Preparing solutions
A solution is a mixture that looks the same everywhere. The solute (for example salt) dissolves in the solvent (for example water).
To prepare a solution of a given concentration:
- Calculate the mass of solute you need.
- Weigh it on a balance using a weighing boat.
- Pour it into a volumetric flask using a funnel.
- Add some water and swirl until the solid dissolves.
- Add water until the bottom of the curved water surface (meniscus) touches the mark. Put the stopper on and turn the flask upside down a few times to mix.
Concentration tells how much solute is in a volume of solution.
Mass concentration = mass of solute (g) ÷ volume of solution (L). Example: 5 g of salt made up to 100 mL (0.100 L) gives 5 ÷ 0.100 = 50 g/L. This is the same as 5 g per 100 mL, or 5 % (m/V).
Dilution: to make a weaker solution from a stronger one, use C1 × V1 = C2 × V2.
- Always wear goggles. Add concentrated acid to water, never water to acid.
- Label every bottle with the name, concentration, date and your name.
Filtering
Filtration separates an insoluble solid from a liquid. Fold a circle of filter paper into a cone and fit it in a funnel. Wet the paper so it sticks to the glass. Pour the mixture slowly down a glass rod. Liquid passes through tiny holes in the paper; solid pieces are too big to pass.
- Residue: what stays on the paper.
- Filtrate: the liquid that goes through.
Keep the paper edge below the rim of the funnel and the funnel tip against the side of the beaker so liquid does not splash. A filter cannot remove a dissolved solute: salt water goes right through the paper. For that you need evaporation or distillation.
Preparing microscope slides
A microscope needs a sample thin enough for light to pass through. Steps for a wet mount:
- Place a clean glass slide on the table.
- Put a small drop of water (or stain) in the middle.
- Use forceps to place a very thin piece of sample (for example onion skin) in the drop. Spread it flat.
- Touch one edge of the coverslip to the drop. Lower it slowly at a slant using a needle. This pushes air out, so there are no bubbles.
- Soak up extra liquid with blotting paper.
A stain (like iodine for onion or methylene blue for cheek cells) colours parts of the cell so you can see the nucleus. Start with the lowest magnification, focus, then go up. Carry the microscope with two hands.
Cells and tissue types
The cell is the basic unit of life. A tissue is a group of similar cells that do one job together. Plant cells have a cell wall, so they look like neat bricks. Animal cells have no wall.
| Animal tissue | What it looks like | Job |
|---|---|---|
| Epithelial | Cells packed tightly in a sheet | Covers and lines (skin, gut) |
| Muscle | Long fibres with stripes | Contracts to make movement |
| Nerve | Cell body with branches and a long fibre | Carries signals |
| Connective | Cells spread in a matrix (blood, bone, fat) | Supports, joins, carries |
Cells are tiny: a typical cell is 10 to 100 micrometres (µm) wide. 1 mm = 1000 µm. That is why we need a microscope. Total magnification = eyepiece power × objective power (for example 10 × 40 = 400).
Comparing research methods
Scientists choose a method to fit the question.
| Method | What you do | Strength | Limit |
|---|---|---|---|
| Observation | Watch and record without changing anything | Natural, good for new questions | Cannot prove cause |
| Experiment | Change one variable, keep others the same | Can show cause and effect | Conditions are artificial |
| Survey | Ask many people or sample many places | Shows patterns in large groups | Answers may be wrong or biased |
| Model / simulation | Use a drawing, equation or computer | Safe, fast, cheap | Only as good as its assumptions |
Example: to find if salt changes how fast ice melts, do an experiment. To find which bird visits a garden, use observation. To find how many students use a library, use a survey.
Try it at home: salt, sand and a strainer
- Mix 1 spoon of sand and 1 spoon of salt in a glass of water. Stir.
- Pour the mixture through a coffee filter or clean cloth into another glass. What stays behind? Which is the residue?
- Look: the filtrate is clear. Is the salt gone? No. It is dissolved and went through the filter. (Never taste things in a lab.)
- Predict, then check: how will you get the salt back from the filtrate? (Heat it gently until the water is gone, with an adult helping.)
Key formulas and definitions
- Concentration (g/L) = mass of solute (g) ÷ volume of solution (L)
- % (m/V) = mass of solute (g) ÷ volume (mL) × 100
- Dilution: C1 × V1 = C2 × V2
- Filtrate = liquid that passes through; residue = solid that stays
- Total magnification = eyepiece × objective
- 1 mm = 1000 µm
Worked examples
1. You dissolve 12 g of sugar in water and make the solution up to 400 mL. Find the concentration in g/L.
400 mL = 0.4 L. Concentration = 12 ÷ 0.4 = 30 g/L.
2. How much sodium chloride is needed to make 250 mL of a 20 g/L solution?
250 mL = 0.25 L. Mass = 20 × 0.25 = 5 g.
3. You have a 100 g/L stock solution. How do you make 200 mL of a 10 g/L solution?
C1V1 = C2V2: 100 × V1 = 10 × 200, so V1 = 20 mL. Measure 20 mL of stock and add water to make 200 mL.
4. A mixture of chalk powder and water is filtered. Name the residue and the filtrate.
Chalk powder is the residue (stays on the paper). The filtrate is clear water.
5. An eyepiece is 10× and the objective is 40×. What is the total magnification? A cell looks 2 mm wide in the view. How wide is it really, in µm?
Total = 10 × 40 = 400×. Real width = 2 mm ÷ 400 = 0.005 mm = 5 µm.
Common mistakes
- Adding the solvent to the mark in the beaker, not the flask. A beaker is not accurate; use a volumetric flask.
- Forgetting to change mL to L before working out g/L.
- Thinking a filter takes out dissolved salt or sugar. It only stops undissolved solids.
- Dropping the coverslip flat so air bubbles are trapped.
- Using the high-power objective first. Always start with the lowest magnification.