A chemical equation is a representation of a chemical reaction with the help of symbols and formulae of the substances involved in the reaction. It is a chemical shorthand for representing the reacting substance or substances combining (the reactants) and the substance or substances formed as a result of the reaction (the products).

Molecular Equations

A Molecular equation is the one which shows the reactants combining and the products formed, in their elemental or molecular forms in a chemical reaction. An example of a molecular equation is the reaction between sodium and water to produce sodium hydroxide solution and hydrogen gas:

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

In this context, sodium (in elemental form) reacts with water (in molecular form) to produce sodium hydroxide (in molecular form) and hydrogen gas (in molecular form).

Word Equations for given Chemical Reactions

Write word equations for given chemical reactions

A word equation is a short form of expressing a chemical reaction by word. Chemical reactions can be summarized by word equations that show all the reactants and the products. This type of equation links together the names of the reactants and the products. For examples, the burning of magnesium in air to produce magnesium oxide can be represented by the following word equation:

Magnesium + Oxygen → Magnesium oxide

Another example is the reaction between sodium and chlorine to give sodium chloride (common salt)

Sodium + Chlorine → Sodium chloride

Equations like these sometimes give us some information about the products formed when different substances are reacted together. But equations can be made even more useful by writing them using chemical symbols and formulae.

Any method for representing a chemical reaction must meet basic certain requirements. These are:

  • the chemical nature of the reactants as well as those of the products must be clear. The reactants can be in solid, gaseous, liquid or aqueous forms.
  • the mole ratios in which the products are combined and the products are formed must be deducible. This means that atoms of the reactants and the products must be balanced.
  • the direction of the reaction must be established. This means that it should be clearly shown which substances are the reactants and which ones are the products. This is normally done by separating the reactants from the products by an arrow. The arrow normally points from the reactants to the products.

Consider the reaction between potassium and water:

2K(s) + 2H2O (l) → 2KOH (aq) + H2 (g).

In this reaction, the three requirements have been met:

  • The chemical nature of the reactants [potassium (solid); water (liquid)] and the products [potassium hydroxide (aqueous); hydrogen (gas)] has been shown.
  • The mole ratios of the reactants and products are clearly shown: 2 moles of potassium combines with 2 moles of2water to produce 2 moles of potassium hydroxide and one mole of hydrogen gas.
  • The reactants (potassium and water) and the products (potassium hydroxide and hydrogen) are separated by an arrow (→) which also indicates the direction of the reaction.


To predict the reaction products precisely, one needs to take into account the type of reaction occurring. Once you identify the type of reaction that is going to take place, then you will be in a position of telling what possible products of reaction would be. A chemical reaction is said to have taken place when two or more chemical substances called reactants are converted into very different chemical substances called products.

There are a few ways to predict the reaction products. Firstly, when the reactants are mixed and then isolated, products can be identified. Prediction can also be made when elements from the same group in the Periodic Table show similar reactions. Finally, chemical reactions can be classified into different categories such as combination (or synthesis), decomposition, displacement, precipitation, and redox reactions as described in details below:

Types of Chemical Reactions

When a chemical reaction occurs, it is obvious that the changes have taken place. However, under ordinary conditions it is not easy to see how a reaction goes on. The neutralization of an acid solution with an alkali produces no change that you can see. However, reaction has happened. The temperature of the mixture increases and the new substances have formed which can be separated and purified. Ideally, we can tell whether a reaction has taken place if one or more of the following changes are observed:

(a) heat change has taken place and can be detected by the change in temperature of the products; (b) a precipitate is formed; (c) there is a change in state of the reactants, i.e. gas, liquid; solid; (d) a colour change has occurred; or (e) a gas is evolved and can be identified by its colour, smell or by effervescence.

  • heat change has taken place and can be detected by the change in temperature of the products;
  • a precipitate is formed;
  • there is a change in state of the reactants, i.e. gas, liquid; solid;
  • a colour change has occurred; or
  • a gas is evolved and can be identified by its colour, smell or by effervescence.

There are very many different chemical reactions. To make it easy to study about these reactions, it is useful to try to group certain types of reactions together. They may be grouped according to certain types of phenomena which accompany them. They can further be subdivided into categories of reactions, each of which has its unique characteristics. Some types of chemical reactions are discussed below:

Combination or synthesis (A + B → C)

Synthesis reaction occurs when two or more simple substances (elements or compounds) are combined to form one new and more complex substance. The general form of a synthesis reaction is:

element or compound + element or compound→ compound.

