Chemistry

Physical Change and Chemical Change

len Alfred Ajibola - Thu, 20th June, 2019 @ 11:22: AM

Topics in Chemistry

Le Chatelier's Principle: Changes in Concentration and Pressure explained in Dynamic Equilibrium Chemical Equilibrium: Dynamic Equilibrium in Chemistry Static and Dynamic Equilibrium explained with their Differences Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures Mixtures: Characteristics of Mixtures Physical Properties of Matter explained with Examples Chemical Properties of Matter explained with Examples Differences between Physical and Chemical Change Ionic Theory and Electrolysis Physical and Chemical Properties of Acids States of Matter: Characteristics of Solids Liquid State of Matter: Characteristics of Liquids Gaseous State of Matter: Characteristics of Gases Molar Mass, Worked Examples and Differences with Molecular Mass Elements: Characteristics of Elements Salts and Types of Salt in Chemistry


Academic Questions in Chemistry

Please check out our Test Your Knowledge page to see all Questions and Answers

Which of the following statement is false concerning hydrocarbons?

  • A. Hydrocarbons can be aromatic

  • B. Hydrocarbons are made up of carbon and hydrogen only

  • C. Alkynes are aliphatic hydrocarbons

  • D. Alkenes have a carbon-carbon double bond in their structure

  • E. Benzene ring has a carbon-carbon triple bond in its structure

  • F. Hydrocarbons are the main constituents of petroleum and natural gas

The ability of carbon to form long chain of itself is called _____.

  • A. Carbon chain reaction

  • B. Catenation

  • C. Carbontion

  • D. Carbontion

  • E. Organic chemistry

  • F. Carbon moleculation

Calculate the relative molecular mass of methane. (Carbon = 12.001, H = 1.00794).

  • A. 13.00894 amu

  • B. 11.00206 amu

  • C. 12.00794 amu

  • D. 14.001 amu

  • E. 15.001 amu

  • F. 16.033 amu

Which of he following is not a fact concerning gold?

  • A. 24 karat gold is the purest gold element

  • B. It is the 50th rarest metal amongst all elements found beneath the earth crust

  • C. It is the most malleable and ductile pure metal ever known

  • D. It is the only metal with a golden color

  • E. Gold is non toxic and can be eaten with food

  • F. It's present inside the earth crust in all seven continents

Which of the following is not an example of a chemical change?

  • A. The burning of wood
  • B. The melting of candle wax
  • C. The decomposition of food
  • D. The rotting of leaves
  • E. The lightening of a match stick
  • F. None of the above

Which of the following is not a fraction of crude oil?

  • A. Gasoline
  • B. Naphtha
  • C. Benzene
  • D. Wax
  • E. Paraffin
  • F. Lubrication Oil

The Separating Funnel is a technique used in the separation of mixtures. Another name for Separating Funnel is the _____ Funnel.

  • A. Partition
  • B. Separatory
  • C. Division
  • D. Liquid
  • E. Half
  • F. Dividing

2NH4+(aq) + Ag+(aq) -> 2H4AgN

The equation above represents _____ acid.

  • A. Svante Arrhenius'

  • B. Brønsted–Lowry's

  • C. Gilbert Lewis'

  • D. Avogadro's

  • E. Sir Isaac Newton's

  • F. Albert Einstein's

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Physical Change:

Having a solid understanding of physical and chemical change is one of the ideal knowledge a student who aspire to become excellent in chemistry should gain.

Recall that Chemistry is the study of matter and the changes they undergo.

Please read on the Concept of Matter here.

A physical change is a change that is easily reversible and no new substance is formed.

As an 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 the salt solution to get back its constituents, (that is, common salt and water).

Please read on Elements, Compounds and Mixtures here.

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.

Salt solution does not have any ideal chemical formula. In short, a mixture have no ideal chemical formula because it is associated with a physical change.

Please read on Differences between Physical Change and Chemical Change here.

This salt solution in our instance above 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 via the separating funnel technique.

Recall that water has a higher density than oil. For this reason, it remains at the bottom while the oil (with lower density) floats on it.

