O-Level Chemistry Video: Energy Changes – Exothermic and Endothermic Reactions


Energy cannot be created or destroyed. However, it can be changed from one form to another. This is what you learned from basic Physics syllabus.

All chemical reactions involve energy changes.

Some physical changes, such as boiling or dissolving a ionic solid in water, also involve energy changes.

Two keywords in this topic called Energy Changes: Exothermic and Endothermic Changes

  • An exothermic change is a change in which heat is given out to the surroundings
  • An endothermic change is a change in which heat is absorbed from the surroundings

Let us take a look at a video to have a better understanding of both Exothermic and Endothermic Changes. I will be using physical state changes of solids, liquids and gases to introduce the concepts involved.

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Length of video: 8.37 mins

Direct link: https://youtu.be/WD58-_SJy5E

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below.

Feel free to share this video with your friends who needs help in mastering Chemistry.

PS: You can do a search on the top right hand corner for more discussions and questions on “Energy Changes” or select from the suggested blog posts under “Related Articles” below.

O-Level Chemistry Video: Use of Catalyst on Rate of Reaction


In the previous blog post, we have watched a video on 5 factors that can affect the rate of a chemical reaction.

Today, we shall discuss the role of a suitable catalyst and how it increases the rate of a chemical reaction.

Many of my students in our Sec 4 GCE O-Level Pure Chemistry tuition classes always share with us that their friends have difficulty to score full marks in Chemistry Examinations whenever they were being asked about how a catalyst work to increase the rate of reaction.

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Length of video: 7.41 mins

Direct link: https://youtu.be/rqozg7_7pBo

Some key points from the video:

  • A catalyst is a substance which increases the speed of a chemical reaction and remains chemically unchanged at the end of the reaction
  • A catalyst increases the speed of a reaction by providing an alternative pathway with a lower activation energy
  • In a catalysed reaction, more colliding particles possess energy equal to or greater than the activation energy and hence the speed of reaction is higher

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below.

Feel free to share this video with your friends who needs help in mastering Chemistry.

PS: You can do a search on the top right hand corner for more discussions and questions on “Rate of Reactions” or select from the suggested blog posts under “Related Articles” below.

O-Level Chemistry Video: Writing Chemical Formulae of Ionic Compounds and Covalent Substances


It is very important for Chemistry students to master the skills of writing the correct chemical formulae for ionic compounds and covalent substances (elements or compounds) because almost every other Chemistry topics are related to it.

For students in Singapore, they will learn them in Sec 3 (some as early as Sec 2) whether they are based in GCE O-Level Pure Chemistry syllabus, or IGCSE and IP programs.

I observed that students who join my weekly O-Level Pure Chemistry tuition classes are generally not confident about writing the correct formula of a substance when they first came in. So i will usually work with them promptly to beef up these fundamentals first. You should do so likewise.

Before we even start, do take a look at the recent blog post about the different types of bonding: Ionic, Covalent and Metallic, and the structures associated with it first.

At the end of the two videos, you will learn the different skills and strategies to write the chemical formulae for both ionic compounds and covalent substances quickly, which are different.

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Feel free to share these videos with your friends who needs help in mastering Chemistry.

1st Video: Writing Chemical Formulae of Ionic Compounds

Length of video: 5.08 mins

Direct link: https://youtu.be/3hyUy6DQWhU

Key points of the video to deduce the formula of ionic compounds:

  1. Write down the respective ions with their charges, e,g, Xm+Yn-
  2. Move the values m and n diagonally but without the charges
  3. Use ( ) when it involves more than 1 unit of polyatomic ion

2nd Video: Writing Chemical Formulae of Covalent Substances

Length of video: 12.13 mins

Direct link: https://youtu.be/w_Y6rIlIkb4

Key points of the video to deduce the formula of covalent substances:

  1. Non-metal atoms share valence electrons to achieve noble/full electronic configuration
  2. Duplet (2 valence electrons) or Octet (8 valence electrons) configuration after bonding
  3. Count the atoms involved to determine the chemical formula

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below.

PS: Under related articles below, there are several blog post discussions and questions related to Chemical Bonding. You can also do a keyword search using the sidebar at the top right hand corner.