Chemistry

Physical Change, Chemical Change, Element, Compound and Mixture

Olawunmi Omosanya - Thu, 20th June, 2019 @ 11:22: AM

Topics in Chemistry

Salts and Types of Salt in Chemistry Separation Techniques Separation Techniques part 2 Separation Techniques part 3 Physical Change, Chemical Change, Element, Compound and Mixture Electrolysis, Electrolyte, Electrode, Anode and Cathode Grahams law of diffusion Physical Properties of Metals Daltons Atomic Theory and its Modifications Periodic table and Classification of Elements



Academic Questions in Chemistry

The only metal that exist as liquid at room temperature is _____.

  1. Nickel
  2. Duralumin
  3. Mercury
  4. Lead
  5. Titanium
  6. Cesium

ANSWER is Mercury

Mercury is a liquid metal at room temperature. Its chemical symbol is Hg. The name mercury comes from its Greek name, hydrargyrum, which means "liquid silver" .  Mercury has an atomic number of 80. Please read on the periodic table of elements here

The element mercury is also considered to be named after the fastest-moving planet in the solar system. You can read on solar system here.

Chemistry

Physical Change:

Having a solid understanding of physical change, chemical change, elements, compounds and mixtures is one of the ideal knowledge a student who aspire to become excellent in chemistry should gain. In this article, we will break down each of these in simple terms.

A physical change is a change that is easily REVERSIBLE and NO NEW substance is formed.

For instance, when we add NaCl (sodium chloride or common salt) into H2O (hydrogen oxide or water), a salt solution will be formed. This change that resulted from the mixture of salt and water is a physical change because we could easily separate it to get back the constituents materials that formed the mixture (that is, NaCl and H2O).

Note: Since no new substance is formed in the mixture of NaCl and H2O, the best name we could attribute to such mixture is a salt solution; meanwhile salt solution does not have any ideal chemical formula. This salt solution can be separated by a technique called Evaporation.

A mixture of NaCl and H2O does not have a chemical name because a new substance is NOT formed.

Another example of a physical change is a mixture of red oil and water. There is no chemical name that can be attributed to this mixture since no new substance is formed. Red oil and water are two immiscible liquid and as result, their mixture can be separated by using a separating funnel technique.

A third example of a physical change is the mixture of sand and water. Since no new substance is formed, no name can be ascribed to such mixture. (The best we can name it is a mixture of sand and water)

Decantation is the technique used to separate water from sand, (that is, an insoluble solid in a liquid).

Note: The freezing of liquid water into ice is also a physical change since it can be easily reversible. Moreover, the chemical formula of water and ice remains the same (H2O) thus, no new substance is formed.

 

Chemical Change

Having a good understand of physical change will give you a clue on what a chemical change is.

A chemical change is a change that is NOT easily reversible and a NEW substance is formed.

Take for instance, when a piece of iron (Fe) is exposed to the atmosphere and rainfall (or water); after some period of time, the iron will gradually begin to rust until it finally erodes away. When this happens, we no longer have our iron (Fe) and unfortunately, there’s no way to recover it back. On the other hand, the rust that formed is an entirely new substance and it has a chemical name called iron oxide or ferric oxide or hematite. We cannot reverse the formed rust (Fe2O3) back to iron (Fe).

Another example of a chemical change is the formation of water (H2O). The constituent elements in the formation of H2O are hydrogen molecule (H2) and oxygen molecule (O2). The reaction is shown below:

2H2(g) + O2(g) -> 2H2O(l)

From the above equation, a new substance has been formed and that’s water (H2O). Water is completely different from hydrogen and similarly, so is it completely different from oxygen. As a matter of fact, water cannot be reversed to its constituent elements which are hydrogen and oxygen respectively.

The new substance formed from the reaction between hydrogen and oxygen has a chemical name and that’s hydrogen oxide or water (H2O).

Note: The burning of wood, rotting of leaves and decomposition of food are all chemical changes.

