What are Mixtures?
Welcome to today’s science lesson. Let shine a bright light an important topic in science today: mixtures. What are they? What different types of mixtures are there? How important are they in our daily lives? Get set and go!
Helen went on a sea voyage with her family. They had a lovely yacht and the weather was beautiful. A problem arose though, in the second week. Their drinking water was over. Helen’s father agonized over how to get drinking water. Helen couldn’t understand the problem. There was water in the sea after all, she thought. “Oh! Helen, sea water is not pure. It’s a mixture,” her father said. “Really? What are mixtures?” Helen asked. “Well, why don’t you ask your science teacher?” her father said, as he got busy preparing a mechanism by which sea water could be purified.
As soon as she reached school after their trip, Helen went to Mr. Henderson, her science teacher. She asked, “Mr. Henderson, what are mixtures?”
Mr. Henderson: Hi Helen, glad to have you back. And yes, I will tell you all about mixtures. As your father said, sea water is indeed a mixture.
Helen: What are some examples of mixtures other than sea water, Mr. Henderson?
Mr. Henderson: Air and crude oil.
Helen: I see.
Mr. Henderson: A mixture, you see, consists of two or more substances that are mixed, but are not chemically combined together.
Helen: Are the substances that make up a mixture elements?
Mr. Henderson: Well, they could be elements. They could also be compounds.
Helen: What are the components of air?
Mr. Henderson: Nitrogen, oxygen, carbon dioxide, water vapour and noble gases.
Helen: Oh! Can these be separated from air?
Mr. Henderson: Definitely. That can be done by physical methods, for example, fractional distillation.
Helen: Ok. Does it mean that when a mixture is formed, there is no chemical reaction?
Mr. Henderson: You are right. There is no chemical reaction. So, the mixture has the properties of its constituent substances.
Helen: That is interesting. I know that oxygen supports combustion. So, air also must support combustion because air contains oxygen.
Mr. Henderson: Excellent. You are right.
Helen: In a mixture, are the constituents present in any fixed proportion?
Mr. Henderson: Not at all. For example, there is more carbon dioxide in the air in a city, compared to the air in a jungle.
Helen: That is interesting. So, this is one difference between mixtures and compounds. I know that in compounds there is fixed proportions of constituents.
Mr. Henderson: That is right. Another difference is that compounds are formed by chemical reaction whereas mixtures are not formed by chemical reactions.
Helen: Right. And I guess compounds cannot be broken down by physical methods too.
Mr. Henderson: Yes. Did you know that your gold ear rings are made of a mixture of metals rather than pure gold?
Helen: Really? I didn’t know that.
Mr. Henderson: Gold ornaments contain metals such as copper, zinc and silver too. But of course, gold is present in a larger amount than these metals.
Helen: Why can’t pure gold be used in making gold ornaments? Wouldn’t it look better?
Mr. Henderson: Pure gold is very soft. Too malleable and ductile. It is made stronger by adding other metals for making jewelry.
Helen: I see. Mr. Henderson, you said crude oil is also a mixture.
Mr. Henderson: Yes, you are right.
Helen: So, crude oil can also be broken down into constituents by fractional distillation, right?
Mr. Henderson: Yes, you are right. In fact, many of the substances that we use in our daily lives are obtained from crude oil by fractional distillation.
Helen: Really?
Mr. Henderson: Yes, we get petroleum gas, petrol, naphtha, kerosene, diesel, lubricating oil and bitumen from it.
Helen: Wow. That is so interesting. I wonder how fractional distillation works.
Mr. Henderson: Fractional distillation is used when a mixture has more than two liquids in it and they have different boiling points. When crude oil is heated, the components with a lower boiling point boil out first.
Helen: Amazing. By the way, my father said he used a method called distillation. To get drinking water from sea water when we were out in the sea.
Mr. Henderson: Oh, I see. He would have boiled the water, and then collected the condensation.
Helen: Yes, that is what he said.
Mr. Henderson: See, chemistry is of use even while you are on vacation!
Helen: Haha. You are right, Mr. Henderson. It is getting late. I have to go. Bye.
Mr. Henderson: Bye Helen. See you in class.