Classic home experiments for an 8-year-old child My 8-year-old daughter's school report says that she's good at understanding the basic science she's doing, but she's having trouble seeing how experimental results lead to conclusions. Specifically, it says she struggles to appreciate how changing parameters in an experiment can be used to prove or rule out a hypothesis.
I'm a biologist/chemist by degree, and I found it hard to think of any parameter-based experiments in my field that we could do at home that wouldn't bore a child. They like creepy-crawlies well enough, but biology experiments tend to need time and repeat observations to get a result. Chemistry tends to need more specialist equipment and reagents.
I've got a couple of books of home science, and I scoured the net as well, but most of the living-room science experiments I found that were suitable for an 8-year-old were more demonstrations than actual experiments. There were few parameters: nothing you could really vary.
So can any of your learned people suggest some simple, parameter-based physics experiments we could do at home or in the garden, without specialist equipment, which involve science at a level an eight-year-old could engage with?
 A: Some basics experiments:
Freezing of water. The parameter is the temperature. After that, you could add some salt in the ice. You can show that the mix ice/water have a lower freezing temperature than just water. So you can observe T_fusion as a function of the mass percentage of salt in your mix.
You can do the same with other stuff : alcohol, vinegar, .....
You can put a iron and an wood spoon in your fridge/Oven put the temperature to something not too hot/cold and after that, try to grab thoses object. The iron one would be a little dangerous (be cautious) and will be very hot. The wood one will be hot, but you will not feel it and you can grab it without danger. THe parameter being the materiel (wood or iron) and the propertie being the feeled temperature.
If you want theoretical insight, look here : http://en.wikipedia.org/wiki/Thermal_effusivity
Optics is very cool for such experiments. You can easily show diffraction, interferences etc.... when it's appear etc, what is the effect of the color etc... 
A: You could see how the mass of an object affects how quickly it falls by dropping balls of various masses (tennis ball, orange, melon, whatever) from a specific height and timing which of these falls fastest. This will allow you to draw the conclusion that the time it takes an object to fall from a given height is independent of the mass. 
You could also vary the shape (dropping a feather, a ping pong ball, and a piece of paper) and measure that the time it takes to fall does vary. You can then conclude, in combination with your previous experiment, that the time it takes for an object to fall is based on its shape/size (air resistance), but not its mass.
A: As someone who has a fondness for atmospheric physics, I do enjoy the "cloud in a bottle" demonstration. I'm sure children would enjoy seeing a cloud in a bottle too.
All you need is:


*

*A 2 litre plastic bottle 

*A small amount of water

*A match


It demonstrates how a cloud forms by the process of adiabatic expansion and evaporates by adiabatic compression. This video will walk you through the process:
https://www.youtube.com/watch?v=MBh6TPYH3XU
A: There are so many things you could do. Here are just two:
Put things on a microwave. See how hot they get after 1 minute on high. Does it matter whether you have one, two, three cups. Does adding salt to the water make a difference. How about vegetable oil and water - do they heat at the same rate. What if you add sugar. Does it heat the same when you remove the turntable and put a cup right in the corner. Etc.
Lenses. Find a few cheap lenses. On a sunny day can you set paper on fire by focusing the sunlight. Does a big lens work better. What if the effect of focal length. Can you cover half the lens with paper - does the shape matter. Does it take longer. How much of the lens can you cover. Does black paper catch fire sooner. 
Baking. Make cakes in flat tins or deep tins. Glass or metal. Do they bake at the same rate. 
Your kitchen is full of physics and chemistry experiments just waiting to happen... 
If you take a look at my answer to another question you will see another nice experiment - if you have an iPhone and three dollars you can do this yourself. Another twenty buys you a scale on Amazon that allows you to weigh objects with mg resolution... A chance to really go overboard!
A: Range of a projectile as a function of launch angle is simple and has a nice "right answer" that you can compare with. I'd start with the simplest: "throw this ball as hard as you can straight forward, then straight up, then at some different angles". Measure the distances. If you can do it easily, set up a video camera and play around with measuring the launch angles. Make a graph, there's sure to be a peak somewhere, hopefully near $45^\circ$. Sure there's some biological uncertainty getting involved ("as hard as you can"), but I bet the results won't be terrible.
If you still have enthusiasm after that you can try coming up with a launching device that throws at the same strength (launch velocity) every time. You can also try different projectiles, tying in with Fenzik's answer about the rate at which objects of different masses and shapes fall.
A: There are a lot of good suggestions here, but I think some of them are missing the crux of the question; the student needs to learn to prove or disprove a hypothesis by varying parameters.
For that, you might need several hypotheses - demo experiments are not going to help for the reason they haven't worked so far; they show a rule working, rather than the process of investigating which rule is correct.
Monkey and the hunter
This is a classic experiment, designed to demonstrate separation of vertical and horizontal motion, but the story gives a lot of opportunity to frustrate intuitions and thus space for multiple reasonable hypotheses.
The story goes like this: "Kiki is a very curious monkey, and one day she decides to ignore her parents and go investigate the humans on the other side of the forest. She is just hanging on a branch when she hears a noise, and sees one of the humans, holding a gun, and aiming straight at her! She has seen people use guns before, and decides to let go the instant the gun fires, so that the bullet flies over her head. The gun fires, and Kiki lets go instantly. What happens to Kiki?"
Ask the student what her hypothesis is. There are a few options here:


*

*Kiki falls far enough that the bullet sails over Kiki's head.