The reaction between iron and sulphur to form iron (II) sulphide is the best example for this kind of reaction. Iron combines directly with sulphur to form iron (II) sulphide:

Fe(s) + S(s) → FeS(s)

Another example is the reaction between hydrogen and oxygen to form water:

Hydrogen + Oxygen → Water

Decomposition (A →B + C)

Decomposition occurs when one compound breaks down into simpler substances. All decomposition reactions have one thing in common: There is only one reactant and it breaks down into two or more simpler products. Decomposition can be brought about by heat, light, electricity and even enzymes or catalysts.

Decomposition by heat

Decomposition caused by heat is termed as thermal decomposition. An example is the decomposition of calcium carbonate (limestone) which breaks down into calcium oxide(quicklime) and carbon dioxide gas when heated.

Calcium carbonate → Calcium oxide + Carbon dioxide

Formula Equations Using Chemical Symbols

Write formula equations using chemical symbols

Essentially, chemical reactions can be expressed in two forms. The chemical reaction can be expressed either as a word equation or as a formula (or symbolic) equation. We have already seen how chemical equations can be represented by words (word equation). The formula equation makes use of chemical symbols and formulae to represent a chemical reaction. An example is the reaction between iron and sulphur to form iron (II) sulphide: Fe + S → FeS

Steps for writing a chemical equation

These are the steps to follows when writing a chemical equation:

  1. State the reaction equation in words, for example, carbon reacts with oxygen to form carbon dioxide.
  2. Write the complete word equation using an arrow to separate the reactants from the products: Carbon + Oxygen → Carbon dioxide. Conventionally, the reactants are placed on the left-hand side and the products on the right-hand side of the equation. An arrow from left to right indicates that the reaction proceeds from reactants to products as shown.
  3. Change the words into the correct symbols and formulae of the reactants and products: C + O2 → CO2
  4. Balance the number of each type of atoms on each side of the equation.It is important to make sure that there is equal number of each kind of atom on the left of a chemical equation as on the right in order for your equation to comply with the Law of Conservation of Mass (or Indestructibility of Matter): Matter can neither be created nor destroyed in the course of a chemical reaction. This means that the total mass of all products of a chemical reaction is equal to the total mass of all reactants. All atoms appearing on the left-hand side must also be presented on the right-hand side. No atom should appear as a product if it is not present as a reactant.
  5. Add the state symbols: Reactants and products may be solids, liquids, gases or solutions. You show their state by adding state symbols to the equation. The state symbol are, (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution (solution in water). For the two reactions above, the equations with the state symbols are: Fe(s) + S(s) → FeS(s)</sub>; C(s) + O2(g) → CO2(g) All state symbols must be bracketed and placed as subscripts after the reactant(s) and product(s).

Balancing Chemical Equations

Balance chemical equations

A balanced chemical equation has an equal number of atoms of different elements of the reactants and the products on both sides of the equation. A balanced equation gives us more information about a reaction than we get from a simple word equation.

Below is a step-by-step approach to working out the balanced equation for the reaction:

  1. Write the chemical equation for the reaction with the correct symbols and formulae of the reactant(s) and the product(s).
  2. Identify different atoms of the different elements of the reactant(s) and the product(s).
  3. Check whether these different atoms are equal on both sides of the equation. Some atoms may balance each other directly.
  4. Balance the atoms on each sides of the equation by Hit and Trial Method.
  5. Add state symbols.

Example 1

The reaction between hydrogen and oxygen to produce water:

Hydrogen + Oxygen → Water

H2 + O2 → H2O (not balanced)

The atoms involved in the reaction are hydrogen and oxygen. It is these atoms that we are going to balance. The atoms must be equal on both sides of the reaction equation. There are two hydrogen atoms on each side of the equation. But, as you can see there are two oxygen atoms on the left-hand side (LHS) of the equation and only one oxygen atom on the right-hand side (RHS). To balance oxygen atoms, we write 2 before water.

H2 + O2 → 2H2O (not balanced yet)

By introducing 2 before water, another problem has been created. Now we have 4 hydrogen atoms on the RHS but only 2 hydrogen atoms on the LHS. To equalize the number of hydrogen atoms we write 2 before hydrogen on the LHS.

2H2 + O2 → 2H2O (balanced).

You can still check to find out whether the atoms are balanced or not. Now look at the number of atoms on each side of the equation:


Now, the number of hydrogen and oxygen atoms is the same on both sides of the equation. This is because the atoms do not disappear during a reaction. They are neither created nor destroyed. They obey the Law of Conservation of Mass. When the numbers of different atoms are the same on the both sides, an equation is said to be balanced. Once the equation is balanced you can now add the state symbols.

2H2(g) + O2(g) → 2H2O(l)

This gives a standard and an acceptable chemical equation.

