Secondary 1 Topic

Exploring the diversity of matter by its chemical properties

Classify into Elements, Compounds and Mixtures

This science revision note aims to help your child understand the must-know substances and to classify them correctly.

 

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Question

(a) Classify the following 6 substances as an element, a compound or a mixture. [3]

carbon dioxide          chromium           seawater           salt            brass            sugar

 STEP 1: Recall

What is the definition of elements, compounds and mixtures?

  • Remember Definitions:
  • Elements: Substances composed of only one type of atom that cannot be broken down into two or more simpler substances by physical or chemical methods.
  • Compounds: Substances composed of two or more different elements chemically combined together.
  • Mixtures: Combination of two or more substances that are physically mixed but not chemically combined.

 

STEP 2: Classify each substance

  • Carbon dioxide: It is composed of carbon and oxygen chemically bonded together, so it’s a compound.
  • Chromium: It’s an element since it consists of only one type of atom. The periodic table has this element!
  • Seawater: A mixture of various salts and water, so it’s a mixture.
  • Salt: A compound composed of sodium and chlorine elements / atoms that are chemically combined together.
  • Brass: A mixture of copper and zinc, so it’s a mixture. You have to memorise this knowledge.  Look at the window frames in your room! Brass is a common material used to make window frames.
  • Sugar: A compound made of carbon, hydrogen, and oxygen atoms chemically combined together.

Did you notice we mentioned the words “chemically combined/joined together” ?

That tells you how important those keywords are!

These chemical bonds are extremely strong!

 

Hmm… so what is the difference between the chemical bonds and the forces of attraction between compounds?

Some of you might have seen those words.

To understand the differences, and why chemical bonds are so strong, we need to VISUALISE these bonds and give ourselves a scenario.

 

Imagine this…

When you boil the water, the water becomes steam.

Yet, we know that steam is made of the same water molecules (H2O) but in its gaseous state.

What is happening at the atomic level?

Let’s zoom in on a water compound / molecule.

It looks like this (do ignore the colour. They are for illustrative purposes😊) :

 

Now imagine a drop of water (it contains hundreds of trillions of water compounds / molecules!)

We are not going to draw all of them.

Let’s focus on a few water molecules here:

In the diagram above, did you notice the chemical bonds and the forces of attraction?

The chemical bonds are INSIDE the water compound/molecule while the forces of attraction are BETWEEN the water compounds / molecules.

These forces of attraction between trillions of water compounds / molecules are what enable the water droplets to remain in their shape – it look like a mini globule.

Onward.

 

To understand further, we need to give a scenario – What happens when the water boils?

When the water in its liquid state gets enough heat, the water compounds / molecules break free from the forces of attraction (this is different from the chemical bonds between the hydrogen and oxygen atoms).

The chemical bonds between the hydrogen and oxygen atoms are not broken even after the water becomes steam.

 

Wheeee! The water molecules are free and can roam about randomly and far away from each other.

 

Because they are so spread out, our eyes cannot see the water in its gaseous state anymore!

Hence, we might think the water has “disappeared”.

That is, of course, not true.

We know that water molecules exist all around us. 

Now that we know the differences between chemical bonds and the forces of attraction, let’s answer the question.

 

STEP 3: Piecing your answer together.

Answers

 

Common mistakes

 

Wrong answers: Brass and seawater are compounds.

 

Teachers’ comments: 

Remember to differentiate between substances that are chemically combined (compounds) and those that are physically mixed (mixtures).

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