Experimental Design and Acids and Bases

What is a chemical reaction?

A chemical reaction occurs when bonds between atoms are broken or new bonds are formed, producing at least one new substance.

What happens to atoms during a chemical reaction?

Atoms are rearranged. No new atoms are created. No atoms are destroyed.

Are new elements formed in a chemical reaction?

No. The same atoms are present before and after the reaction. They are simply arranged differently.

What is formed in a chemical reaction?

At least one new substance with different properties is formed.

What happens to bonds during a chemical reaction?

Existing bonds are broken. New bonds are formed. This creates a new arrangement of atoms.

What is a physical change?

A physical change is a change in which no new substance is formed.

What can happen during a physical change?

Substances may:

• Change state (solid, liquid, gas)

• Be mixed together

• Be separated

What happens to atoms and bonds during a physical change?

The arrangement of atoms stays the same. No chemical bonds are broken or formed.

How is a chemical reaction different from a physical change?

Chemical Reaction:

• New substance formed

• Bonds broken and formed

• Atoms rearranged

Physical Change:

• No new substance formed

• Bonds unchanged

• Atoms arranged the same

Key fact about chemical reactions

Atoms are rearranged, but no new atoms or elements are made.

Key fact about physical changes

No new substance is formed, and the arrangement of atoms stays the same.

What is an acid?

A substance that donates H^+ (hydrogen) ions in solution.

What are the properties of acids?

Sour taste

Corrosive

Conduct electricity

pH less than 7

Can be neutralised by bases

Give examples of household acids.

Vinegar - Ethanoic acid
Lemons and limes - Citric acid
Milk - Lactic acid

What is a base (alkali)?

A substance that accepts H^+ (hydrogen) ions.

What are the properties of bases?

Slippery or soapy

Bitter taste

Conduct electricity

Caustic

pH greater than 7

Give examples of household bases.

Detergent - Sodium hydroxide

Baking soda - Sodium hydrogen carbonate

Bleach - Mixture of bases

What happens when an acid reacts with a base?

Neutralisation occurs.

What is a dilute solution?

A solution containing: Small amount of solute and a large amount of solvent

What is a concentrated solution?

A solution containing: Large amount of solute Small amount of solvent

What does the pH scale measure?

The strength of acids and bases on a scale from 0-14.

What does a change of 1 pH unit mean?

A tenfold change in acidity.

What is accuracy?

How close a measurement is to the true value.

What is an error?

Any difference between a measured value and the true value.

What is a systematic error?

An error that follows a pattern and consistently affects results.

What is a random error?

An error that does not follow a pattern.

Can averaging measurements reduce systematic errors?

No. Systematic errors shift results in the same direction and must be identified and corrected.

How can random errors be reduced?

By taking multiple measurements and calculating the average.

What is reliability?

The ability to obtain the same results when an investigation is repeated.

What is repeatability?

Getting the same results when YOU repeat the investigation.

What is reproducibility?

Getting the same results when SOMEONE ELSE repeats the investigation.

What is validity?

Whether an investigation is actually testing the correct hypothesis.

Can an investigation be reliable but not valid?

Yes. You may get consistent results, but still be testing the wrong thing.

Can an investigation be valid if it is not reliable?

No. An investigation must be reliable to be valid.

How can scientists reduce errors?

Use the correct equipment. Take multiple measurements and average them. Follow a detailed method.

Why are detailed methods important?

They allow others to replicate the investigation and verify the results.

pH of acids?

Less than 7.

pH of bases?

Greater than 7.

What ion do acids produce?

H^+ (hydrogen ions).

Acids are substances that ______ hydrogen ions.

Donate.

Bases are substances that ______ hydrogen ions.

Accept.

Acid + Base →

Salt + Water.

What is precision?

Precision is how close repeated measurements are to one another. It measures the consistency of measurements, even if they are not accurate.