 

The table below shows the differences between a PHYSICAL CHANGE and a CHEMICAL CHANGE

Physical Change

Chemical Change

 It is easily reversible

 It is not easily reversible

 No new substance is formed

 A new substance is formed

 The change is not accompanied with a new chemical formula

 This change is accompanied with a new chemical formula

 The original matter can be recovered by using a separation technique

 The original matter cannot be recovered

 It results to a change in the physical properties of the substance, e,g color and shape

 It results to a change in the chemical composition and properties of the new substance

 This change only result in the rearrangement of molecules but the internal composition of the substance remains the same

 This change results in the formation of  new substance with an entirely different chemical composition

 

 

Element

Everything in our world is composed elements and compounds. For instance:

  • The atmosphere is composed of elements and compounds like Nitrogen, Oxygen, Carbon(IV)oxide and so on. Please read on the constituents of the atmosphere here.
  • Humans are composed of elements like Carbon (C), Iron (Fe), Calcium (Ca), Sodium (Na), Chlorine (Cl) and so on
  • Various materials or objects are made of elements and compounds for instance, plastic and nylons contain the elements carbon, hydrogen, sulphur, chlorine and so on. Sand contain the elements silicon and oxygen. Cotton is made up of elements like oxygen, carbon, hydrogen, nitrogen and so on.

In chemistry, an element is the simplest form of a substance that cannot be broken down by chemical means.  

For instance, when one attempts to split a pure gold (element) in particles, all they see is the same thing (gold) on each particle, nothing else.

Elements are naturally occurring in the universe; in fact, life is present on earth because of the type of elements found in it. In other planets, the elements that are necessary for life are not present in sufficient quantities, for instance most will have little or no oxygen in their atmospheres.

Think of it, we can wear gold because there is an element called gold. We have water because of the elements hydrogen and oxygen. We have wires because of the elements copper, silver and aluminium.

Below are some notable points on elements

  • Each element has a unique name and is displayed in an organized manner on a table called the periodic table. Please read on the periodic table here.
  • All elements are composed of atoms with the same number of protons. It is these numbers of protons that defines or make unique an element. For instance, Hydrogen is hydrogen simply because it has only one proton. Oxygen is oxygen because it has eight protons.
  • Elements may have the same number of protons and different number of neutrons giving rise to a phenomenon called isotopy. Please read on Dalton’s atomic theory and its modification here.
  • There are 118 known elements as at present. There is a ongoing research to discover or create the 120th element; and once it’s verified, the periodic table will be altered.

 

Compound

A compound is formed from two or more elements chemically combined.

Compounds are not elements; rather, they are formed from elements.

H2O (water) is a compound formed from hydrogen and oxygen. It is neither hydrogen nor oxygen.

Once a compound is formed, it is very difficult to revert it to the constituents’ elements that formed it. As a result, the change that accompanies the formation of a compound is a chemical change.

Below are some reactions that show the formation of compounds:

Na(s) + Cl(g) -> NaCl(s) *Sodium Chloride*

2Fe(s) + 3O2 -> Fe2O3(s) *Iron(III)oxide*

C(s) + O2 -> CO2(g) *Carbon(IV)oxide*

 

Mixture

A mixture is a combination of two or more elements physically combined. A mixture can be homogeneous or heterogeneous.

Homogeneous mixture is seen when it has a uniform composition all through. For instance, the mixture of salt and water is considered to be a homogeneous when properly mixed together. The solution of a homogeneous mixture appears to be pure.

Heterogeneous mixture is seen when different sections of the mixture have different composition of the substances that it’s made of. Examples are:

  • The mixture of sand and water
  • The mixture of oil and water
  • Petroleum or Crude oil and so on

A mixture is always an attribute of a physical change.

When the term mixture is used on a substance, consider such substance to be easily reversible to it whatever formed it.



Amazing facts in Chemistry

Gold and copper are the only two non-silvery colored metals. Please read some interesting facts about gold here

Copper is the only metal that is naturally antibacterial. This is why you would see some children carry copper water bottles to school

Water actually freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy.
Please read on modification of Dalton's atomic theory here.

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre


NOTABLE POINTS IN Chemistry

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