practical-assessment — Chemistry (1.3. ELEMENTS, COMPOUNDS AND MIXTURES)

MINISTRY OF EDUCATION

KASEMPA DAY SECONDARY

NATURAL SCIENCE

INSTRUCTIONS:

1. Answer ALL questions in the spaces provided.

2. Show all working for calculations.

3. Use correct units where applicable.

4. Follow all safety precautions during activities.

5. Marks for each question are shown in brackets.

PART A: PRACTICAL EXPERIMENTS [20 marks]

EXPERIMENT 1: PREPARING AND OBSERVING SIMPLE MIXTURES

AIM:

To prepare three simple mixtures and record observations to classify them as homogeneous or heterogeneous.

THEORY:

A mixture is a substance made of two or more substances physically combined. Mixtures may appear uniform (homogeneous) or non-uniform (heterogeneous). This activity asks learners to make and observe simple mixtures without performing chemical reactions or separations.

APPARATUS AND MATERIALS:

250 mL beakers — 3 per station

Measuring cylinder, 100 mL — 1 per station

Distilled or clean water — 200 mL per station

Table salt (NaCl) — 5 g per sample

Sand — 5 g per sample

Sugar — 5 g per sample

Stirring rod or spoon — 1 per station

Labels and marker — 3 labels per station

White tile or plain white paper — 1 per station

PROCEDURE:

1. Label three clean beakers A, B and C.

2. Beaker A: add 50 mL water. Add 2 g table salt and stir until no crystals are visible.

3. Beaker B: add 50 mL water. Add 2 g sand and stir for 10 seconds. Allow the mixture to stand for 1 minute.

4. Beaker C: on a white tile or paper, mix 2 g table salt and 2 g sugar thoroughly with a spoon (dry solid-solid mixture).

5. Observe the appearance of each beaker/sample immediately after mixing and after standing for one minute. Record observations in the results table.

OBSERVATIONS AND RESULTS:

PRECAUTIONS:

CONCLUSION:

[Experiment 1 Total: 12 marks]

EXPERIMENT 2: USING MODELS AND DIAGRAMS TO REPRESENT MIXTURES AND COMPARE WITH COMPOUNDS

AIM:

To model and draw particle diagrams of homogeneous and heterogeneous mixtures and state differences between mixtures and compounds.

THEORY:

Particle diagrams show the positions and arrangements of particles in substances. Diagrams help distinguish mixtures (two or more types of particles not chemically joined) from compounds (different atoms chemically bonded so particles are identical molecules). This activity uses models and drawings to practise these ideas.

APPARATUS AND MATERIALS:

Coloured counters or beads — two colours, ~40 pieces per station

Plain paper and pencils — 2 sheets and 1 pencil per station

Small trays or cups to hold beads — 1 per station

Labels — 3 small labels per station

PROCEDURE:

1. Using two colours of beads, make a model of a homogeneous mixture: mix beads so both colours are evenly distributed across the tray.

2. Make a model of a heterogeneous mixture: arrange the beads in groups or clusters so the two colours are not evenly mixed.

3. On paper, draw labelled particle diagrams of both models and answer the comparison questions.

OBSERVATIONS AND RESULTS:

PRECAUTIONS:

CONCLUSION:

[Experiment 2 Total: 8 marks]

RESULTS SUMMARY TABLE:

PART B: ANALYSIS AND CONCLUSIONS [13 marks]

DOCUMENT 2: LABORATORY ASSISTANT MATERIALS REQUISITION

Total Students: 60 Number of Stations: 60 Students per Station: up to 1

MATERIALS REQUIRED:

STATION SETUP INSTRUCTIONS:

Set out one full station per pupil spaced to avoid cross-contamination.

Provide pre-measured sachets of solids if time-saving is needed.

Place labels, goggles and paper at each station before the lesson.

Arrange class waste bucket and provide instructions for waste disposal.

