Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
24 questions
Unit 9 · Matter
1. Fill in the blanks with suitable words from the box
A substance made of two or more substances mixed physically is called a __________.
Answer
mixture
The substances that make up a mixture are called the __________ of the mixture.
Answer
components
The components are mixed evenly in a __________ mixture.
Answer
homogeneous
Raksi is made in villages by the __________ method.
Answer
distillation
In the centrifuging method, the heavy substances collect at the __________ of the test tube.
Answer
bottom
(evaporation is the word left over.)
3. Write true or false
Air is a mixture of different gases.
Answer
True. Air is mostly nitrogen and oxygen, with argon, carbon dioxide, water vapor and other gases.
A solution of salt water is a heterogeneous mixture.
Answer
False. Salt water is a homogeneous mixture: the salt is spread evenly and can’t be seen.
Colors can’t be separated by the centrifuging method.
Answer
True. Dissolved colors are spread evenly, and they don’t settle when spun in a centrifuge. They are separated by chromatography.
Alcohol can be made by the distillation method.
Answer
True. Alcohol is separated from fermented grain mash by distillation, as in making raksi. (The alcohol itself forms during fermentation; distillation collects it.)
Centrifuging is used to separate blood cells.
Answer
True. In a centrifuge, the heavier blood cells go to the bottom of the tube, and the plasma stays on top.
4. Differentiate between
Homogeneous mixture and heterogeneous mixture
Answer
| Homogeneous mixture | Heterogeneous mixture |
|---|---|
| The components are mixed evenly. | The components are not mixed evenly. |
| The components can’t be seen or told apart with the naked eye. | The components can be seen and told apart with the naked eye. |
| Every part is the same. | Different parts differ. |
| Example: salt water, sugar water, air. | Example: muddy water, sandy water, rice with stones. |
Solvent and solute
Answer
| Solvent | Solute |
|---|---|
| The substance in which another substance dissolves. | The substance that dissolves in the solvent. |
| Usually there is more of it in a solution. | Usually there is less of it in a solution. |
| A solution has one solvent. | A solution can have one or more solutes. |
| Example: water in sugar water. | Example: sugar in sugar water. |
5. Give reasons
Sugar water is called a homogeneous (uniform) mixture.
Answer
In sugar water, the sugar dissolves and spreads evenly all through the water, so every part is the same and the sugar can’t be seen with the naked eye. A mixture whose components are mixed evenly like this is a homogeneous mixture.
A mixture of different colors is separated by chromatography.
Answer
When a liquid moves through paper, it carries the colors along with it, but each color travels at its own speed. The fast ones go further and the slow ones stay behind, so the colors come apart. Dissolved colors can’t be separated by filtering, settling or spinning, but chromatography can separate them.
The centrifuging method is used to separate light and heavy substances in a mixture of a liquid and solids.
Answer
When the mixture is spun very fast, the heavier substances are pushed outwards hardest, to the bottom of the container, while the lighter ones stay above them. So the heavy and light substances separate, much faster than if the mixture were simply left to settle.
The evaporation method is used to separate salt from salt water.
Answer
Salt is dissolved in the water, so it can’t be caught by a filter. But water turns into vapor easily when heated, while salt doesn’t (it would need well over 1000 °C to boil). So heating the salt water drives the water off as vapor and leaves the salt behind.
6. Answer the following questions
What is a mixture?
Answer
A mixture is two or more substances mixed together physically, without joining to form a new substance. Each keeps its own properties, and they can be separated by simple methods. Examples: soil, salt water, milk, air.
What kind of mixture is called a uniform (homogeneous) mixture? Give two examples.
Answer
A mixture whose components are mixed evenly all through, so that they can’t be seen or told apart with the naked eye, is a uniform or homogeneous mixture. Examples: salt water and sugar water (also soda water, air).
Which of muddy river water and salt water is a heterogeneous mixture? Give your reason.
Answer
Muddy river water is the heterogeneous mixture. The soil in it is not mixed evenly: you can see its bits, the water is cloudier in some places than others, and when it is left to stand, the mud settles at the bottom. In salt water, the salt is spread evenly and can’t be seen, so salt water is homogeneous.
What kind of mixture can be separated by distillation? Write its properties.
Answer
Distillation separates a mixture in which a liquid is mixed with something that boils at a very different temperature. For example:
- a solid dissolved in a liquid, such as salt in water: the water boils off long before the salt, and is cooled back and collected as pure water;
- two liquids that mix evenly but boil at different temperatures, such as alcohol (about 78 °C) and water (100 °C): the one that boils first is collected first.
The liquid we want must turn into vapor and back into liquid without being harmed by the heat.
Explain where and why the centrifuging method is used.
Answer
Centrifuging is used where we need to separate heavy and light substances in a liquid quickly:
- in dairies, to separate cream from milk (and, by churning, butter from curd);
- in hospitals and laboratories, to separate blood cells from plasma for blood tests, and to separate the solid parts of urine and other samples;
- in a washing machine, to spin water out of clothes.
It is used because spinning very fast pushes the heavier parts to the bottom much faster and more completely than leaving the mixture to settle on its own.
Which method would you use to separate the different chemicals in blood? Give a reason.
Answer
Chromatography. The chemicals dissolved in blood are mixed evenly and can’t be filtered out or spun down. In chromatography, a liquid carries the chemicals along a strip (or a tube), and each travels at its own speed, so they come apart and can be identified. (To separate only the blood cells from the liquid plasma, centrifuging would be used.)
Discuss the uses of solutions in our daily life.
Answer
- Plants take in water and mineral salts from the soil as a solution, through their roots, to make their food.
- The food we eat and the medicines we take are absorbed into the blood as solutions after digestion.
- Animals and plants that live in water breathe the oxygen dissolved in it.
- Many medicines, drinks and juices are solutions: Jeevan Jal saves people with diarrhea; tea and sarbat are solutions; salt water is used for gargling; spirit and sanitizer kill germs.
You are given salt water in one glass and pure water in another. Without tasting them, how can you find which is pure water? Use a method of separating mixtures.
Answer
Use evaporation. Pour a little from each glass into two clean, dry spoons or pans (or two clean pieces of glass), and heat them gently, or leave them in the sun, until all the water has gone.
- The one that leaves a white crust of salt behind held salt water.
- The one that leaves nothing behind held pure water.