An equation which is not balanced is not correct. An unbalanced equation implies that the atoms have been created or destroyed. It is therefore, wrong and calculations based on it are certainly unreliable.

Remember that we cannot change the formulae of the substances involved in the reaction. These are fixed by the bonding in the substance itself. For instance, in attempt to balance the number of oxygen in water, H2O, we cannot write H2O2. We can only put a multiplying numbers before symbols and formulae, e.g. 2H2O.

Example 2

Hydrogen burns in oxygen to form water. The equation for the reaction is:

2H2(g) + O2(g) →2H2O(l)

  1. How much oxygen is needed to burn 1g of hydrogen?
  2. How much water is formed when 5g of hydrogen is completely burned in oxygen? (Atomic weights: H = 1, O = 16)


a. Reaction equation:2H2(g)+ O2(g)→2H2O(l)

Atoms present: H : O

Molecular weights: 4 : 32

Reacting weights: 1g : Xg

The weight, X, of oxygen = 1×32⁄4= 8g

So, 1g of hydrogen needs 8g of oxygen


Ionic Equations

The Different Between Molecular Equations and Ionic Equations

Differentiate between molecular equations and ionic equations

Ionic equations are equations in which the reacting substances are represented in ionic forms after the elimination of spectator ions. In other words, ionic equations are those equations represented in such a way that spectator ions are not included in the final equation. Spectator ions refer to those ions, which do not change during the reaction i.e. they do not take part in a chemical reaction.

In order to be able to derive an ionic equation from a molecular equation, one must be acquainted with the solubility rules as outlined below:

  • All sodium, potassium and ammonium salts are soluble.
  • All nitrates, chlorates and acetates are soluble.
  • All binary compounds of the halogens (other than F) with metals are soluble, except those of silver, copper, lead and mercury (lead halides are soluble in hot water).
  • All sulphates are soluble except those of silver, lead, mercury (I), barium, strontium and calcium.
  • All carbonates, sulphites and phosphates are insoluble except those of ammonium and alkali metal (Group I) cations.
  • All hydroxides are insoluble except those of ammonium, barium and alkali metal (Group I) cations.
  • All sulphides are insoluble except those of ammonium, alkali metal (Group I) cations and alkali earth metal (Group II) cations.
  • All oxides are insoluble except those of calcium, barium and alkali metal (Group 1) cations; these soluble ones actually react with the water (hydrolyse) to form hydroxides.

Balanced Ionic Equations

Write balanced ionic equations

Steps for writing balanced ionic equations

  1. Write a balanced formula equation for the reaction.
  2. Split all soluble reactants and products into individual ions, clearly indicating their state symbols. Remember that substances that exists as molecules such as water, gasses and concentrated mineral acids, precipitates and neutral atoms do not consist of ions and hence do not ionize in water.
  3. Cancel out all the ions which appear on both sides of the equation (spectator ions). These are the ions which remain unchanged in the reaction.
  4. Re-write the remaining ions. This is the net ionic equation for that reaction.

Example 3

Consider the reaction for neutralization of hydrochloric acid with sodium hydroxide.

  1. Step 1: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  2. Step 2:H+(aq)+Cl(aq)+Na+(aq)+OH(aq)→ Na+(aq)+Cl(aq) + H2O(l)
  3. Step 3: :H+(aq)+Cl(aq)+Na+(aq)+OH (aq) → Na+(aq)+Cl(aq) + H2O(l)
  4. Step 4: H+(aq)+ OH(aq) → H2O(l)


What are the topics for S 3 chemistry? ›

Generally speaking, students can expect to learn about topics such as atomic structure, chemical bonding, acids and bases, mole concept and chemical calculations, and so on.

What is the chemical equation answers? ›

Chemical equations are symbolic representations of chemical reactions in which the reactants and the products are expressed in terms of their respective chemical formulae.

How to memorize chemistry answers? ›

Read them regularly and at the end of the day, try to write at least five reactions a time with their mechanisms. Try to reach examples where that reaction takes place. You can look after the conversion reactions too. This way, you can find a loop between different reactions and memorise them easily.

What are the 3 parts of a chemical equation? ›

  • The three main parts of a chemical reaction are reactants, arrow, and products.
  • Reactants: During a reaction, these compounds or elements combine and undergo some chemical changes. ...
  • Arrow: In other words, this indicates the formation of products from the reactant, as well as separating the reactant and product.

What is Chapter 3 in chemistry? ›


What is the hardest topic in chemistry A level? ›

The hardest questions in General Chemistry focus on Titrations, Electrochemistry, and Thermodynamics/Kinetics purely because they're multi-step, math heavy, topics.

