What is a chemical formula?
A representation of a substance using symbols for the elements it contains.
AQA GCSE Combined Science: Trilogy (8464) — Chemistry — Higher · 5.1.1.1
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What is a chemical formula?
A representation of a substance using symbols for the elements it contains.
What does a compound’s chemical formula show about its constituent atoms?
Which elements are present and the fixed proportions in which their atoms are chemically combined.
What are chemical formulae used for?
To identify substances and name them correctly.
What is the periodic table used for in this course?
To identify element names and symbols.
Which sets of elements should you be able to use the names and symbols for in this course?
What is the role of a chemical symbol in chemistry?
A chemical symbol is the standard shorthand used to represent an element.
What does a word equation show in a chemical reaction?
The names of the reactants and products.
How does a symbol equation represent a chemical reaction?
It uses chemical symbols and formulae to show the reaction more precisely than a word equation.
What makes a chemical equation balanced?
It has the same number of each type of atom on both sides.
What does a balanced chemical equation show about the fate of atoms in a reaction?
Atoms are rearranged, not created or destroyed.
What are ionic equations and half equations used to represent more specifically?
Reactions involving ions and electron transfer.
What makes a chemical equation balanced?
Each element has the same number of atoms on the reactant and product sides.
What do coefficients in a chemical equation indicate, and where are they written?
They indicate the number of whole particles and are written in normal script before a formula.
What do subscripts in a chemical formula indicate?
The number of atoms of each element in one particle of the substance.
When balancing a chemical equation, which numbers may be changed and which must remain unchanged?
Change the coefficients; leave the subscripts in the formulas unchanged.
What are all substances made from?
Atoms.
About how many different elements are there?
About 100.
What are the basic building blocks from which other substances are made?
Elements.
What is an atom?
The smallest part of an element that can exist as that element.
What is an element?
A pure substance made from only one type of atom.
What is a compound?
A substance made of atoms of two or more different elements chemically combined.
What does it mean that elements in a compound are joined in fixed proportions?
A given compound always has the same composition.
How does a compound differ from a physical mixture of its elements?
A compound is a new substance in which the elements are chemically combined.
How can a compound be represented?
By a chemical formula.
How can a compound be separated back into its elements?
Only by chemical reactions.
What happens to reactants in a chemical reaction?
One or more reactants change into one or more new substances called products.
What happens to atoms during a chemical reaction?
They are rearranged, not created or destroyed.
What can happen to chemical bonds during a reaction?
Bonds can be broken and new bonds formed.
How can compounds form from elements?
Elements can react chemically to form compounds.
How can a chemical reaction transfer energy relative to its surroundings?
It can transfer energy to or from the surroundings.
What detectable temperature change can accompany a chemical reaction?
The temperature may increase or decrease.
What does an ionic equation represent?
A reaction in terms of the particles that undergo chemical change, including other reacting or product particles where appropriate.
What are spectator ions, and how are they treated in an ionic equation?
They are ions that remain unchanged on both sides and are omitted from the ionic equation.
What two quantities must be balanced when choosing coefficients for an ionic equation?
The number of atoms of each element and the total electrical charge must be equal on both sides.
What does a half-equation represent?
One part of a reaction involving electron transfer.
In an oxidation half-equation, what happens to electrons and where are they shown?
Electrons are lost and appear among the products.
In a reduction half-equation, what happens to electrons and where are they shown?
Electrons are gained and appear among the reactants.
What two conditions must a balanced half-equation satisfy?
Both sides must have equal numbers of each type of atom and equal total electrical charge.
What does the number of electrons in a half-equation account for?
The change in charge.