Acids - Definitions of Acids, Physical and Chemical properties of Acids

len Alfred Ajibola - Thu, 24th January, 2019 @ 17:08:41 PM

Topics in Chemistry

Elements, Compounds and Mixtures with their Characteristics Salts and Types of Salt in Chemistry Mixtures and Separation of Mixtures Mixtures and Separation of Mixtures - part 2 Mixtures and Separation of Mixtures - part 3 Physical and Chemical Change explained with their Differences Electrolysis, Electrolyte, Electrode, Anode, Cathode and Ionic Theory Diffusion - Grahams Law of diffusion and Importance of Graham's Law of Diffusion Metals - Physical Properties of Metals Daltons Atomic Theory and its Modifications Periodic table: Classification of Elements in the Periodic Table Emperical Formula - Explanation and Worked Examples of Emperical Formula Organic Chemistry - Hydrocarbon, Carbon and its Compounds Explanation, Worked Examples and Differences between Molecular Mass and Molar Mass Gold: Interesting Facts about Gold Matter, Mass, Weight and Characteristics of Solids, Liquids, Gases Acids - Definitions of Acids, Physical and Chemical properties of Acids

Academic Questions in Chemistry

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

The reaction involving an acid and a base is termed _____.

  • A. Hydrolysis
  • B. Neutralization
  • C. Combustion
  • D. Displacement
  • E. Decomposition
  • F. Sublimation

Which of the following is NOT a type of salt?

  • A. Acid salt
  • B. Basic salt
  • C. Double salt
  • D. Tripple salt
  • E. Complex salt
  • F. Normal Salt

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

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

What are Acids?

Acids have been defined in a variety of ways. All of these definitions are correct but some are more corect when compared to others.

Below are some of the definitions of an acid by various scientists

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

According to Arrhenius, when Hydrogen Chloride gas is dissolved in water (to form aqueous HCl) , H+ will be produced. Below is the equation:

HCl(aq) -> H+ + Cl-


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

Bronsted Thomas Martin Lowry definition of Acid

Note: Recall that a proton has an atomic number of 1. Based on this fact, we can conclude that Hydrogen ion is a proton (since it has an atomic number of 1).

Consider the equation below

HCl(aq) + NaOH(aq) -> NaCl(s) + H2O(l)

Notice from the equation that the acid HCl donated it's proton (H+) to NaOH, forming water (H2O) in the process. Simultaneously (At the same time), NaOH donated it's Na+ to HCl to form the salt NaCl.

Please read on the Periodic Table here.


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

Gilbert Newton Lewis definition of Acid

Note: Recall that valence electron(s) are electrons present in the outermost shell of an atom.

Consider the equation below

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

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Image Credit: cnx

Notice that silver ion Ag+ accepts a pair of electron (2 electrons or 2H+) from 2 moles ammonium molecule (2NH4+) to form ammonium silver (2H4AgN).

The fact that silver ion (Ag+) accepts a pair of electron from ammonium qualifies it to being a Lewis acid. In short, an substance that accepts a pair of hydrogen ion (2H+ which is equivalent to 2 electrons is a LEWIS ACID.

Please read on Physical Change, Chemical Change, Elements, Compounds and Mixtures here.

Note: In any redox (oxidation and reduction) reaction, electron is always lost and gain.

Lewis definition of acid is quite encompassing because it accommodates for both oxidation and reduction.

Always remember that Oxidation is the loss of electron(s) during a reaction while Reduction is the gain of electrons(s).


Acids are also defined as substances that gives out hydronium ion (H3O+) when dissolved in water.

The equation below explains the definition

HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(g)

From the above equation, dissolving the acid (HCl) in water yields hydronium ion (H3O+), sometimes called oxonium ion; as it's only positive ion.

Note: Cations are positively charged ions while Anions are negatively charged ions.

Please read more on ions here.

All acids usually 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 anion.


Physical properties of Acids

  1. They have sour taste. (Acid in Latin means sour).

  2. They will turn a blue litmus paper to red.

  3. On a pH scale, they have a value less than 7 (<7). The pH scale is used to measure the degree of acidity or alkalinity substances. It's values ranges from 0-14.

  4. They will change the color of methyl orange from orange/yellow to pink.

  5. They will change color of phenolphthalein from pink to colourless.

  6. They are corrosive. A corrosive substance can "rust metals" and "burn the human skin".

  7. Aqueous form of acids can conduct electricity.


Chemical properties of Acids

1. Neutralization reaction: This is a reaction in which an acid reacts with a base to form salt and water only.

HCl(aq) + NaOH(aq) -> NaCl(s) + H2O(l)

HNO3(aq) + NaOH(aq) -> NaNO3(s) + H2O(l)

Note: Normal salt is a type of salt formed from a Neutralization reaction.

NaCl and NaNO3 are Normal salts formed from the above reaction.

Please read on Salt and Type of Salts here.


2. Reaction with Metals: When an acid reacts with metals higher than hydrogen in the activity or electrochemical series, hydrogen gas will be evolved or discharged.

Electrochemical Series - Len Academy

Image Credit: Redefining Knowledge

Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)

2Al(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2(g)

Notice that Zinc (Zn) and Aluminium (Al) are higher than hydrogen (H) in the electrochemical series, hence displacing it from the acid "HCl".

Please read on the Physical Properties of Metals here.


3. Reaction with carbonates: When an acids reacts with carbonates, salt and water is produced as carbon(IV)oxide (CO2) is given off.

Na2CO3(s) + H2SO4(aq) -> Na2SO4(s) + H2O(l) + CO2(g)

CaCO3(s) + H2SO4(aq) -> CaSO4(s) + H2O(l) + CO2(g)


4. Reaction with bicarbonates: It is similar to the reaction with carbonates in the sense that salt, water and carbon(IV)oxide is produced.

KHCO3(s) + HCl(aq) -> KCl(aq) + H2O(l) + CO2(g)

NaHCO3(s) + HCl(aq) -> NaCl(aq) + H2O(l) + CO2(g) + H2O(l)

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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.

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 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