ADDITIONAL REQUIREMENTS:

Laboratory bench or table for each station

Access to running water and a sink

Good ventilation and a clear working area

First aid kit and eye-wash facility accessible

PREPARATION NOTES FOR LABORATORY ASSISTANT:

Pre-measure solids into labelled sachets for each station (optional but recommended for timing).

Fill shared supply bottles of water or ensure access to a tap.

Provide bead sets (two colours) in small cups for each station.

Ensure all safety equipment (goggles) is available at each station.

DOCUMENT 3: MARKING KEY (TEACHER ONLY)

TOTAL: 38 MARKS

General guidance: All marks must be awarded as per the mark breakdown provided in each question's model answer. Award full marks only where the student's response meets the criteria given. Partial credit should reflect proximity to the expected scientific idea.

Observational marks: For observation items, credit clear, specific statements about appearance (e.g. 'clear', 'cloudy', 'particles suspended', 'particles settled') rather than vague terms.

Diagrams: For drawing questions award marks for correct conceptual arrangement and labelling. For the homogeneous diagram look for even distribution of both particle types; for the heterogeneous diagram look for visible clustering or separation. Minor artistic variation is acceptable.

Terminology: Accept simple, correct vocabulary from Form 1 level (e.g. 'dissolved', 'suspended', 'settled', 'uniform', 'non-uniform', 'homogeneous', 'heterogeneous').

Safety/Precautions: Credit any sensible safety precaution (no taste, goggles, clean spills).

Skills assessment: Observe each learner performing the mixing and recording tasks. Use the stated criteria to award skill marks; record observations on the pupil's practical sheet.

Acceptable alternatives: Where answers give scientifically correct alternatives (e.g. naming different everyday examples), award marks provided the student correctly identifies the mixture type and offers correct reasoning.

Incorrect substance identification: If a student misidentifies a sample but gives consistent reasoning (e.g. calls a sand-water mixture a solution because it looked 'mixed'), award observation marks only where the recorded observation is correct; do not award classification marks unless justification matches correct science.

Part A total: 20 marks. Part B total: 15 marks. Part C skills total: 5 marks. Ensure adding matches the paper totals.

Recordkeeping: Teachers should annotate practical sheets to indicate partial credit and the reason for each awarded or withheld mark to support moderation.

EXPERIMENT 1: Preparing and observing simple mixtures

1(a). Expected answer (1 mark): Clear, uniform liquid; salt crystals have dissolved and cannot be seen (1 mark). Alternative phrasing: 'a clear solution, no visible particles' (1 mark). [1 marks]

1(b). Expected answer (2 marks): Immediately after stirring: water looks cloudy or contains visible suspended particles (1 mark). After standing: sand settles to the bottom, forming a layer and clearer water above (1 mark). Explanation: sand particles do not dissolve so they remain as solid particles which settle due to gravity. [2 marks]

1(c). Expected answer (3 marks): Appearance: visible separate crystals or grains of two different solids; not uniform in appearance (1 mark). Classification: heterogeneous (1 mark). Explanation: components remain as distinct particles and are not uniformly mixed at the visible scale (1 mark). Acceptable alternative: if student notes 'appears uniform if finely ground' award first mark for appearance and allow 1 mark for recognising particle differences if justified. [3 marks]

1(d). Expected answers (2 marks total): One homogeneous example (1 mark): e.g. sugar dissolved in tea, salt solution, air (acceptable alternatives: soft drink after fully mixed). One heterogeneous example (1 mark): e.g. sand in water, oil and water, cereal in milk. Award the mark when the example is correct and labelled homogeneous or heterogeneous. [2 marks]

1(e). Expected drawing (3 marks): Beaker A (1.5 marks): uniform distribution of small dots representing water with a uniform distribution of tiny dots or different-coloured dots representing dissolved salt spread evenly — label 'salt ions/particles' and 'water'. Beaker B (1.5 marks): larger distinct particles (sand) shown as clusters or separate dots within a continuous medium (water) with some particles settled toward bottom — label 'sand particles' and 'water'. Marking: award proportional credit for correct arrangement and labelling; 1.5 marks each, allow 0.5–1.5 for partial diagrams showing correct concept. [3 marks]

