Language of Chemistry
ICSE Class 7 · Chemistry · Chapter 5
What this chapter covers
- Why chemistry needs its own language
- Chemical reactions
- Signs of a chemical reaction
- Reactants and products
- Word equations
- Writing word equations from what you observe
- The usefulness of a word equation
- Symbols and formulae: the shorthand behind equations
Key points
- The language of chemistry uses symbols for elements, formulae for compounds and equations for reactions.
- A chemical reaction forms one or more new substances with different properties.
- A reaction takes place when substances come in contact and energy is transferred.
- Signs of a reaction: change of colour, state or smell; a gas given off; a precipitate formed; heat given out or taken in.
- One sign alone is a clue; the real test is whether a new substance has formed.
- Reactants are the starting substances; products are the new substances formed.
- A word equation writes reactants on the left, products on the right, plus signs between names, and an arrow pointing to the products.
- Conditions such as heat or sunlight may be written on the arrow.
- A word equation is short, clear and the first step towards a chemical equation.
- A symbol has one capital letter, or a capital and a small letter; a formula shows the number of atoms of each element in a compound.
Questions answered
What is a chemical reaction?
A chemical reaction is a change in which one or more substances react to form one or more new substances with different properties.
Name any four signs that show a chemical reaction has taken place.
Any four of: a change of colour, a change of state, a change of smell, the evolution of a gas, the formation of a precipitate, and a change of heat.
Two colourless solutions, potassium iodide and lead acetate, are mixed. What do you see, and what is formed?
A bright yellow precipitate forms. It is lead iodide, a new substance.
Bubbles appear when a bottle of soda water is opened. Is this a chemical reaction? Give a reason.
No. The carbon dioxide was already dissolved in the water and only escapes; no new substance is formed.
In the burning of magnesium in air, name the reactants and the product.
Reactants: magnesium and oxygen. Product: magnesium oxide.
Write a word equation for: "Sulphur burns in oxygen to form sulphur dioxide."
sulphur + oxygen → sulphur dioxide
Write a word equation for heating calcium carbonate.
calcium carbonate → (heat) → calcium oxide + carbon dioxide
Which way must the arrow in a word equation point, and what does it mean?
It points from the reactants to the products, and it means "react to form" or "give".