Pure and impure substances Flashcards
Evaporation occurs when a ___ slowly turns into a ___ below its boiling point.
liquid
gas
Crystallisation is a ___ technique used to obtain ___ of a ___ solute
separation
crystals
solid
When a ____ is heated, the ___ evaporates and ___ of the ___ are left behind
solution
solvent
crystals
solute
What would be left behind if a bowl of salty water was left out in the sun?
Salt crystals. The water would evaporate so the dissolved salt would end up left behind in the bowl.
Why do salt crystals appear faster in the salt solution when it is heated using the Bunsen burner?
Because the hotter a liquid is, the more energy the particles have, so the faster the liquid will evaporate.
What is evaporation?
Evaporation describes the process of a liquid turning into a gas.
What is faster? Evaporation or Boiling?
Evaporation is a slower process than boiling because it occurs at temperatures below the boiling point of the liquid.
Why do puddles dry up?
Puddles dry up because of evaporation. Puddles do not boil because the temperature on earth is always below 100°C - the boiling point of water.
How is evaporation involved in the water cycle?
Water evaporates from the seas, oceans, lakes and rivers. It condenses in clouds to form rain.
What is a technique to separate a solid solute from a solvent in a solution?
Evaporation
The solvent is the liquid, and when the solution is heated, the solvent evaporates. The solute is left behind as crystals.
Describe and explain how evaporation can be used to obtain copper sulfate crystals from copper sulfate solution.
Copper sulfate solution is placed into an evaporating basin and heated using a Bunsen burner. The water (the solvent) heats up and evaporates. The copper sulfate (the solute) does not evaporate. The copper sulfate turns into dry crystals when all the water has evaporated.
What is a solute?
The solid (or occasionally a gas) which dissolves into a solvent (liquid) in order to make a solution. For example, the main solute in sea water is sodium chloride.
What is a solvent?
The liquid in a solution which dissolves the solute. For example, the solvent in sea water is water.
What is a solution?
A mixture made when a solute (usually a solid) dissolves into a solvent (a liquid). Sea water is a solution of salt dissolved into water.
What is are two techniques to separate a solid solute from a solvent in a solution?
Evaporation
Crystallisation
Which technique to separate a solute from a solvent is faster? Evaporation or Crystallisation?
Evaporation is faster.
Crystallisation is similar to the evaporation technique used to separate a solid solute from a solution but is done over a longer period of time.
What produces larger crystals? Evaporation or Crystallisation?
Crystallisation produces the largest possible crystals of the solute, because the slower the evaporation process, the larger the crystals.
Desribe the 4 steps of the Crystallisation process
- Rather than heating the evaporating basin directly with a Bunsen burner, it is often better to warm it gently and slowly over a beaker of boiling water.
- Heat the solution until half the solvent (water) has evaporated, which makes the solution more saturated.
- Leave the solution to cool and large crystals will form slowly.
- Put the evaporating basin in a warm place so the rest of the water evaporates.
What is the main difference between what is produced in crystallisation and evaporation?
Crystal size.
This is because crystallisation produces larger crystals than evaporation.
What term is used to describe a mixture which contains a solid dissolved in a liquid?
Solution
What term describes the liquid in a solution?
Solvent
What is the name of the process when a liquid turns into a gas below the boiling point of the liquid?
Evaporation
What term describes a solution which contains the maximum amount of solute?
Saturated
What is the best way to get the largest possible crystals when evaporating a solution?
Warm the solution gently for a long time
What piece of apparatus is usually used to heat up a solution during evaporation?
Bunsen burner
Which word is used for the solid which has dissolved to make a solution?
Solute
Distillation is a separation technique used to separate a ___ from a ____.
solvent
mixture
Water can be separated from salt solution by ____.
distillation
What two processes occur in the water cycle to turn sea water into rain water?
Evaporation and condensation.
Water evaporates from the surface of the sea. The salt gets left behind in the sea. Then, the water vapour in the clouds cools and condenses to form rain.
What is the name of the piece of apparatus that cools down the hot gas after the mixture has been heated?
Condenser.
Distillation involves___ the solution and then ___ the___ back into a liquid by ___ it down.
boiling
condensing
vapour
cooling
For example,___ water can be distilled to produce ___ water, which is safe to drink.
dirty
pure
Name 6 Separation Techniques
Evaporation
Crystallisation
Distillation
Fractional Distillation
Filtration
Chromatography
Which two changes of state occur during distillation?
Boiling (liquid to gas), and then condensing (gas to liquid).
Desribe the steps for distillling a salt solution
The salt solution is placed into a flask and heated until it boils.
The water turns into a gas but the salt stays behind in the flask.
The steam passes into the condenser. The condenser is a tube which is surrounded by a layer of cold water. This cools the steam, which turns it back into a liquid.
The distillate is pure water.
When a solution made from salt dissolved in water is distilled, the temperature reading on the thermometer will be 100°C.
Why is this?
100°C is the boiling point of the water.
Ethanol boils at 78°C, whereas water boils at 100°C. A mixture of ethanol, water and salt can be separated using ___.
Ethanol boils at 78°C, whereas water boils at 100°C. A mixture of ethanol, water and salt can be separated using fractional distillation.
Describe the 3 steps to separate Water and ethanol
- Heat the mixture in a flask until the gas above the mixture is 78°C. The ethanol boils and can be re-captured using a condenser in the same way as the water in the salt water example.
- Keep the temperature at 78°C until the liquid ethanol coming from the condenser stops or slows down to almost nothing.
- The water will remain in the original flask.
___ is a technique also used in the chemical industry to obtain petrol and diesel from crude oil (a fossil fuel).
Fractional distillation