What are 5 chemical equations? ›

The five basic types of chemical reactions are combination, decomposition, single-replacement, double-replacement, and combustion.

What are 5 examples of a chemical equation? ›

Balancing Chemical Equations Practice Problems
  • CO2 + H2O → C6H12O6 + O2
  • SiCl4 + H2O → H4SiO4 + HCl.
  • Al + HCl → AlCl3 + H2
  • Na2CO3 + HCl → NaCl + H2O + CO2
  • C7H6O2 + O2 → CO2 + H2O.
  • Fe2(SO4)3 + KOH → K2SO4 + Fe(OH)3
  • Ca3(PO4)2 + SiO2 → P4O10 + CaSiO3
  • KClO3 → KClO4 + KCl.
Mar 14, 2023

What is a chemical equation with example? ›

Reactants are converted to products, and the process is symbolized by a chemical equation. For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). Fe(s) + S(s) → FeS(s) The plus sign indicates that iron reacts with sulfur.

What is a simple chemical equation? ›

A chemical equation is a graphic representation that shows how chemical reactions produce products. Such equations format chemical substances; for example, instead of writing sodium chloride, NaCl is written. Coefficients are used to show the number of moles of reactants and products.

How do you memorize answers quickly? ›

Simple memory tips and tricks
  1. Try to understand the information first. Information that is organized and makes sense to you is easier to memorize. ...
  2. Link it. ...
  3. Sleep on it. ...
  4. Self-test. ...
  5. Use distributed practice. ...
  6. Write it out. ...
  7. Create meaningful groups. ...
  8. Use mnemonics.

How can I get chemistry easy? ›

How to study Chemistry
  1. Space out the work. ...
  2. Read before lecture. ...
  3. Focus on the main points in lecture. ...
  4. Rework your notes after lecture. ...
  5. Keep up with the readings. ...
  6. Memorize essential information including reactions. ...
  7. Conceptual understanding is the goal. ...
  8. Work problems until you can teach the material to someone else.

How to easily pass chemistry? ›

Be Smart on Tests
  1. Don't cram for a test. Don't put yourself in a position where you have to stay up all night studying. ...
  2. Get sleep before a test. Eat breakfast. ...
  3. Read through the test before answering any questions. ...
  4. Be sure to answer the high-point questions. ...
  5. Review returned tests.
Jan 19, 2019

What is the name of 3 in chemistry? ›

calcium chloride. titanium dioxide. ammonium nitrate. sodium bisulfate (the common name for sodium hydrogen sulfate)

Which chapter is hard in chemistry? ›

Organic Chemistry:

Organic chemistry for JEE mains is one of the most difficult sections in the chemistry syllabus due to its vastness and complexity. Organic reactions involve chemical changes where atoms come together to form molecules.

What is Group 3 in science? ›

Group 3 is the first group of transition metals in the periodic table. This group is closely related to the rare-earth elements. It contains the four elements scandium (Sc), yttrium (Y), lutetium (Lu), and lawrencium (Lr). The group is also called the scandium group or scandium family after its lightest member.

Is chemistry grade 12 hard? ›

Grade 12 Chemistry is one of the most challenging courses in Ontario high schools, and it's the same with Physics.

Is it hard to get an A * in A Level Chem? ›

Chemistry a-level is extremely hard compared to other subjects, with only 13.6% achieving an A* in 2022 and 24.4% getting an A. Many students struggle with chemistry in school and wonder if they have what it takes to handle the rigours of an A-Level chemistry course.

Is chemistry 1 hard in high school? ›

General Chemistry is so hard because it's very different from most other classes and subjects. The primary reason chemistry is so hard is because of the topic progression. You really have to fully understand several topics before you can fully understand other topics.

What are 20 examples of chemical changes? ›

  • burning of paper.
  • cooking of food.
  • burning of wood.
  • ripening of fruits.
  • rotting of fruits.
  • frying egg.
  • rusting of iron.
  • mixing acid and base.

What are 5 chemical changes? ›

The five conditions of chemical change: color change, formation of a precipitate, formation of a gas, odor change, temperature change.

What are the 5 reaction types? ›

This becomes much easier for students to do when they learn the pattern of 5 basic categories of chemical reactions: synthesis, decomposition, single replacement, double replacement, and combustion.

What are 10 examples of a chemical reaction? ›

Here are 10 Chemical Reactions That Take Place In Our Everyday Lives
  • Rusting. Rusting is the process of oxidation, which is the result of a reaction that takes place because of oxygen. ...
  • Digestion. ...
  • Photosynthesis. ...
  • Detergent and Soap Reactions. ...
  • Aerobic Cellular Respiration. ...
  • Anaerobic Cellular Respiration. ...
  • Acid-Base Reactions.
Mar 24, 2023

What are the 4 chemical formulas? ›

The four types of chemical formulas are: molecular, structural, condensed, and empirical.

