O lvl Chem: Particulate Nature of Matter

O Lvl Chem: Nov 2003/Paper 3/ Section A/ Q7

Some properties of four solid, I, J, K, L, are given in Fig. 7.1. Use this information to complete the last column of the table.

solid	percentage composition by mass	solid conducts electricity	strong heat in air	element or mixture or compound
I	constant	no	decomposes
J	varies	no	burns
K	varies	yes	melts
L	constant	yes	oxidises to one substance

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Answer:

solid	percentage composition by mass	solid conducts electricity	strong heat in air	element or mixture or compound
I	constant	no	decomposes	COMPOUND
J	varies	no	burns	ELEMENT
K	varies	yes	melts	MIXTURE
L	constant	yes	oxidises to one substance	COMPOUND

I is a compound. Firstly, its percentage composition by mass is constant. Hence it is not a mixture. Secondly, it does not conducts electricity in the solid state. This information does not tell us anything. However, we know that it decomposes in strong heat in air. Element do not decomposes in strong heat. Hence I is a compound. Examples includes sugar.

J is an element. Firstly, its percentage composition by mass varies. This tell us that it is not a compound. For compound, the percentage composition by mass is fixed. Hence it can be an element(with isotopes) or it can be a mixture. It does not conduct electricity when in the solid state. This tell us that it is not a metal. Next, it burns when heated strongly in air. This two of information give us the information that it is an element. Burning is a phenomenon of oxidation. This means that a new substance (compound) is being form. Elements cannot be broken down into two or more simpler substances by ordinary chemical methods. Examples includes Arsenic, Bromine and so forth.

K is a mixture. Firstly, it percentage composition by mass varies. This means that it is not a compound. It conduct electricity in the solid state. This means that it might be a metal(conduct electricity) or a mixture. However, it melts when heat strongly in air. A mixture melts over a range of temperature. Metals do not melts when burn in air. Instead it is oxidised. Melting means that it is a mixture, since it cannot be an element (metal)

L is a compound. Firstly, its percentage by composition is constant. Hence, it is not a mixture. The next piece of information is that it oxidised to one substance when heated strongly in air. Mixture will not be oxidised to a single substance but several substance. Hence it is a compound.

O lvl Chem: Particulate Nature of Matter

RI Prelims 1999 P2 Section A Q 7

Flourine forms useful compounds called hexaflourides.

(a) Uranium forms 2 types of gaseous uranium hexaflouride, ²³⁵UF₆ and ²³⁸UF₆, based on the 2 isotopes of uranium.
(i) What are isoptopes?
(ii) Which isotope of uranium is radioactive? [out of syllabus]
(iii) Which gas, ²³⁵UF₆ or ²³⁸UF₆, diffuses faster? Explain your answer.

(b) Sodium aluminium hexaflouride or cryolite is used in the extraction of aluminium by electrolysis.
(i) Explain the role of cryolite in the extraction of aluminium. [out of syllabus]
(ii) Why is it not possible to extract aluminium by electrolysing a solution of an aluminium salt, like aluminium chloride, using carbon electrodes?

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Answer:

(a)
(i) They are elements with the same chemical properties and number of protons, but with different number of neutrons.

(ii) Uranium-235, ²³⁵₉₂U [out of syllabus]

(iii) ²³⁵UF₆ diffuses faster. This is because it has a smaller molecular mass than ²³⁸UF₆.


(b)
(i) The mixture of cryolite and aluminium oxide has a lower melting point than pure aluminium oxide. [out of syllabus]

(ii) Aluminium is very reactive; it is higher in the reactivity series than hydrogen. Thus, hydrogen ions will be preferably discharged, not aluminium.

O lvl Chem: Particulate Nature of Matter

Nov98/Paper 2/Section A/Q2

Use the information in the table to answer the following questions.

Substance	when solid Conducts electricity	Melting point/oC	Dissolves in water
Sodium Chloride	No	808	Yes
Sulphur	No	113	No
Tungsten	Yes	3377	No
Wax	No	35-50	No
Aluminium	Yes	660	No

(a) (i) Name two metals from the table
(ii) How can you tell from the table that the substances you have chosen in (i) are metals

(b) How can you tell from the table that wax is a mixture?

(c) (i)Name a compound from the table.
(ii) Explain the meaning of the word compound

(d) Describe hoe the arrangement and movement of particles in aluminium change as the temperature rises from 650 °C to 661 °C?

(e) Suggest how you could separate a mixture of sulphur and sodium chloride to produce a sample of each?

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Answer:

(a) (i) Tungsten and Aluminium
(ii) Metals conduct electricity in the solid state (due to the availability of free electrons) and have high melting points (strong electrostatic forces of attraction between the positive cations in a sea of negative electrons)

(b) Wax melts over a range of temperatures. Mixture are defined by the fact that they will melt over a range of temperature due to the fact that each component of the mixture has a different melting point.

(c) (i)Sodium Chloride. (It is a compound that is formed from Group I Na and Group VII Chlorine via electron transfer.)
(ii) A compound consists of 2 or more elements chemically combined with each other (Compare this to mixture. A mixture consists of 2 or more elements/mixture physically combined with each other)

(d) The melting point of Al is 660 °C. At 659 °C, it has not reached its melting point yet. Hence, it is in its solid state. The particles are closely packed together and can only vibrate about a mean position. At 660 °C, it is its melting point. The strong metallic bonds are being broken. The aluminium particles are able to move about. At 661 °C, all the particles are in continuous motion but are still closely packed together.

(e) Sulphur is hydrophobic while sodium chloride is hydrophilic. Hence, dissolve the mixture in water. Sodium chloride will form a mixture with water while sulphur will not. Pass the solution/suspension through a filter. The residue is sulphur. Evaporate the filtrate to the dryness

O lvl Chem: Particulate Nature of Matter

1) Consider the following list of common substances.

air	brass	glass	limestone
salt	sugar	water

Choose from this list a substance which
(a) is a mixture containing both elements and compounds.
(b) is a mixture of compounds
(c) is a mixture of elements
(d) is a compound containing only two element
(e) is a compound which contains carbon

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Answer:

(a) air
Air is a mixture of elements such as oxygen, nitrogen,and compounds such as carbon dioxide and water vapor.

(b) glass
Glass is an amorphous mixture of compounds including sand/silica, sodium oxide, calcium oxide and so forth.

(c) brass
Brass is a mixture of 2 elements of Cu and Zn.

(d) water
Water is a compound in which hydrogen and oxygen chemically react together to form H2O.

(e) sugar
Sugar is an organic compound. (Organic implies carbon containing) For example, table sugar consists of sucrose (C12H22O11)