3.3 (untitled)activity: to describe the composition of substances

Natural sciences

Matter, measuring and reactions

Educator section

Memorandum

Water:

• H ₂ O
• Hydrogen and oxygen
• The elements oxygen and hydrogen joined. This formed a new substance, namely water, that consists of two types of atoms. The atoms joined to form a water molecule.
• Yes, the elements are combined in a fixed relation to form a new substance with new characteristics.

Assignment 2:

1.

• 500 oxygen atoms
• 1 000 oxygen atoms

2. Combination

Leaner section

Content

Activity: to describe the composition of substances in terms of chemical symbols and formulas [lo 2.3]

When we know the physical and chemical properties of a substance we can use our knowledge to create another substance. Water, a compound, is the best solvent because many substances can dissolve in water. We can use the compound water to obtain gases like hydrogen and oxygen. We can also produce water by mixing these gases.

We make use of chemical symbols to identify elements and use chemical formulas to identify compounds. Here are some chemical symbols:

• Ag (symbol) is used for silver
• Fe is used for iron
• Pb is used for lead
• Au is used for gold

Try to illustrate the composition of a water molecule:

• Write the chemical formula for water.

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• Name the two elements that form water?

__________________________________ and _______________________________

• What had to happen to these elements to form water?

Use the words: elements, atoms and molecules in your explanation.

_____________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________

• Would this compound depend on a fixed ratio or could it change at random? Motivate your answer.

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• Provide a graphic representation of the atomic composition of water:

Assignment 2

Now try to analyse the chemical formula for carbon dioxide (carbonic acid gas):

Carbon dioxide is a compound of two kinds of atoms. The chemical formula is:

CO2

CO 2

This formula signifies that one atom of carbon combines with two atoms of oxygen to form carbon dioxide. We can therefore say that the ratio of carbon atoms to oxygen atoms in carbon dioxide molecules is 1:2.

1. How many atoms of oxygen will have to be present for:

• 250 carbon atoms?__________________________________________________
• 500 carbon atoms?__________________________________________________

2. Work out the combinations that are present in the following formulas:

Assessment

Learning Outcome 2: The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

Assessment Standard 2.3: We know this when the learner interprets information by identifying key ideas in text, finding patterns in recorded data, and making inferences from information in various forms (e.g. pictures, diagrams, text).