Video: GCSE Chemistry Foundation Tier Pack 1 • Paper 1 • Question 1

GCSE Chemistry Foundation Tier Pack 1 • Paper 1 • Question 1

11:11

Video Transcript

This question is about the physical properties of different substances. Draw one line from each statement to the diagram illustrating the relevant structure. The statements are: can scratch most materials; forms electrically conductive solutions; can be compressed easily; and can flow into a container without filling it.

The best way of approaching this question is to think of materials that correspond to the diagrams. The top diagram is of a diatomic gas. The dots represent atoms and the line between them a bond. You can see that it’s filling the box the way a gas would fill a container. An example of such a gas is chlorine.

The second diagram depicts a solid composed of two different elements. It is an example of a binary solid. These two elements are also arranged in a lattice. One such material is sodium chloride. The third diagram should look familiar. It’s a unit of a giant covalent structure. The substance most commonly associated with this diagram is diamond.

The fourth diagram is of a molecular liquid, a substance that takes the shape of its container but does not expand to fill it. Based on the structure where we have one dark spot attached to two lights spots, the likely structure it is depicting is water. The final structure depicts a two-dimensional lattice and it should be recognizable as graphite.

Now we can look at the statements one by one and see which material they correspond to. What substance can scratch most materials? Something hard, something with a lattice work of covalent bonds, diamond. Now, on to the second statement. What substance will form electrically conductive solutions? Electrically conductive, we’re talking about mobile ions in solution. We’re talking about an ionic lattice.

Now on to the third statement. Something that can be compressed easily, something with a lot of space. Something that takes the shape of its container. We’re talking about a gas here. Now, on to “can flow into a container without filling it”. That’s going to be a liquid. Only one of the diagrams depicts a liquid.

Therefore, we have four answers for our four statements. The key here was to look at the diagrams and think of what substances are associated with those structures. And then look at the statements and think about what statement is associated with what material.

Figure one shows the structure of a common substance. What is the name of element A? Carbon, oxygen, silicon, or sulphur.

Let’s have a look at the structure in figure one. We can see that element B forms two bonds while element A forms four bonds. This structure represents a continuous lattice. So you have to think about the structure that’s represented here, patterning on itself repeatedly in all directions. If you look at your periodic table, you’ll be able to find these elements easily.

Carbon and silicon are in group four. Oxygen and sulphur are in group six. Being in group four means that carbon and silicon have four electrons in their outer shell. They are therefore four electrons short of a full shell. They therefore form four bonds. Oxygen and sulphur, on the other hand, have six electrons in their outer shell and therefore are two electrons short of the full eight and therefore form two bonds. We can therefore eliminate oxygen and sulphur from this because we know that element A forms four bonds.

Now, we are left with carbon and silicon. To choose between these two, it might be quicker if we work out what element B is. We already know that it’s either oxygen or sulphur because oxygen and sulphur form two bonds and element B forms two bonds. Oxygen combines with carbon and with silicon to form very common compounds, carbon dioxide and silicon dioxide. On the other hand, the equivalent compounds of sulphur, carbon disulfide and silicon disulfide, are not common substances. Therefore, element B is oxygen.

Now, the two substances left in the competition are carbon dioxide and silicon dioxide. Carbon dioxide is a simple molecular gas whereas silicon dioxide is a lattice with this structure. Therefore, carbon is not the correct answer and silicon is. You could have shortcut this question by simply recalling that this is the structure of silicon dioxide. And therefore, the two elements involved are silicon and oxygen and that silicon is the element that forms four bonds.

Why does this substance have a high melting point? It contains layers of atoms; it contains delocalized electrons; it contains a lattice of oppositely-charged ions; or it contains strong bonds between atoms.

The key to this question is that having a high melting point suggests that there are strong internal bonds. The stronger the internal bonds, the higher the energy required to break those bonds and liquify the substance. This quickly highlights the fourth option as the correct answer. But, let’s go through the other statements one by one.

The first statement is invalid because the structuring question does not have layers. What the statement might apply to is a substance like graphite. The second statement, it contains delocalized electrons, is also invalid because there are no delocalized electrons. This statement would apply if we were looking at a metal. The third statement is not correct because a lattice of oppositely charged ions is not covalent. And what we’re dealing with here is a covalent structure. This statement would apply to something like sodium chloride. It’s actually the strong bonds between the atoms in this covalent lattice that give it such a high boiling point. Therefore, this is the correct answer.

Figure two shows the average composition of one type of bronze. Bronze is an alloy of copper and tin that is commonly used in statues and other sculptures. Atoms of tin are larger than those of copper. Explain why the difference in atom size makes bronze more suitable than copper for the production of sculptures.

First, let’s reassure ourselves that the diagram is correct. Tin atoms are in fact larger than copper atoms. We can see that these larger tin atoms are getting in the way of the neat organization that is natural for the copper atoms. This is how the copper atoms would be arranged if the tin was not there. In pure copper, because of the regular arrangement of atoms, any force applied to a row or a sheet will force that row or sheet along.

This compliance with external forces is what makes pure copper soft. A similar factor also has some part to play in the strength of the material. So pure copper is weaker than when the tin atoms are added. Something we really want from a sculpture is for it to keep its shape and for it to be hard wearing. So we have the first part of our answer. Sculptures must be hard and strong to resist weathering and other damage over time.

The introduction of these tin atoms disrupts the layers of copper atoms and makes them far more resistant to being moved by external forces, making the material harder and stronger. We therefore have the second part of our answer. Bronze is harder and stronger than copper because the tin atoms disrupt the layers of copper atoms making it more difficult for the layers of atoms to slide over each other.

What percentage of atoms in this alloy are copper atoms?

The percentage of copper atoms in this alloy is equal to the number of copper atoms divided by the total number of atoms multiplied by 100 percent. So what we need to do is count the total number of copper atoms in figure two and then the total number of atoms in figure two and plug those into the equation.

The total number of copper atoms is equal to one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22. Meanwhile, the number of tin atoms is equal to one, two, three. The total number of atoms is equal to the number of copper atoms plus the number of tin atoms. This is equal to 22 plus three which is equal to 25. The percentage of atoms that are copper is equal to 22 divided by 25 multiplied by 100 percent, which is equal to 88 percent. So the percentage of atoms in the alloy that are copper atoms is 88 percent.

What type of bonding occurs between the atoms in this alloy? Covalent, ionic, metallic, or intermolecular.

For this question, we have to look back at figure two and consider what exactly makes up the alloy. The alloy is a mixture of tin and copper. In their pure forms, tin and copper are both metals. Therefore, the bonding between these atoms should be metallic. Therefore, the answer to this is metallic. Between two metals, we would not see covalent or ionic bonding. And since we aren’t dealing with molecules, we can’t have intermolecular forces.

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