Please read more on Density and Specific Gravity here.

In a separating funnel technique, the water (at the bottom of the mixture) is first tapped out, leaving behind the oil.

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 for separating water from sand, (that is, an insoluble solid in a liquid).

Please read more on Separation Techniques here.

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.

Please read on the Physical Properties of Matter explained here.

 

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, if we exposed a piece of iron (Fe) to the atmosphere and rainfall (or water) for some period of time; we will observe that 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.

What we have now is referred to as rust and the process leading to the formation of rust is referred to as corrosion.

Please read on the Chemical Properties of Matter here.

On the other hand, the rust formed from the exposed iron is an entirely new substance and it has a chemical name called iron (III) oxide or ferric oxide or hematite with a chemical formula Fe2O3.

We cannot reverse the formed rust (Fe2O3) into iron (Fe).

This irreversibility of the new substance formed is the main idea behind a chemical change.

Please read on the Physical Properties of Metals here.

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; with a chemical formula (H2O).

Note: Other examples of chemical changes seen around us include:

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.


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Amazing facts in Chemistry

The only letters that failed to appear on the periodic table are letters:

J     &     Q

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 utilize 'copper' water bottles in schools

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

A major challenge encountered when calculating molecular mass is that it becomes difficult or impossible to calculate especially when the relative molecular mass of large molecules, polymers and macromolecules are involved.

Examples of large molecules (with indefinite molecular masses) include carbohydrates, cellulose and complex sugars.

The large molecules (above) have no specific chemical formula throughout their volume.

Please read more on Carbohydrates and Sugars here

Understand that Relative Molecular Mass prove to be useful only when we calculate substances with small and definite molecular sizes. This was proven through the Modifications of Dalton's Atomic Theory.

Please read on Dalton's Atomic Theory and its Modifications here.

Calculate the Relative Molecular Mass of Methane (CH4). (Carbon=12.001 and H=1.00794)

  • Molecular Mass = Atomic mass of carbon + Atomic mass of Hydrogen
  • Methane is composed of one atom of Carbon and 4 atoms of Hydrogen.
  • 12.001 + (1.00794 x 4)

Molecular Mass of Methane = 16.033 amu

Please read more on molecular mass here


Calculate the Relative Molecular Mass of Water (H2O). (Oxygen=15.9994 and Hydrogen=1.00794)

  • Molecular Mass = Atomic mass of Hydrogen + Atomic mass of Oxygen
  • Water contains 2 atoms of Hydrogen and 1 atom of oxygen.
  • (1.00794 x 2) + 15.9994

Molecular Mass of Water = 18.015 Da

Please read on Emperical Formula and its Calculations here

Gold ; Len Academy

Below are some facts about gold

  • It is the only metal that has a gold (golden) color.

  • Gold is present inside the earth crust in all the seven continents.

  • Pure gold is soft and very stretchable (It can even be stretched into threads).

  • The Latin name of Gold is aurum which means 'shining dawn'. For this reason, its chemical symbol is 'Au'.

  • Gold is a very good conductor of electricity. This is why it’s sometimes used in making USB and audio cables.

  • Gold is non toxic and can be eaten with food. It is sometimes added to desserts in some expensive restaurants.

  • The term Gold originated from an ancient English word 'geolu' which means yellow.

  • 24karat gold is the purest gold element.

Please read more interesting facts about gold here

An acid is a substance that produces Hydrogen ions (H+) when dissolved in water (or when in the form of an aqueous solution).

Svante Arrhenius definition of Acid

 

An acid is a substance that donates a proton in a chemical reaction.

Brønsted Thomas Martin Lowry definition of Acid

 

An acid is a substance that accept an electron pair in a chemical reaction.

Gilbert Newton Lewis definition of Acid

 

Finally, Acids can also be also defined as substances that gives out hydronium ion (H3O+) when dissolved in water.

 

In summary, all acids will always contain the positive H+ (cation) and an anionic group in their formula. For instance, the acid HCl (Hydrogen Chloride) contains H+ as cation and Cl- as its anion.

Please read the explanations of the above definitions of acid here