1(f). Expected answer (1 mark): e.g. 'Wear safety goggles to protect eyes from splashes' or 'Do not taste any samples' or 'Clean up spills immediately to avoid slipping'. Award 1 mark for any appropriate safety precaution. [1 marks]

EXPERIMENT 2: Using models and diagrams to represent mixtures and compare with compounds

2(a). Expected drawings (4 marks): (a) Homogeneous: two-colour particles evenly and randomly mixed throughout the box; label each colour/component (2 marks). (b) Heterogeneous: visibly separated regions or clusters of one colour and the other colour with non-uniform distribution; label components (2 marks). Award partial credit for correct idea and labelling. [4 marks]

2(b). Expected answers (4 marks): Award 1 mark for each correct, distinct difference up to 4 marks. Typical differences: 1) Composition: A mixture contains two or more substances physically combined in any ratio; a compound contains elements chemically combined in fixed proportions (1 mark). 2) Separation: mixtures can be separated by physical methods (e.g. filtration, evaporation, picking) while compounds require chemical reactions to separate elements (1 mark). 3) Properties: a mixture's components retain their original properties; a compound has different properties from its constituent elements (1 mark). 4) Particle level: in a mixture different particles remain as distinct types; in a compound particles (molecules or units) are chemically bonded and identical (1 mark). Accept any two of these, awarding 2 marks each if answers are complete; if student lists more, mark best two. [4 marks]

PART B: ANALYSIS AND CONCLUSIONS

B1. Expected answer (2 marks): A mixture is a substance made of two or more substances physically combined but not chemically joined, where each substance keeps its own properties (2 marks). Partial credit (1 mark) for 'two or more substances combined but not chemically joined' or equivalent phrasing. [2 marks]

B2. Expected answer (2 marks): Example (1 mark): e.g. sugar dissolved in tea, salt dissolved in water, air. Reason (1 mark): the dissolved substance is evenly distributed at the visible/particle scale and the mixture looks the same throughout (uniform). [2 marks]

B3. Expected answer (2 marks): Example (1 mark): e.g. sand in water, oil and water, salad. Reason (1 mark): different components are not evenly distributed and you can see or detect different parts or layers (non-uniform). [2 marks]

B4. Expected drawings (4 marks): (a) Homogeneous: small symbols for two particle types evenly and randomly mixed throughout — label each type (2 marks). (b) Heterogeneous: regions or clusters dominated by one particle type and separate regions for the other — label each (2 marks). Award partial credit for correct concepts and labelling. [4 marks]

B5. Expected answer (3 marks): Mixtures are physical combinations of substances whose components are not chemically bonded, so physical methods (e.g. sieving, decanting or evaporation) can separate them (1 mark). Compounds consist of atoms chemically joined and require chemical reactions to break the bonds and separate elements (1 mark). A short example helps (1 mark) — e.g. salt mixed with sand can be separated physically, but salt chemically combined with chlorine would need chemical change to separate elements. Award partial credit if reasoning is correct but example omitted. [3 marks]

PART C: SKILLS ASSESSMENT MARKING GUIDE

Preparation of a simple mixture (mixing measured amounts): Award 2 marks if the learner measures and mixes the components as instructed without contaminating samples and uses correct volumes/masses; award 1 mark for partially correct mixing or minor measurement error that does not affect classification; 0 marks if procedure not followed. [maximum 2 marks]

Accurate observation and recording: Award 2 marks for clear, correct observations recorded for all three samples (immediate and after standing) using appropriate terms (e.g. 'clear', 'suspended particles', 'settled'); 1 mark for partially complete or slightly unclear records; 0 for no useful observations. [maximum 2 marks]

Clear labelled particle diagram: Award 1 mark for a clear, labelled particle diagram that correctly distinguishes homogeneous and heterogeneous arrangements; 0 marks if diagram unclear or labels missing/incorrect. [maximum 1 marks]

MARK ALLOCATION CHECK: 20 + 13 + 5 = 38 marks

END OF MARKING KEY.