Grade 7 Science · Chapter 7 · Seven hands-on experiments you can run on screen
Welcome to the lab
Pick an experiment. In each one, change something, watch what happens, record what you see, then check your thinking with the question at the bottom.
1
Pins on a Heated Strip
Four pins are stuck below a strip with wax. The right end is heated by a candle. Which pin falls first, and how does the material change things?
Observations
0 stime heating
25°Chot end
Material
I
II
III
IV
Try this: run copper, then iron, then wood. Compare when pin I falls.
Why does pin I fall before pin II, and why don't the pins fall from a wooden strip?
Heat travels along the strip particle by particle from the hot end (conduction), so it reaches pin I first. Metals are good conductors; copper is fastest. Wood and glass are poor conductors, so very little heat reaches the wax.
2
Convection Currents
A crystal of purple potassium permanganate sits at the bottom of a beaker of water. Heat the centre of the base and watch the colour move. Then see which way incense smoke goes.
What to look for
25°Cwater at bottom
25°Cwater at top
Where does the purple streak go first?
Which way does it come back down?
Turn the incense stick upside down. Does the smoke follow the stick?
Why does the coloured water rise in the middle and sink at the sides?
Water at the bottom centre gets hot, expands, becomes lighter and rises. Cooler, heavier water at the sides sinks to take its place. The moving water itself carries the heat: this is convection. Smoke is hot gas, so it always rises, whichever way the stick points.
3
Land and Sea Breeze
Move through a whole day at the beach. Watch the land and sea temperatures and the direction of the wind.
Readings
--land
--sea
Breeze now: --
Try this: stop at 2 pm and at 2 am. Which way does the flag point each time?
Why does the breeze reverse at night?
Land heats up and cools down faster than water. By day the land is warmer, air above it rises and cool air comes in from the sea (sea breeze). At night the land is cooler, air above the warmer sea rises and cool air flows from land to sea (land breeze).
4
Colours in the Sun
Two identical bottles of water stand in sunlight. Choose their colours, switch on the Sun and compare their temperatures.
Bottle ABottle B
Thermometers
25.0°Cbottle A
25.0°Cbottle B
0 mintime
Try this: black vs white, then white vs silver. Then switch the Sun off and watch them cool.
Why do we wear white in summer and dark clothes in winter?
The Sun's heat reaches the bottles by radiation, which needs no medium. Dark surfaces absorb more of it, so they warm up faster. Light and shiny surfaces reflect most of it and stay cooler.
5
Keep the Tea Hot
Hot tea at 80°C is poured into different cups in a room at 25°C. Pick the cups to test and see which keeps the tea hot longest.
After 30 minutes
Cup
Temperature
Why does the kulhad do better than the steel glass, and the flask best of all?
Steel is a good conductor, so heat leaves the tea quickly. Clay and porcelain are poor conductors. A flask has a vacuum between its walls, so heat cannot leave by conduction or convection, and its shiny walls reduce radiation.
6
Seepage: Clay, Sand and Gravel
Pour 200 mL of water into each bottle and collect what comes out in 10 minutes. Make your predictions first!
Predict, then observe
Bottle
Prediction
Collected
0 mintime
Why does water pass through gravel fastest?
Gravel has wide, open and connected spaces between its particles, so water seeps through easily. Clay particles are tiny and tightly packed. This seeping of water through soil and rocks is called infiltration.
7
Water Cycle Machine
Turn up the Sun, grow forests or pave the land with concrete, and track where the water goes. Watch the total.
Water accounts (units)
0sea and lakes
0vapour and clouds
0groundwater
0total water
--
Try this: switch to the concrete city and watch the groundwater line. Then set the Sun to 0.
What happens to the total amount of water?
The total never changes: water only moves and changes state (evaporation, condensation, precipitation, collection). Concrete blocks infiltration, so less rain reaches the aquifer and groundwater is not recharged.