الأسبوع 15 · مراجعة الفصل الدراسي الأول: العلاقات الغذائية بين الكائنات الحية وحركة الجسيمات · 3 دروس
3
Review: properties, mixtures and changes
Discovery Education Science P5: Term 1, Unit 2, pp. 147–205
GoalReview measuring, properties of materials, mixtures, and physical and chemical changes.
New words
property · something about a material that we can observe or measurephysical change · a change in size, shape or state with no new substancechemical change · a change that makes a new substance
Explanation
We measure mass with a balance in grams (g) or kilograms (kg), volume with a measuring jug in millilitres (mL) or litres (L), length with a ruler in centimetres (cm), and temperature with a thermometer in degrees Celsius (°C). The properties of a material explain its uses: copper is used for wires, glass for windows and rubber for boots. A mixture has two or more parts that keep their own properties, and we can separate it by picking, sieving, filtering or evaporating. If a mixture is made in a closed container, its mass equals the total of the masses of its parts. A physical change changes size, shape or state, and no new substance forms. A chemical change, such as rusting, baking or rotting, makes a new substance, shows signs such as gas bubbles, a new colour, heat or light, and is hard to reverse. Scientists explain with a claim, evidence and reasoning, and they test fairly by changing only one thing.
Question
Physical change
Chemical change
Is a new substance made?
no
yes
Easy to reverse?
usually yes
usually no
Examples
melting, cutting, dissolving
rusting, baking, rotting
Worked examples
Step
Mass
whole bag
2000 g
used
450 g
left
1550 g
A bag of sugar has a mass of 2 kg. Since 1 kg is 1000 g, the bag is 2 × 1000 = 2000 g. After 450 g is used, 2000 − 450 = 1550 g is left.
sand + salt 125 g→add water, stir→filter: sand stays→evaporate: salt left
Farida mixes 80 g of sand with 45 g of salt: 80 + 45 = 125 g. To separate them, she adds water to dissolve the salt, filters out the sand, and leaves the salt water in the sun until the water evaporates and salt is left. She never tastes it.
Sorting by asking: is a new substance made? Melting, tearing and crushing do not make one. A banana going brown, a nail rusting and a cooked egg do.
Class activity
Change journal (a grown-up helps). For three days, write down three changes you notice each day, at home or outside. For each one, write “physical” or “chemical” and the evidence: is there a new substance, and can it be changed back?
Practice
1
Which tool and which unit would you use for the volume of juice in a glass?
A measuring jug, in millilitres (mL).
2
A jar holds 60 g of rice and 40 g of lentils. What is the mass of the mixture, and how could you separate it?
60 + 40 = 100 g. Pick the lentils out by hand, or shake it through a sieve with holes that let the rice fall through and keep the lentils.
3
Physical or chemical? (a) a rusty bicycle, (b) a puddle drying, (c) a cake baking, (d) a wire bending.
Plan a fair test to find out if sugar dissolves faster in warm water than in cold water. What do you change, keep the same and measure?
Change the temperature of the water (a grown-up pours the warm water). Keep the same amount of water, the same amount of sugar and the same stirring. Measure the time the sugar takes to dissolve.
