Video: GCSE Chemistry Foundation Tier Pack 1 • Paper 2 • Question 1

GCSE Chemistry Foundation Tier Pack 1 • Paper 2 • Question 1

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Video Transcript

This question is about elements, mixtures, and analysis. Which two of the following substances are not mixtures? Tick two boxes.

The first thing we need to answer is “what is a mixture?” A mixture is two or more mixed substances not chemically bonded together. Air is a good example of a mixture. Nitrogen and oxygen gases are mixed together, but they are not chemically bonded.

Let’s look at the first option, milk. Milk is a suspension of fat in water. Even though they’re mixed, they are not chemically bonded together. Therefore, milk is a mixture. Since milk is a mixture, it cannot be a correct answer.

Now, let’s have a look at Steel. Steel is a little bit more of a tricky one. Steel is a combination of iron atoms with carbon atoms. The truth is that there is some level of interaction between these atoms. So in the purest sense, steel doesn’t fit the definition of a mixture. However, it is very common for it to be called a mixture because the interactions between the iron and the carbon aren’t purely covalent or ionic. Since steel is commonly called a mixture, it is not the correct answer.

Now, let’s have a look at diamond. Diamond is what’s called an allotrope of carbon, one of the potential structures of the pure element. It is made exclusively out of carbon atoms and therefore is not a mixture. Therefore, diamond is one of the correct answers.

Now, let’s look at calcium. Calcium is made up of calcium atoms. It is only made out of calcium atoms. Therefore, it is not a mixture; it is an element. Therefore, calcium is the second correct answer.

Just to be safe let’s have a look at syrup. Syrup is a mixture of sugar and water. They are not chemically bonded; they are, therefore, a mixture. Therefore, syrup is not a correct answer.

Therefore, the two substances which are not mixtures are diamond and calcium.

Draw one line from each context to the correct meaning. The contexts are elements in chemistry and elements in everyday life. The meanings are a substance composed of only one type of atom, the reactants in a chemical reaction, a pure substance, a part of something, or a substance composed of two or more chemically bonded types of atom.

What this question is testing is your understanding of chemical terminology, but also your understanding that words have different meanings in different contexts. The word elements in chemistry has a very specific technical meaning. But in everyday life, the word elements is used more loosely.

Let’s start with elements in chemistry. At the moment, there are 118 chemical elements. They’re all neatly arranged in the periodic table. They have properties like mass number, symbols, and atomic number.

Now, which meaning probably defines an element in chemistry? Is it a substance composed of only one type of atom? Yes, an element is an example of a pure substance, where only one type of atom with only one type of nucleus makes it up.

But let’s look at the other statements just to be safe. The reactants in a chemical reaction have a special name; they’re called reactants. So we need not search for another name for them.

As for being a pure substance, well elements are pure substances. But not all pure substances are elements. Compounds can be pure substances too if they aren’t mixed with other substances.

What about elements being a part of something? Well, we’re going to come back to that in a second when we talk about elements in everyday life. As for a substance composed of two or more chemically bonded types of atom, well you should recognise that as the definition of a compound.

Now that we’ve reassured ourselves that elements and chemistry are substances composed of only one type of atom, let’s look at elements in everyday life. Another name for element is part, like the sugar that goes into a cake; the sugar is part of the cake. So the meaning of elements in everyday life is a part of something.

So we’ve just drawn one line from each context to the correct meaning.

What is the test for carbon dioxide gas? Tick one box. Turning damp litmus paper white, generating a pop when exposed to a lit splint, relighting a glowing splint, or turning limewater cloudy.

Let’s start by having a think about what we know about carbon dioxide. Firstly, we know that it’s composed of one carbon atom, two oxygen atoms and has the symbol CO2. Secondly, we know that it’s produced when something containing carbon burns. Given that it’s also used in fire extinguishers, we know that it’s not flammable.

Now, let’s have a look at each of the options and see whether this corresponds to the behaviour we expect from carbon dioxide. Turning damp litmus paper white is result of bleaching. This is not a property commonly associated with carbon dioxide. Therefore, this is not the correct answer. This is in fact the test for chlorine gas which is bleaching.

Now, let’s have a look at generating a pop when exposed to a lit splint. This is suggestive of something that’s flammable, something that reacts with oxygen when given enough energy, for instance, by a flame. Since we know carbon dioxide to be not flammable, this is not a correct answer. In fact, generating a pop when exposed to a lit splint is the test for hydrogen.

Now, let’s have a look at relighting a glowing splint. This is suggestive of something that supports burning. Since we know that carbon dioxide is actually used to suppress burning, this is not the correct answer. Relighting a glowing splint is in fact the test for oxygen.

This leaves us with turning limewater cloudy. Lime water is a solution of calcium hydroxide. When carbon dioxide is bubbled into limewater, the solution turns cloudy due to the production of calcium carbonate. Carbon dioxide gas turns limewater cloudy. Therefore, this is the correct answer.

A student mixed an unknown chloride salt with dilute sodium hydroxide. The gas produced turned damp red litmus paper blue. Which cation must have been present in the salt? Tick one box. Iron(II) cation, sodium cation, ammonium cation, or magnesium cation.

Now, if you look closely, there are four bits of information in the question that will help you answer it. The first piece of information is that you can form a chloride salt. Let’s check against all the options. Iron(II) forms iron(II) chloride, sodium forms sodium chloride, ammonium forms ammonium chloride, and magnesium forms magnesium chloride.

We haven’t really got any further because of this information. So let’s look at the second piece. The second piece of information is that the salt reacts with dilute sodium hydroxide. Iron(II) reacts with sodium hydroxide to form a green precipitate of iron hydroxide.

If you mix sodium chloride with dilute sodium hydroxide, all you get is a mixed solution of the two. There is no reaction. However, if you mix ammonium chloride with sodium hydroxide, you produce ammonia, which brings us to our third piece of information whether the reaction produces a gas.

In the case of ammonium, the reaction does produce a gas. But the reaction of iron chloride does not produce a gas; it produces a precipitate of iron hydroxide. Now, let’s have a look at magnesium.

When magnesium chloride is mixed with dilute sodium hydroxide, you get a precipitate, a white precipitate of magnesium hydroxide. Again, this is a solid precipitate, not a gas. Therefore, it doesn’t pass the gas test.

So far, the only ion to pass all the tests is the ammonium cation. And we didn’t even have to use the fourth piece of information: the fact that the gas turns damp red litmus paper blue. This is in fact the test for basic ammonia gas. Red litmus paper turns blue when exposed to a base.

So our answer for which cation must have been present in the salt is the ammonium cation because it produced ammonia gas when reacting with dilute sodium hydroxide.

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