Atoms, Elements and Compounds
The difference between elements, compounds and mixtures, reading chemical formulae, and writing and balancing symbol equations with state symbols.
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All 26 questions and answers
1.What is an atom?
The smallest part of an element that can still exist.
2.What is an element?
A substance made of only one type of atom. There are about 100 different elements.
3.Where are the elements shown?
In the periodic table.
4.How is an element represented?
By a symbol of one or two letters, for example O for oxygen and Na for sodium.
5.What is a compound?
A substance containing two or more different elements chemically combined in fixed proportions.
6.How are compounds formed?
By chemical reactions, which involve atoms sharing or transferring electrons.
7.Can a compound be separated back into its elements by physical means?
No. Compounds can only be separated by chemical reactions.
8.Are the properties of a compound the same as the elements in it?
No. A compound has completely different properties from the elements it is made from.
9.What does a chemical formula tell you?
Which elements are present and how many atoms of each there are.
10.How many atoms are in one formula unit of CaCO3?
Five: one calcium, one carbon and three oxygen.
11.How many atoms are in Ca(OH)2?
Five: one calcium, two oxygen and two hydrogen. The bracket subscript applies to everything inside it.
12.What is a mixture?
Two or more elements or compounds that are not chemically combined together.
13.Do the substances in a mixture keep their own properties?
Yes. Because there is no chemical bond between them, each one behaves as it normally would.
14.How can a mixture be separated?
By physical processes such as filtration, crystallisation, distillation and chromatography. None of these involve chemical reactions.
15.What is a word equation?
A way of representing a reaction using the names of the reactants and products, for example magnesium + oxygen gives magnesium oxide.
16.What are reactants and products?
Reactants are the substances you start with, on the left. Products are the substances made, on the right.
17.Why must a symbol equation be balanced?
Atoms are never created or destroyed in a chemical reaction, so there must be the same number of each type of atom on both sides.
18.Balance this equation: H2 + O2 gives H2O
2H2 + O2 gives 2H2O
19.Balance this equation: Mg + O2 gives MgO
2Mg + O2 gives 2MgO
20.Balance this equation: Na + Cl2 gives NaCl
2Na + Cl2 gives 2NaCl
21.Balance this equation: CH4 + O2 gives CO2 + H2O
CH4 + 2O2 gives CO2 + 2H2O
22.When balancing, what are you allowed to change?
Only the big numbers in front of each formula. Never change the small subscript numbers, because that would change the substance itself.
23.What are the four state symbols?
(s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous, meaning dissolved in water.
24.What does aqueous mean?
Dissolved in water.
25.What is the law of conservation of mass?
No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
26.Why might the mass appear to decrease in an open flask?
A gas has been produced and escaped into the air, so it is no longer being weighed.
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