What is a formula equation? ›

An equation is made up of expressions that equal each other. A formula is an equation with two or more variables that represents a relationship between the variables. A linear example is a line of the form y = m x + b where m is the slope and b is the y-intercept.

What are the 2 types of chemical equations? ›

Table of Contents
Types of Chemical ReactionsExplanationGeneral Reaction
Combination reactionTwo or more compounds combine to form one compound.A + B → AB
Decomposition reactionThe opposite of a combination reaction – a complex molecule breaks down to make simpler ones.AB → A + B
4 more rows

What are all chemical reactions called? ›

Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is referred to as metabolism. Metabolism includes both exothermic (heat-releasing) chemical reactions and endothermic (heat-absorbing) chemical reactions.

What are the rules for chemical equations? ›

The Rules for Writing Chemical Equations is first to write the symbols with positive charge valency. Next, write the valency of each atom at the top of its symbol. Finally, split the valency number by their highest common factor, ignoring the positive or negative radicals. The radical's valency should be switched.

What is a full chemical equation? ›

A chemical equation (see an example below) consists of a list of reactants (the starting substances) on the left-hand side, an arrow symbol, and a list of products (substances formed in the chemical reaction) on the right-hand side.

What are the major topics of chemistry? ›

General chemistry is the study of matter, energy, and the interactions between the two. The main topics in chemistry include acids and bases, atomic structure, the periodic table, chemical bonds, and chemical reactions.

What are the main topics of surface chemistry? ›

The important topics in surface chemistry are adsorption, catalysis, colloids and emulsions.

What are the 5 major branches of chemistry enumerate 3 topics? ›

Fundamentally, chemistry is the study of matter and change. The way that chemists study matter and change and the types of systems that are studied varies dramatically. Traditionally, chemistry has been broken into five main subdisciplines: Organic, Analytical, Physical, Inorganic and Biochemistry.

What are the 5 main types of chemistry? ›

In a more formal sense, chemistry is traditionally divided into five major subdisciplines: organic chemistry, biochemistry, inorganic chemistry, analytical chemistry, and physical chemistry.

What are the 7 types of chemistry? ›

Types of chemistry
  • Agrochemistry. Chemists in this field study chemistry as it pertains to crops and livestock. ...
  • Analytical chemistry. ...
  • Astrochemistry. ...
  • Biochemistry. ...
  • Environmental chemistry. ...
  • Forensic chemistry. ...
  • Geochemistry. ...
  • Industrial chemistry.
Feb 3, 2023

How many main topics are in chemistry? ›

The five primary branches of chemistry are physical chemistry, organic chemistry, inorganic chemistry, analytical chemistry, and biochemistry.

What is 10 called in chemistry? ›

Greek Prefixes
6 more rows
Aug 15, 2020

What chemical has 3 bonds? ›

triple bond, in chemistry, a covalent linkage in which two atoms share three pairs of electrons, as in the nitrogen molecule, N2, or acetylene, C2H2.

What is 5 in chemistry? ›

1mono- or hen-*deca-
4 more rows

What is surface chemistry short notes? ›

Introduction to Adsorption The branch of chemistry that deals with the physical and chemical processes occurring on surfaces is called surface chemistry. The phenomenon of accumulation of a substance on the surface rather than in the bulk of a solid or liquid is termed as adsorption.

What is surface in chemistry? ›

surface, in chemistry and physics, the outermost layer of a material or substance.

Why is surface area important in chemistry? ›

Surface area analysis refers to the measurement of a particle's available surface. It is important because it is the means by which a solid interacts with its surroundings, whether they are gases, liquid, or other solids.

What are 5 examples to explain branches of chemistry? ›

Branches of Chemistry with Examples
Branches of ChemistryReal Life Examples
Organic ChemistryWashing detergents, Plastics, Hair dyes
Inorganic ChemistryFloor cleaner, Soap
Physical ChemistryWater boiling, Water freezing
Analytical ChemistryWater and Soil Testing
6 more rows

What are the 4 branches of chemistry? ›

There are 5 major branches of chemistry - organic, inorganic, analytical, physical, and biochemistry.

Why is chemistry important? ›

Chemistry is essential for meeting our basic needs of food, clothing, shelter, health, energy, and clean air, water, and soil. Chemical technologies enrich our quality of life in numerous ways by providing new solutions to problems in health, materials, and energy usage.


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