The term pH means potential hydrogen, and is a logarithmic scale that measures the acidity or alkalinity of a solution. Strong acids may have a pH below 1 and strong alkalis can be above 14. A neutral solution has a pH of 7 (denoting equal amounts of H+ and OH- ions) but trace amounts of acidic or alkaline components can change the measured pH significantly.
So for example, rainwater can have a pH as low as 5.5 because of dissolved carbon dioxide, milk is usually slightly acidic with a pH of 6.5, human blood has a pH close to 7.4, and sea water is typically pH 7.5 to 8.4. Vinegar and lemon juice are acidic with a pH between 2.5 and 3, whilst wines may have a pH range of 3 to 4.
The pH of a solution may be estimated using a pH indicator strip, but is more accurately measured with a glass electrode and pH meter, that is suitably calibrated.
The pH of tap water can vary. Water quality regulations specify a pH of water from 6.5 to 9.5, and soft water is usually slightly acidic, whilst hard water is rendered alkaline by the dissolved salts. Acidic water can leach metal salts, (including corrosion products) from pipework giving water a metallic taste, this can be problematic when pipework is constructed of lead. Water with a high pH is more likely to deposit scale.
Most water boards will supply water with a pH value of 7-8, so if the pH of your tap water in your property is outside these limits, and different to that of your neighbour, it is worth checking to see if faulty plumbing is causing a problem. This could include washing machines or garden hoses, connected without a non-return valve, allowing contaminated water to flow back into the system when water pressure is low.