joining three, four, five, or even more carbon atoms by
C - C single bonds, we can generate the large family of molecules called alkanes.

Alkanes are often described as saturated hydrocarbons: hydrocarbons

because they contain only carbon and hydrogen; saturated because they
have only C - C and C - H single bonds and thus contain the maximum possible
number of hydrogens per carbon. They have the general formula
CnH2n12, where n is an integer. Alkanes are also occasionally called aliphatic
compounds, a name derived from the Greek aleiphas, meaning “fat.” We’ll
see many animal fats contain long carbon chains similar
to alkanes.

Think about the ways that carbon and hydrogen might combine to make

alkanes. With one carbon and four hydrogens, only one structure is possible:
methane, CH4. Similarly, there is only one combination of two carbons with six
hydrogens (ethane, CH3CH3) and only one combination of three carbons with
eight hydrogens (propane, CH3CH2CH3). When larger numbers of carbons and
hydrogens combine, however, more than one structure is possible. For example,
there are two substances with the formula C4H10: the four carbons can all
be in a row (butane), or they can branch (isobutane). Similarly, there are three
C5H12 molecules, and so on for larger alkanes.

Compounds like butane and pentane, whose carbons are all connected in a
row, are called straight-chain alkanes, or normal alkanes. Compounds like
2-methylpropane (isobutane), 2-methylbutane, and 2,2-dimethylpropane,
whose carbon chains branch, are called branched-chain alkanes.
Compounds like the two C4H10 molecules and the three C5H12 molecules,
which have the same formula but different structures, are called isomers, from
the Greek isos 1 meros, meaning “made of the same parts.” Isomers are compounds
that have the same numbers and kinds of atoms but differ in the way
the atoms are arranged. Compounds like butane and isobutane, whose atoms
are connected differently, are called constitutional isomers. We’ll see shortly
that other kinds of isomers are also possible, even among compounds whose
atoms are connected in the same order.
Constitutional isomerism is not limited to alkanes—it occurs widely throughout
organic chemistry. Constitutional isomers may have different carbon skeletons
(as in isobutane and butane), different functional groups (as in ethanol
and dimethyl ether), or different locations of a functional group along the
chain (as in isopropylamine and propylamine). Regardless of the reason for the
isomerism, constitutional isomers are always different compounds with different
properties but with the same formula.

A given alkane can be drawn in many ways. For example, the straight-chain,
four-carbon alkane called butane can be represented by any of the structures
shown in Figure 3.2. These structures don’t imply any particular threedimensional
geometry for butane; they indicate only the connections among
atoms. In practice, , chemists rarely draw all the bonds
in a molecule and usually refer to butane by the condensed structure,
CH3CH2CH2CH3 or CH3(CH2)2CH3. Still more simply, butane can be represented
as n-C4H10, where n denotes normal (straight-chain) butane.

Straight-chain alkanes are named according to the number of carbon atoms
they contain, as shown in Table 3.3. With the exception of the first four
compounds—methane, ethane, propane, and butane—whose names have historical
roots, the alkanes are named based on Greek numbers. The suffix -ane is
added to the end of each name to indicate that the molecule identified is an
alkane. Thus, pentane is the five-carbon alkane, hexane is the six-carbon alkane,
and so on. We’ll soon see that these alkane names form the basis for naming all
other organic compounds, so at least the first ten should be memorized.

sp3 Hybrid Orbitals and the Structure of Ethane

The same kind of orbital hybridization that accounts for the methane structure
also accounts for the bonding together of carbon atoms into chains and rings
to make possible many millions of organic compounds. Ethane, C2H6, is the
simplest molecule containing a carbon–carbon bond.

We can picture the ethane molecule by imagining that the two carbon
atoms bond to each other by s overlap of an sp3 hybrid orbital from each
(Figure 1.12). The remaining three sp3 hybrid orbitals on each carbon overlap
with the 1s orbitals of three hydrogens to form the six C ] H bonds. The
C ] H bonds in ethane are similar to those in methane, although a bit
weaker—421 kJ/mol (101 kcal/mol) for ethane versus 439 kJ/mol for methane.
The C ] C bond is 154 pm long and has a strength of 377 kJ/mol (90 kcal/mol).
All the bond angles of ethane are near, although not exactly at, the tetrahedral
value of 109.5°.

sp3 Hybrid Orbitals and the Structure of Methane

The bonding in the hydrogen molecule is fairly straightforward, but the situation
is more complicated in organic molecules with tetravalent carbon
atoms. Take methane, CH4, for instance. As we’ve seen, carbon has four
valence electrons (2s2 2p2) and forms four bonds. Because carbon uses two
kinds of orbitals for bonding, 2s and 2p, we might expect methane to have
two kinds of C ] H bonds. In fact, though, all four C ] H bonds in methane are
identical and are spatially oriented toward the corners of a regular tetrahedron
(Figure 1.6). How can we explain this?
An answer was provided in 1931 by Linus Pauling, who showed mathematically
how an s orbital and three p orbitals on an atom can combine, or hybridize,
to form four equivalent atomic orbitals with tetrahedral orientation.
Shown in Figure , these tetrahedrally oriented orbitals are called
sp3 hybrids. Note that the superscript 3 in the name sp3 tells how many of
each type of atomic orbital combine to form the hybrid, not how many electrons
occupy it.

The concept of hybridization explains how carbon forms four equivalent
tetrahedral bonds but not why it does so. The shape of the hybrid orbital
suggests the answer. When an s orbital hybridizes with three p orbitals, the
resultant sp3 hybrid orbitals are unsymmetrical about the nucleus. One of
the two lobes is larger than the other and can therefore overlap more effectively
with an orbital from another atom to form a bond. As a result,
sp3 hybrid orbitals form stronger bonds than do unhybridized s or
p orbitals.

The asymmetry of sp3 orbitals arises because, as noted previously, the two
lobes of a p orbital have different algebraic signs, 1 and 2, in the wave function.
Thus, when a p orbital hybridizes with an s orbital, the positive p lobe adds
to the s orbital but the negative p lobe subtracts from the s orbital. The resultant
hybrid orbital is therefore unsymmetrical about the nucleus and is strongly
oriented in one direction.
When each of the four identical sp3 hybrid orbitals of a carbon atom overlaps
with the 1s orbital of a hydrogen atom, four identical C ] H bonds are formed
and methane results. Each C ] H bond in methane has a strength of 439 kJ/mol
(105 kcal/mol) and a length of 109 pm. Because the four bonds have a specific
geometry, we also can define a property called the bond angle. The angle
formed by each H ] C ] H is 109.5°, the so-called tetrahedral angle. Methane thus
has the structure shown in Figure

What is chemistry ?

Chemistry is a branch of physical science, is the study of the composition, properties and behavior of matter.Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds. Chemistry is also concerned with the interactions between atoms (or groups of atoms) and various forms of energy (e.g. photochemical reactions, changes in phases of matter, separation of mixtures, properties of polymers, etc.).

Chemistry is sometimes called "the central science" because it bridges other natural sciences like physics, geology and biology with each other. Chemistry is a branch of physical science but distinct from physics.

The etymology of the word chemistry has been much disputed. The genesis of chemistry can be traced to certain practices, known as alchemy, which had been practiced for severalmillennia in various parts of the world, particularly the Middle East.