*The bullet flies so fast that Kiki has barely moved, and Kiki meets an unfortunate end

*Both Kiki and the bullet fall together, and Kiki meets the same fate


Hopefully, the student will be able to come up with their own ideas of what they could do to verify or falsify one hypothesis versus another. In case they don't, try suggesting some of the following:


*

*The distance between the hunter and the monkey (can be used to falsify #2)

*The speed of the bullet (can be used to falsify #2)

*Plotting the distance fallen vs the distance between hunter and monkey will show quadratic relationship, verifying #3; she is falling further as the gap widens, and it's not linear, but both have the same vertical component of their motion.

*Adding a delay between the shot and Kiki letting go will make the hunter miss (falsify #1, #2)


Running the experiment once will falsify #1 (sorry, Kiki doesn't make it).
The experiment itself I still remember from an A-level Physics class (I remember the teacher, the room, the equipment vividly). It requires a steel tin, an electromagnet, a spring-based launcher, some aluminium foil and some stands to hold everything up. Set up the launcher to fire the projectile (ball bearing?) directly at the tin, which will be held up by the electromagnet. Clip the foil loosely on the end of the gun, in the path of the projectile, and use it to complete the electromagnet's circuit. As the projectile leaves the gun, it will break the connection and switch off the magnet, precipitating the monkey's fall. 
I remember the flat metallic clatter of ball bearing off cream chalk tin as our gruff Physics teacher looked on with gruff glee. Mr Gander, if you're out there, thanks for that, I've got a doctorate now.
A: How big is an atom?
Fill a sink with water. Find a chemical which, when dropped into water, forms a contiguous floating disk. Drop one drop of this chemical into water. Measure volume of drop and area of floating disk. This provides an upper bound on the size of an atom.
Falling speed versus mass/shape
Drop a book and a piece of paper at the same time. Book hits floor first. Now place paper on top of book and drop book. They fall together. This suggests that air resistance is the reason the paper normally falls slower, and that it has nothing to do with mass.
You can also crumple the paper and then drop it with the book to get the same effect.
A: One that jumps to mind is Hooke's law (extension of a spring). Hang a spring or thick elastic band and load it with increasing weights. See that extension is proportional to load at least initially. A natural extension of that is to also measure oscillation time/frequency.
Another one would be Archimedes principle, and play with floating/sinking different size/density objects to a tub of water and try and arrive at the correct law.
This that go bang are always good, so you could do bottle rockets or baking soda and vinegar and look at the optimal quantities to get the highest/biggest reactions.
If you are looking for experiments, this thread has many (nominally) home experiments. Some of which might be appropriate for you (some people have a different idea of home experiment!).
Edit: As you mention you're a chemist there are loads of good chemistry experiments with household products. Vinegar and baking soda is one. You can do homemade indicators, red cabbage is one IIRC. You can put small coins in coke or vinegar and they get cleaned. You can make a simple electrolysis cell to get hydrogen and oxygen (don't use table salt or you make chlorine gas...). I'm sure you could adapt some of these to look at some parameter and prove a theory.
A: The Non-Newtonian fluid experiment:  
Slowly add an equal amount of water to a mixing bowl of cornstarch. 500ml of each should do nicely.

Continue to add water until the cornstarch acts like a liquid when you stir it slowly. When you tap on the liquid with your finger, it shouldn't splash, but rather will become hard. If your mixture is too liquid, add more cornstarch. Your goal is to create a mixture that feels like a stiff liquid when you stir it slowly, but feels like a solid when you tap on it with your finger or a spoon. 


Pictorial recipe here and a text only version. 

"Struggles to appreciate how changing parameters in an experiment can be used to prove or rule out a hypothesis" Reads to me as: doesn't have a firm grasp of the scientific method and misunderstands the importance of the control group. This may be a little advanced for 3rd grade, but its worth a shot:
http://askabiologist.asu.edu/teaching-scientific-method
A: Here's an experiment with multiple parameters that simulates meteor impacts. Bonus if your daughter is interested in astronomy and space.
Fill a box or basin with sand and smooth the surface. Then drop rocks onto the sand and measure the diameter or depth of the resulting crater (diameter is probably easier). You can vary the mass (different sized rocks) and speed (different starting heights) of the projectiles and observe how the size of the crater varies. Make sure to resmooth the surface between each drop. Let your daughter do the dropping and record the initial conditions and results.
Once you have some data, you can teach her how to plot data and look for trends. See if your daughter can predict the size of the next crater given a rock with a different mass or one dropped from a different height (changing both at once would probably be too difficult). No equations needed, just let try to guess where points would be that haven't been measured yet. Try interpolation and extrapolation.
As a finale, take her on a trip to some real craters (http://meteorcrater.com/) and talk about how big and fast the rock that created it must have been.
